Removed ancient FAQ from distribution
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-Subject: FAQ: Python -- an object-oriented language
-Newsgroups: comp.lang.python,comp.answers,news.answers
-Followup-to: comp.lang.python
-From: guido@cnri.reston.va.us (Guido van Rossum)
-Reply-to: guido@cnri.reston.va.us (Guido van Rossum)
-Expires: Sun, 1 Dec 1996 00:00:00 GMT
-Supersedes: <DxJ3t1.CJv@cwi.nl>
-Approved: news-answers-request@MIT.Edu
-
-Archive-name: python-faq/part1
-Submitted-by: Guido van Rossum <guido@cnri.reston.va.us>
-Version: $Revision$
-Last-modified: $Date$
-
-This article contains answers to Frequently Asked Questions about
-Python (an object-oriented interpreted programming language -- see
-the answer to question 1.1 for a short overview).
-
-Copyright 1993-1996 Guido van Rossum. Unchanged electronic
-redistribution of this FAQ is allowed. Printed redistribution only
-with permission of the author. No warranties.
-
-Author's address:
- Guido van Rossum
- C.N.R.I.
- 1895 Preston White Drive
- Reston, VA 20191
- U.S.A.
-Email: <guido@python.org>, <guido@cnri.reston.va.us>
-
-The latest version of this FAQ is available by anonymous ftp from
-<URL:ftp://ftp.python.org/pub/python/doc/FAQ>. It will also be posted
-regularly to the newsgroups comp.answers <URL:news:comp.answers> and
-comp.lang.python <URL:news:comp.lang.python>.
-
-Many FAQs, including this one, are available by anonymous ftp
-<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/>. The name under
-which a FAQ is archived appears in the Archive-name line at the top of
-the article. This FAQ is archived as python-faq/part1
-<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/python-faq/part1>.
-
-There's a mail server on that machine which will send you files from
-the archive by e-mail if you have no ftp access. You send a e-mail
-message to <mail-server@rtfm.mit.edu> containing the single word help
-in the message body to receive instructions.
-
-This FAQ is divided in the following chapters:
-
- 1. General information and availability
- 2. Python in the real world
- 3. Building Python and Other Known Bugs
- 4. Programming in Python
- 5. Extending Python
- 6. Python's design
- 7. Using Python on non-UNIX platforms
-
-To find the start of a particular chapter, search for the chapter number
-followed by a dot and a space at the beginning of a line (e.g. to
-find chapter 4 in vi, type /^4\. /).
-
-Here's an overview of the questions per chapter:
-
- 1. General information and availability
- 1.1. Q. What is Python?
- 1.2. Q. Why is it called Python?
- 1.3. Q. How do I obtain a copy of the Python source?
- 1.4. Q. How do I get documentation on Python?
- 1.5. Q. Are there other ftp sites that mirror the Python distribution?
- 1.6. Q. Is there a newsgroup or mailing list devoted to Python?
- 1.7. Q. Is there a WWW page devoted to Python?
- 1.8. Q. Is the Python documentation available on the WWW?
- 1.9. Q. Is there a book on Python, or will there be one out soon?
- 1.10. Q. Are there any published articles about Python that I can quote?
- 1.11. Q. Are there short introductory papers or talks on Python?
- 1.12. Q. How does the Python version numbering scheme work?
- 1.13. Q. How do I get a beta test version of Python?
- 1.14. Q. Are there copyright restrictions on the use of Python?
- 1.15. Q. Why was Python created in the first place?
-
- 2. Python in the real world
- 2.1. Q. How many people are using Python?
- 2.2. Q. Have any significant projects been done in Python?
- 2.3. Q. Are there any commercial projects going on using Python?
- 2.4. Q. How stable is Python?
- 2.5. Q. What new developments are expected for Python in the future?
- 2.6. Q. Is it reasonable to propose incompatible changes to Python?
- 2.7. Q. What is the future of Python?
- 2.8. Q. What is the PSA, anyway?
- 2.9. Q. How do I join the PSA?
- 2.10. Q. What are the benefits of joining the PSA?
-
- 3. Building Python and Other Known Bugs
- 3.1. Q. Is there a test set?
- 3.2. Q. When running the test set, I get complaints about floating point
- operations, but when playing with floating point operations I cannot
- find anything wrong with them.
- 3.3. Q. Link errors after rerunning the configure script.
- 3.4. Q. The python interpreter complains about options passed to a
- script (after the script name).
- 3.5. Q. When building on the SGI, make tries to run python to create
- glmodule.c, but python hasn't been built or installed yet.
- 3.6. Q. I use VPATH but some targets are built in the source directory.
- 3.7. Q. Trouble building or linking with the GNU readline library.
- 3.8. Q. Trouble with socket I/O on older Linux 1.x versions.
- 3.9. Q. Trouble with prototypes on Ultrix.
- 3.10. Q. Other trouble building Python on platform X.
- 3.11. Q. How to configure dynamic loading on Linux.
- 3.12. Q: I can't get shared modules to work on Linux 2.0 (Slackware96)?
- 3.13. Q: Trouble when making modules shared on Linux.
- 3.14. Q. How to use threads on Linux.
- 3.15. Q. Errors when linking with a shared library containing C++ code.
- 3.16. Q. I built with tkintermodule.c enabled but get "Tkinter not found".
- 3.17. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
- 3.18. Q. Link errors for Tcl/Tk symbols when linking with Tcl/Tk.
- 3.19. Q. I configured and built Python for Tcl/Tk but "import Tkinter"
- fails.
- 3.20. Q. Tk doesn't work right on DEC Alpha.
- 3.21. Q. Several common system calls are missing from the posix module.
- 3.22. Q. ImportError: No module named string, on MS Windows.
- 3.23. Q. Core dump on SGI when using the gl module.
-
- 4. Programming in Python
- 4.1. Q. Is there a source code level debugger with breakpoints, step,
- etc.?
- 4.2. Q. Can I create an object class with some methods implemented in
- C and others in Python (e.g. through inheritance)? (Also phrased as:
- Can I use a built-in type as base class?)
- 4.3. Q. Is there a curses/termcap package for Python?
- 4.4. Q. Is there an equivalent to C's onexit() in Python?
- 4.5. Q. When I define a function nested inside another function, the
- nested function seemingly can't access the local variables of the
- outer function. What is going on? How do I pass local data to a
- nested function?
- 4.6. Q. How do I iterate over a sequence in reverse order?
- 4.7. Q. My program is too slow. How do I speed it up?
- 4.8. Q. When I have imported a module, then edit it, and import it
- again (into the same Python process), the changes don't seem to take
- place. What is going on?
- 4.9. Q. How do I find the current module name?
- 4.10. Q. I have a module in which I want to execute some extra code
- when it is run as a script. How do I find out whether I am running as
- a script?
- 4.11. Q. I try to run a program from the Demo directory but it fails
- with ImportError: No module named ...; what gives?
- 4.12. Q. I have successfully built Python with STDWIN but it can't
- find some modules (e.g. stdwinevents).
- 4.13. Q. What GUI toolkits exist for Python?
- 4.14. Q. Are there any interfaces to database packages in Python?
- 4.15. Q. Is it possible to write obfuscated one-liners in Python?
- 4.16. Q. Is there an equivalent of C's "?:" ternary operator?
- 4.17. Q. My class defines __del__ but it is not called when I delete the
- object.
- 4.18. Q. How do I change the shell environment for programs called
- using os.popen() or os.system()? Changing os.environ doesn't work.
- 4.19. Q. What is a class?
- 4.20. Q. What is a method?
- 4.21. Q. What is self?
- 4.22. Q. What is a unbound method?
- 4.23. Q. How do I call a method defined in a base class from a derived
- class that overrides it?
- 4.24. Q. How do I call a method from a base class without using the
- name of the base class?
- 4.25. Q. How can I organize my code to make it easier to change the base
- class?
- 4.26. Q. How can I find the methods or attributes of an object?
- 4.27. Q. I can't seem to use os.read() on a pipe created with os.popen().
- 4.28. Q. How can I create a stand-alone binary from a Python script?
- 4.29. Q. What WWW tools are there for Python?
- 4.30. Q. How do I run a subprocess with pipes connected to both input
- and output?
- 4.31. Q. How do I call a function if I have the arguments in a tuple?
- 4.32. Q. How do I enable font-lock-mode for Python in Emacs?
- 4.33. Q. Is there an inverse to the format operator (a la C's scanf())?
- 4.34. Q. Can I have Tk events handled while waiting for I/O?
- 4.35. Q. How do I write a function with output parameters (call by reference)?
- 4.36. Q. Please explain the rules for local and global variables in Python.
- 4.37. Q. How can I have modules that mutually import each other?
- 4.38. Q. How do I copy an object in Python?
- 4.39. Q. How to implement persistent objects in Python? (Persistent ==
- automatically saved to and restored from disk.)
- 4.40. Q. I try to use __spam and I get an error about _SomeClassName__spam.
- 4.41. Q. How do I delete a file? And other file questions.
- 4.42. Q. How to modify urllib or httplib to support HTTP/1.1?
- 4.43. Q. Unexplicable syntax errors in compile() or exec.
- 4.44. Q. How do I convert a string to a number?
- 4.45. Q. How do I convert a number to a string?
-
- 5. Extending Python
- 5.1. Q. Can I create my own functions in C?
- 5.2. Q. Can I create my own functions in C++?
- 5.3. Q. How can I execute arbitrary Python statements from C?
- 5.4. Q. How can I evaluate an arbitrary Python expression from C?
- 5.5. Q. How do I extract C values from a Python object?
- 5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length?
- 5.7. Q. How do I call an object's method from C?
- 5.8. Q. How do I catch the output from print_error()?
- 5.9. Q. How do I access a module written in Python from C?
- 5.10. Q. How do I interface to C++ objects from Python?
-
- 6. Python's design
- 6.1. Q. Why isn't there a switch or case statement in Python?
- 6.2. Q. Why does Python use indentation for grouping of statements?
- 6.3. Q. Why are Python strings immutable?
- 6.4. Q. Why don't strings have methods like index() or sort(), like
- lists?
- 6.5. Q. Why does Python use methods for some functionality
- (e.g. list.index()) but functions for other (e.g. len(list))?
- 6.6. Q. Why can't I derive a class from built-in types (e.g. lists or
- files)?
- 6.7. Q. Why must 'self' be declared and used explicitly in method
- definitions and calls?
- 6.8. Q. Can't you emulate threads in the interpreter instead of
- relying on an OS-specific thread implementation?
- 6.9. Q. Why can't lambda forms contain statements?
- 6.10. Q. Why don't lambdas have access to variables defined in the
- containing scope?
- 6.11. Q. Why can't recursive functions be defined inside other functions?
- 6.12. Q. Why is there no more efficient way of iterating over a dictionary
- than first constructing the list of keys()?
- 6.13. Q. Can Python be compiled to machine code, C or some other language?
- 6.14. Q. Why doesn't Python use proper garbage collection?
-
- 7. Using Python on non-UNIX platforms
- 7.1. Q. Is there a Mac version of Python?
- 7.2. Q. Are there DOS and Windows versions of Python?
- 7.3. Q. Is there an OS/2 version of Python?
- 7.4. Q. Is there a VMS version of Python?
- 7.5. Q. What about IBM mainframes, or other non-UNIX platforms?
- 7.6. Q. Where are the source or Makefiles for the non-UNIX versions?
- 7.7. Q. What is the status and support for the non-UNIX versions?
- 7.8. Q. I have a PC version but it appears to be only a binary.
- Where's the library?
- 7.9. Q. Where's the documentation for the Mac or PC version?
- 7.10. Q. The Mac (PC) version doesn't seem to have any facilities for
- creating or editing programs apart from entering it interactively, and
- there seems to be no way to save code that was entered interactively.
- How do I create a Python program on the Mac (PC)?
-
-To find a particular question, search for the question number followed
-by a dot, a space, and a Q at the beginning of a line (e.g. to find
-question 4.2 in vi, type /^4\.2\. Q/).
-
-
-1. General information and availability
- =======================================
-
-1.1. Q. What is Python?
-
-A. Python is an interpreted, interactive, object-oriented programming
-language. It incorporates modules, exceptions, dynamic typing, very
-high level dynamic data types, and classes. Python combines
-remarkable power with very clear syntax. It has interfaces to many
-system calls and libraries, as well as to various window systems, and
-is extensible in C or C++. It is also usable as an extension language
-for applications that need a programmable interface. Finally, Python
-is portable: it runs on many brands of UNIX, on the Mac, and on PCs
-under MS-DOS, Windows, Windows NT, and OS/2.
-
-To find out more, the best thing to do is to start reading the
-tutorial from the documentation set (see a few questions further
-down).
-
-1.2. Q. Why is it called Python?
-
-A. Apart from being a computer scientist, I'm also a fan of "Monty
-Python's Flying Circus" (a BBC comedy series from the seventies, in
-the -- unlikely -- case you didn't know). It occurred to me one day
-that I needed a name that was short, unique, and slightly mysterious.
-And I happened to be reading some scripts from the series at the
-time... So then I decided to call my language Python. But Python is
-not a joke. And don't you associate it with dangerous reptiles
-either! (If you need an icon, use an image of the 16-ton weight from
-the TV series or of a can of SPAM :-)
-
-1.3. Q. How do I obtain a copy of the Python source?
-
-A. The latest complete Python source distribution is always available
-by anonymous ftp, e.g.
-<URL:ftp://ftp.python.org/pub/python/src/python1.3.tar.gz>. It is a
-gzipped tar file containing the complete C source, LaTeX
-documentation, Python library modules, example programs, and several
-useful pieces of freely distributable software. This will compile and
-run out of the box on most UNIX platforms. (See section 7 for
-non-UNIX information.)
-
-Sometimes beta versions of a newer release are available; check the
-subdirectory "beta" of the above-mentioned URL (i.e.
-<URL:ftp://ftp.python.org/pub/python/src/beta/>). (At the time of
-writing, beta3 for Python 1.4 is available there, and should be
-checked before reporting problems with version 1.3.)
-
-Occasionally a set of patches is issued which has to be applied using
-the patch program. These patches are placed in the same directory,
-e.g. <URL:ftp://ftp.python.org/pub/python/src/patch1.1.1>. (At the time
-of writing, no patches exist.)
-
-An index of said ftp directory can be found in the file INDEX. An
-HTML version of the index can be found in the file index.html,
-<URL:ftp://ftp.python.org/pub/python/index.html>.
-
-1.4. Q. How do I get documentation on Python?
-
-A. The LaTeX source for the documentation is part of the source
-distribution. If you don't have LaTeX, the latest Python
-documentation set is always available by anonymous ftp, e.g.
-<URL:ftp://ftp.python.org/pub/python/doc/postscript.tar.gz>. It is a
-gzipped tar file containing PostScript files of the reference manual,
-the library manual, and the tutorial. Note that the library manual is
-the most important one of the set, as much of Python's power stems
-from the standard or built-in types, functions and modules, all of
-which are described here. PostScript for a high-level description of
-Python is in the file nluug-paper.ps (a separate file on the ftp
-site).
-
-1.5. Q. Are there other ftp sites that mirror the Python distribution?
-
-A. The following anonymous ftp sites keep mirrors of the Python
-distribution:
-
-USA:
-
- <URL:ftp://ftp.python.org/pub/python/>
- <URL:ftp://gatekeeper.dec.com/pub/plan/python/>
- <URL:ftp://ftp.uu.net/languages/python/>
- <URL:ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python/>
- <URL:ftp://ftp.sterling.com/programming/languages/python/>
- <URL:ftp://uiarchive.cso.uiuc.edu/pub/lang/python/>
- <URL:ftp://ftp.pht.com/mirrors/python/python/>
- <URL:ftp://ftp.cdrom.com/pub/python/>
-
-Europe:
-
- <URL:ftp://ftp.cwi.nl/pub/python/>
- <URL:ftp://ftp.funet.fi/pub/languages/python/>
- <URL:ftp://ftp.sunet.se/pub/lang/python/>
- <URL:ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python/>
- <URL:ftp://ftp.ibp.fr/pub/python/>
- <URL:ftp://sunsite.cnlab-switch.ch/mirror/python/>
- <URL:ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python/>
-
-Australia:
-
- <URL:ftp://ftp.dstc.edu.au/pub/python/>
-
-Or try archie on the string "python".
-
-1.6. Q. Is there a newsgroup or mailing list devoted to Python?
-
-A. There is a newsgroup, comp.lang.python <URL:news:comp.lang.python>,
-and a mailing list. The newsgroup and mailing list are gatewayed into
-each other -- if you can read news it's unnecessary to subscribe to
-the mailing list. Send e-mail to <python-list-request@cwi.nl> to
-(un)subscribe to the mailing list. Hypermail archives of (nearly)
-everything posted to the mailing list (and thus the newsgroup) are
-available on our WWW server,
-<URL:http://www.cwi.nl/~guido/hypermail/index.html>. The raw archives
-are also available by ftp, e.g.
-<URL:ftp://ftp.python.org/pub/python/mail/mailinglist.gz>. The
-uncompressed versions of these files can be read with the standard
-UNIX Mail program ("Mail -f file") or with nn ("nn file"). To read
-them using MH, you could use "inc -file file". (The archival service
-has stopped archiving new articles around the end of April 1995. I
-hope to revive it on the PSA server www.python.org sometime in the
-future.)
-
-1.7. Q. Is there a WWW page devoted to Python?
-
-A. Yes, <URL:http://www.python.org/> is the official Python home page.
-At the time of writing, this page is not yet completely operational;
-you may have a look at the old Python home page:
-<URL:http://www.cwi.nl/~guido/Python.html> or at the U.S. copy:
-<URL:http://www.python.org/~guido/Python.html>.
-
-1.8. Q. Is the Python documentation available on the WWW?
-
-A. Yes, see <URL:http://www.python.org/> (Python's home page). It
-contains pointers to hypertext versions of the whole documentation set
-(as hypertext, not just PostScript).
-
-If you wish to browse this collection of HTML files on your own
-machine, it is available bundled up by anonymous ftp,
-e.g. <URL:ftp://ftp.python.org/pub/python/doc/html.tar.gz>.
-
-An Emacs-INFO set containing the library manual is also available by
-ftp, e.g. <URL:ftp://ftp.python.org/pub/python/doc/lib-info.tar.gz>.
-
-1.9. Q. Is there a book on Python, or will there be one out soon?
-
-A. Mark Lutz is writing a Python book for O'Reilly and Associates, to
-be published early 1996. See the outline (in PostScript):
-<URL:http://www.python.org/workshops/1995-05/outlinep.eps>.
-
-1.10. Q. Are there any published articles about Python that I can quote?
-
-A. So far the only refereed and published article that describes
-Python in some detail is:
-
- Guido van Rossum and Jelke de Boer, "Interactively Testing Remote
- Servers Using the Python Programming Language", CWI Quarterly, Volume
- 4, Issue 4 (December 1991), Amsterdam, pp 283-303.
-
-LaTeX source for this paper is available as part of the Python source
-distribution.
-
-See also the next section (supposedly Aaron Watters' paper has been
-refereed).
-
-1.11. Q. Are there short introductory papers or talks on Python?
-
-A. A recent, very entertaining introduction to Python is the tutorial by
-Aaron Watters in UnixWorld Online:
-
- Aaron R. Watters: "The What, Why, Who, and Where of Python",
- <URL:http://www.wcmh.com/uworld/archives/95/tutorial/005.html>
-
-An olded paper is:
-
- Guido van Rossum, "An Introduction to Python for UNIX/C
- Programmers", in the proceedings of the NLUUG najaarsconferentie
- 1993 (dutch UNIX users group meeting November 1993).
-
-PostScript for this paper and for the slides used for the accompanying
-presentation is available by ftp as
-<URL:ftp://ftp.python.org/pub/python/doc/nluug-paper.ps> and
-<URL:ftp://ftp.python.org/pub/python/doc/nluug-slides.ps>, respectively.
-
-Slides for a talk on Python that I gave at the Usenix Symposium on
-Very High Level Languages in Santa Fe, NM, USA in October 1994 are
-available as <URL:ftp://ftp.python.org/pub/python/doc/vhll-slides.ps>.
-
-1.12. Q. How does the Python version numbering scheme work?
-
-A. Python versions are numbered A.B.C or A.B. A is the major version
-number -- it is only incremented for major changes in functionality or
-source structure. B is the minor version number, incremented for less
-earth-shattering changes to a release. C is the patchlevel -- it is
-incremented for each new patch release. Not all releases have patch
-releases. Note that in the past, patches have added significant
-changes; in fact the changeover from 0.9.9 to 1.0.0 was the first time
-that either A or B changed!
-
-Beta versions have an additional suffix of "betaN" for some small
-number N. Note that (for instance) all versions labeled 1.4betaN
-*precede* the actual release of 1.4. 1.4b3 is short for 1.4beta3.
-
-1.13. Q. How do I get a beta test version of Python?
-
-A. If there are any beta releases, they are published in the normal
-source directory (e.g. <URL:ftp://ftp.python.org/pub/python/src/>).
-
-1.14. Q. Are there copyright restrictions on the use of Python?
-
-A. Hardly. You can do anything you want with the source, as long as
-you leave the copyrights in, and display those copyrights in any
-documentation about Python that you produce. Also, don't use the
-author's institute's name in publicity without prior written
-permission, and don't hold them responsible for anything (read the
-actual copyright for a precise legal wording).
-
-In particular, if you honor the copyright rules, it's OK to use Python
-for commercial use, to sell copies of Python in source or binary form,
-or to sell products that enhance Python or incorporate Python (or part
-of it) in some form. I would still like to know about all commercial
-use of Python!
-
-1.15. Q. Why was Python created in the first place?
-
-A. Here's a *very* brief summary of what got me started:
-
-- I had extensive experience with implementing an interpreted language
-in the ABC group at CWI, and from working with this group I had
-learned a lot about language design. This is the origin of many
-Python features, including the use of indentation for statement
-grouping and the inclusion of very-high-level data types (although the
-details are all different in Python).
-
-- I had a number of gripes about the ABC language, but also liked many
-of its features. It was impossible to extend the ABC language (or its
-implementation) to remedy my complaints -- in fact its lack of
-extensibility was one of its biggest problems.
-
-- I had some experience with using Modula-2+ and talked with the
-designers of Modula-3 (and read the M3 report). M3 is the origin of
-the syntax and semantics used for exceptions, and some other Python
-features.
-
-- I was working in the Amoeba distributed operating system group at
-CWI. We needed a better way to do system administration than by
-writing either C programs or Bourne shell scripts, since Amoeba had
-its own system call interface which wasn't easily accessible from the
-Bourne shell. My experience with error handling in Amoeba made me
-acutely aware of the importance of exceptions as a programming
-language feature.
-
-- It occurred to me that a scripting language with a syntax like ABC
-but with access to the Amoeba system calls would fill the need. I
-realized that it would be foolish to write an Amoeba-specific
-language, so I decided that I needed a language that was generally
-extensible.
-
-- During the 1989 Christmas holidays, I had a lot of time on my hand,
-so I decided to give it a try. During the next year, while still
-mostly working on it in my own time, Python was used in the Amoeba
-project with increasing success, and the feedback from colleagues made
-me add many early improvements.
-
-- In February 1991, after just over a year of development, I decided
-to post to USENET. The rest is in the Misc/HISTORY file.
-
-
-2. Python in the real world
-===========================
-
-2.1. Q. How many people are using Python?
-
-A. I don't know, but the maximum number of simultaneous subscriptions
-to the Python mailing list before it was gatewayed into the newsgroup
-was about 180 (several of which were local redistribution lists). I
-believe that many active Python users don't bother to subscribe to the
-list, and now that there's a newsgroup the mailing list subscription
-is even less meaningful. I see new names on the newsgroup all the
-time and my best guess is that there are currently at least several
-thousands of users.
-
-Another statistic is the number of accesses to the Python WWW server.
-Have a look at <URL:http://www.python.org/stats/>.
-
-2.2. Q. Have any significant projects been done in Python?
-
-A. Here at CWI (the home of Python), we have written a 20,000 line
-authoring environment for transportable hypermedia presentations, a
-5,000 line multimedia teleconferencing tool, as well as many many
-smaller programs.
-
-The University of Virginia uses Python to control a virtual reality
-engine. Contact: Matt Conway <conway@virginia.edu>.
-
-The ILU project at Xerox PARC can generate Python glue for ILU
-interfaces. See <URL:ftp://ftp.parc.xerox.com/pub/ilu/ilu.html>.
-
-The University of California, Irvine uses a student administration
-system called TELE-Vision written entirely in Python. Contact: Ray
-Price <rlprice@uci.edu>.
-
-See also the next question.
-
-If you have done a significant project in Python that you'd like to be
-included in the list above, send me email!
-
-2.3. Q. Are there any commercial projects going on using Python?
-
-A. Yes, there's lots of commercial activity using Python. See
-<URL:http://www.python.org/python/Users.html> for a list.
-
-2.4. Q. How stable is Python?
-
-A. Very stable. While the current version number would suggest it is
-in the early stages of development, in fact new, stable releases
-(numbered 0.9.x through 1.4) have been coming out roughly every 3 to
-6 or 12 months for the past four years.
-
-2.5. Q. What new developments are expected for Python in the future?
-
-A. Follow the newsgroup discussions! The workshop proceedings
-(<URL:http://www.python.org/workshops/>) may also contain interesting
-looks into the future.
-
-2.6. Q. Is it reasonable to propose incompatible changes to Python?
-
-A. In general, no. There are already millions of lines of Python code
-around the world, so any changes in the language that invalidates more
-than a very small fraction of existing programs has to be frowned
-upon. Even if you can provide a conversion program, there still is
-the problem of updating all documentation. Providing a gradual
-upgrade path is the only way if a feature has to be changed.
-
-2.7. Q. What is the future of Python?
-
-A. If I knew, I'd be rich :-)
-
-Seriously, the formation of the PSA (Pyton Software Activity, see
-<URL:http://www.python.org/psa/>) ensures some kind of support even in
-the (unlikely! event that I'd be hit by a bus (actually, here in the
-US, a car accident would be more likely :-), were to join a nunnery,
-or would be head-hunted. A large number of Python users have become
-experts at Python programming as well as maintenance of the
-implementation, and would easily fill the vacuum created by my
-disappearance.
-
-In the mean time, I have no plans to disappear -- rather, I am
-committed to improving Python, and my current benefactor, CNRI (see
-<URL:http://www.cnri.reston.va.us>) is just as committed to continue
-its support of Python and the PSA. In fact, we have great plans for
-Python -- we just can't tell yet!
-
-2.8. Q. What is the PSA, anyway?
-
-A. The Python Software Activity <URL:http://www.python.org/psa/> was
-created by a number of Python aficionados who want Python to be more
-than the product and responsibility of a single individual. It has
-found a home at CNRI <URL:http://www.cnri.reston.va.us>. Anybody who
-wishes Python well should join the PSA.
-
-2.9. Q. How do I join the PSA?
-
-A. The full scoop is available on the web, see
-<URL:http://www.python.org/psa/Joining.html>. Summary: send a check
-of at least $50 to CNRI/PSA, 1895 Preston White Drive, Suite 100, in
-Reston, VA 20191. Full-time students pay $25. Companies can join for
-a mere $500.
-
-2.10. Q. What are the benefits of joining the PSA?
-
-A. Like National Public Radio, if not enough people join, Python will
-wither. Your name will be mentioned on the PSA's web server.
-Workshops organized by the PSA <URL:http://www.python.org/workshops/>
-are only accessible to PSA members (you can join at the door). The
-PSA is working on additional benefits, such as reduced prices for
-books and software, and early access to beta versions of Python.
-
-
-3. Building Python and Other Known Bugs
-=======================================
-
-3.1. Q. Is there a test set?
-
-A. Yes, simply do "import testall" (or "import autotest" if you aren't
-interested in the output). The standard modules whose name begins
-with "test" together comprise the test. The test set doesn't test
-*all* features of Python but it goes a long way to confirm that a new
-port is actually working. The Makefile contains an entry "make test"
-which runs the autotest module. NOTE: if "make test" fails, run the
-tests manually ("import testall") to see what goes wrong before
-reporting the error.
-
-3.2. Q. When running the test set, I get complaints about floating point
-operations, but when playing with floating point operations I cannot
-find anything wrong with them.
-
-A. The test set makes occasional unwarranted assumptions about the
-semantics of C floating point operations. Until someone donates a
-better floating point test set, you will have to comment out the
-offending floating point tests and execute similar tests manually.
-
-3.3. Q. Link errors after rerunning the configure script.
-
-A. It is generally necessary to run "make clean" after a configuration
-change.
-
-3.4. Q. The python interpreter complains about options passed to a
-script (after the script name).
-
-A. You are probably linking with GNU getopt, e.g. through -liberty.
-Don't. The reason for the complaint is that GNU getopt, unlike System
-V getopt and other getopt implementations, doesn't consider a
-non-option to be the end of the option list. A quick (and compatible)
-fix for scripts is to add "--" to the interpreter, like this:
-
- #! /usr/local/bin/python --
-
-You can also use this interactively:
-
- python -- script.py [options]
-
-Note that a working getopt implementation is provided in the Python
-distribution (in Python/getopt.c) but not automatically used.
-
-3.5. Q. When building on the SGI, make tries to run python to create
-glmodule.c, but python hasn't been built or installed yet.
-
-A. Comment out the line mentioning glmodule.c in Setup and build a
-python without gl first; install it or make sure it is in your $PATH,
-then edit the Setup file again to turn on the gl module, and make
-again. You don't need to do "make clean"; you do need to run "make
-Makefile" in the Modules subdirectory (or just run "make" at the
-toplevel).
-
-3.6. Q. I use VPATH but some targets are built in the source directory.
-
-A. On some systems (e.g. Sun), if the target already exists in the
-source directory, it is created there instead of in the build
-directory. This is usually because you have previously built without
-VPATH. Try running "make clobber" in the source directory.
-
-3.7. Q. Trouble building or linking with the GNU readline library.
-
-A. Consider using readline 2.0. Some hints:
-
-- You can use the GNU readline library to improve the interactive user
-interface: this gives you line editing and command history when
-calling python interactively. You need to configure and build the GNU
-readline library before running the configure script. Its sources are
-no longer distributed with Python; you can ftp them from any GNU
-mirror site, or from its home site
-<URL:ftp://slc2.ins.cwru.edu/pub/dist/readline-2.0.tar.gz> (or a
-higher version number -- using version 1.x is not recommended). Pass
-the Python configure script the option --with-readline=DIRECTORY where
-DIRECTORY is the absolute pathname of the directory where you've built
-the readline library. Some hints on building and using the readline
-library:
-
-- On SGI IRIX 5, you may have to add the following
-to rldefs.h:
-
- #ifndef sigmask
- #define sigmask(sig) (1L << ((sig)-1))
- #endif
-
-- On most systems, you will have to add #include "rldefs.h" to the
-top of several source files, and if you use the VPATH feature, you
-will have to add dependencies of the form foo.o: foo.c to the
-Makefile for several values of foo.
-
-- The readline library requires use of the termcap library. A
-known problem with this is that it contains entry points which
-cause conflicts with the STDWIN and SGI GL libraries. The STDWIN
-conflict can be solved (and will be, in the next release of
-STDWIN) by adding a line saying '#define werase w_erase' to the
-stdwin.h file (in the STDWIN distribution, subdirectory H). The
-GL conflict has been solved in the Python configure script by a
-hack that forces use of the static version of the termcap library.
-
-- Check the newsgroup gnu.bash.bug <URL:news:gnu.bash.bug> for
-specific problems with the readline library (I don't read this group
-but I've been told that it is the place for readline bugs).
-
-3.8. Q. Trouble with socket I/O on older Linux 1.x versions.
-
-A. Once you've built Python, use it to run the regen.py script in the
-Lib/linux1 directory. Apparently the files as distributed don't match
-the system headers on some Linux versions.
-
-3.9. Q. Trouble with prototypes on Ultrix.
-
-A. Ultrix cc seems broken -- use gcc, or edit config.h to #undef
-HAVE_PROTOTYPES.
-
-3.10. Q. Other trouble building Python on platform X.
-
-A. Please email the details to <guido@cnri.reston.va.us> and I'll look
-into it. Please provide as many details as possible. In particular,
-if you don't tell me what type of computer and what operating system
-(and version) you are using it will be difficult for me to figure out
-what is the matter. If you get a specific error message, please email
-it to me too.
-
-3.11. Q. How to configure dynamic loading on Linux.
-
-A. This is now automatic as long as your Linux version uses the ELF
-object format (all recent Linuxes do).
-
-3.12. Q: I can't get shared modules to work on Linux 2.0 (Slackware96)?
-
-A: This is a bug in the Slackware96 release. The fix is simple:
-
-Make sure that there is a link from /lib/libdl.so to /lib/libdl.so.1
-so that the following links are setup:
-
- /lib/libdl.so -> /lib/libdl.so.1
- /lib/libdl.so.1 -> /lib/libdl.so.1.7.14
-
-You may have to rerun the configure script, after rm'ing the
-config.cache file, before you attempt to rebuild python after this
-fix.
-
-3.13. Q: Trouble when making modules shared on Linux.
-
-A. This happens when you have built Python for static linking and then
-enable *shared* in the Setup file. Shared library code must be
-compiled with "-fpic". If a .o file for the module already exist that
-was compiled for static linking, you must remove it or do "make clean"
-in the Modules directory.
-
-3.14. Q. How to use threads on Linux.
-
-A. [Greg Stein] I built myself a libpthreads.so from the libc.5.3.12
-distribution (the binary distribution doesn't have pthreads in
-it). Then, I configured Python with --with-threads and then tweaked
-config.h to include a #define _MIT_POSIX_THREADS (or something like
-that, see /usr/include/pthreads.h). It worked fine at that point.
-
-Note that I couldn't get threading to "operate well" with any of the
-other thread packages. Prior libc versions didn't integrate well with
-threads, either, so I couldn't use them (e.g. sleep() blocked all
-threads :-( ).
-
-3.15. Q. Errors when linking with a shared library containing C++ code.
-
-A. Link the main Python binary with C++. Change the definition of
-LINKCC in Modules/Makefile to be your C++ compiler. You may have to
-edit config.c slightly to make it compilable with C++.
-
-3.16. Q. I built with tkintermodule.c enabled but get "Tkinter not found".
-
-A. Tkinter.py (note: upper case T) lives in a subdirectory of Lib,
-Lib/tkinter. If you are using the default module search path, you
-probably didn't enable the line in the Modules/Setup file defining
-TKPATH; if you use the environment variable PYTHONPATH, you'll have to
-add the proper tkinter subdirectory.
-
-3.17. Q. I built with Tk 4.0 but Tkinter complains about the Tk version.
-
-A. Several things could cause this. You most likely have a Tk 3.6
-installation that wasn't completely eradicated by the Tk 4.0
-installation (which tends to add "4.0" to its installed files). You
-may have the Tk 3.6 support library installed in the place where the
-Tk 4.0 support files should be (default /usr/local/lib/tk/); you may
-have compiled Python with the old tk.h header file (yes, this actually
-compiles!); you may actually have linked with Tk 3.6 even though Tk
-4.0 is also around. Similar for Tcl 7.4 vs. Tcl 7.3.
-
-3.18. Q. Link errors for Tcl/Tk symbols when linking with Tcl/Tk.
-
-Quite possibly, there's a version mismatch between the Tcl/Tk header
-files (tcl.h and tk.h) and the tck/tk libraries you are using (the
-"-ltk4.0" and "-ltcl7.4" arguments for _tkinter in the Setup file).
-If you have installed both versions 7.4/4.0 and 7.5/4.1 of Tcl/Tk,
-most likely your header files are for The newer versions, but the
-Setup line for _tkinter in some Python distributions references
-7.4/4.0 by default. Changing this to 7.5/4.1 should take care of
-this.
-
-3.19. Q. I configured and built Python for Tcl/Tk but "import Tkinter"
-fails.
-
-A. Most likely, you forgot to enable the line in Setup that says
-"TKPATH=:$(DESTLIB)/tkinter".
-
-3.20. Q. Tk doesn't work right on DEC Alpha.
-
-A. You probably compiled either Tcl, Tk or Python with gcc. Don't.
-For this platform, which has 64-bit integers, gcc is known to generate
-broken code. The standard cc (which comes bundled with the OS!)
-works. If you still prefer gcc, at least try recompiling with cc
-before reporting problems to the newsgroup or the author; if this
-fixes the problem, report the bug to the gcc developers instead. (As
-far as we know, there are no problem with gcc on other platforms --
-the instabilities seem to be restricted to the DEC Alpha.) See also
-question 3.6.
-
-3.21. Q. Several common system calls are missing from the posix module.
-
-A. Most likely, *all* test compilations run by the configure script
-are failing for some reason or another. Have a look in config.log to
-see what could be the reason. A common reason is specifying a
-directory to the --with-readline option that doesn't contain the
-libreadline.a file.
-
-3.22. Q. ImportError: No module named string, on MS Windows.
-
-A. Most likely, your PYTHONPATH environment variable should be set to
-something like:
-
-set PYTHONPATH=c:\python;c:\python\lib;c:\python\scripts
-
-(assuming Python was installed in c:\python)
-
-3.23. Q. Core dump on SGI when using the gl module.
-
-There are conflicts between entry points in the termcap and curses
-libraries and an entry point in the GL library. There's a hack of a
-fix for the termcap library if it's needed for the GNU readline
-library, but it doesn't work when you're using curses. Concluding,
-you can't build a Python binary containing both the curses and gl
-modules.
-
-
-
-4. Programming in Python
-========================
-
-4.1. Q. Is there a source code level debugger with breakpoints, step,
-etc.?
-
-A. Yes. Check out module pdb; pdb.help() prints the documentation (or
-you can read it as Lib/pdb.doc). If you use the STDWIN option,
-there's also a windowing interface, wdb. You can write your own
-debugger by using the code for pdb or wdb as an example.
-
-4.2. Q. Can I create an object class with some methods implemented in
-C and others in Python (e.g. through inheritance)? (Also phrased as:
-Can I use a built-in type as base class?)
-
-A. No, but you can easily create a Python class which serves as a
-wrapper around a built-in object, e.g. (for dictionaries):
-
- # A user-defined class behaving almost identical
- # to a built-in dictionary.
- class UserDict:
- def __init__(self): self.data = {}
- def __repr__(self): return repr(self.data)
- def __cmp__(self, dict):
- if type(dict) == type(self.data):
- return cmp(self.data, dict)
- else:
- return cmp(self.data, dict.data)
- def __len__(self): return len(self.data)
- def __getitem__(self, key): return self.data[key]
- def __setitem__(self, key, item): self.data[key] = item
- def __delitem__(self, key): del self.data[key]
- def keys(self): return self.data.keys()
- def items(self): return self.data.items()
- def values(self): return self.data.values()
- def has_key(self, key): return self.data.has_key(key)
-
-A2. See Jim Fulton's ExtensionClass for an example of a mechanism
-which allows you to have superclasses which you can inherit from in
-Python -- that way you can have some methods from a C superclass (call
-it a mixin) and some methods from either a Python superclass or your
-subclass. See <URL:http://www.digicool.com/papers/ExtensionClass.html>.
-
-4.3. Q. Is there a curses/termcap package for Python?
-
-A. Yes -- Lance Ellinghaus has written a module that interfaces to
-System V's "ncurses". If you know a little curses and some Python,
-it's straightforward to use. It is part of the standard Python
-distribution, but not configured by default -- you must enable it by
-editing Modules/Setup. It requires a System V curses implementation.
-
-You could also consider using the "alfa" (== character cell) version
-of STDWIN. (Standard Window System Interface, a portable windowing
-system interface by myself <URL:ftp://ftp.cwi.nl/pub/stdwin/>.) This
-will also prepare your program for porting to windowing environments
-such as X11 or the Macintosh.
-
-4.4. Q. Is there an equivalent to C's onexit() in Python?
-
-A. Yes, if you import sys and assign a function to sys.exitfunc, it
-will be called when your program exits, is killed by an unhandled
-exception, or (on UNIX) receives a SIGHUP or SIGTERM signal.
-
-4.5. Q. When I define a function nested inside another function, the
-nested function seemingly can't access the local variables of the
-outer function. What is going on? How do I pass local data to a
-nested function?
-
-A. Python does not have arbitrarily nested scopes. When you need to
-create a function that needs to access some data which you have
-available locally, create a new class to hold the data and return a
-method of an instance of that class, e.g.:
-
- class MultiplierClass:
- def __init__(self, factor):
- self.factor = factor
- def multiplier(self, argument):
- return argument * self.factor
-
- def generate_multiplier(factor):
- return MultiplierClass(factor).multiplier
-
- twice = generate_multiplier(2)
- print twice(10)
- # Output: 20
-
-An alternative solution uses default arguments, e.g.:
-
- def generate_multiplier(factor):
- def multiplier(arg, fact = factor):
- return arg*fact
- return multiplier
-
- twice = generate_multiplier(2)
- print twice(10)
- # Output: 20
-
-4.6. Q. How do I iterate over a sequence in reverse order?
-
-A. If it is a list, the fastest solution is
-
- list.reverse()
- try:
- for x in list:
- "do something with x"
- finally:
- list.reverse()
-
-This has the disadvantage that while you are in the loop, the list
-is temporarily reversed. If you don't like this, you can make a copy.
-This appears expensive but is actually faster than other solutions:
-
- rev = list[:]
- rev.reverse()
- for x in rev:
- <do something with x>
-
-If it isn't a list, a more general but slower solution is:
-
- i = len(list)
- while i > 0:
- i = i-1
- x = list[i]
- <do something with x>
-
-A more elegant solution, is to define a class which acts as a sequence
-and yields the elements in reverse order (solution due to Steve
-Majewski):
-
- class Rev:
- def __init__(self, seq):
- self.forw = seq
- def __len__(self):
- return len(self.forw)
- def __getitem__(self, i):
- return self.forw[-(i + 1)]
-
-You can now simply write:
-
- for x in Rev(list):
- <do something with x>
-
-Unfortunately, this solution is slowest of all, due to the method
-call overhead...
-
-4.7. Q. My program is too slow. How do I speed it up?
-
-A. That's a tough one, in general. There are many tricks to speed up
-Python code; I would consider rewriting parts in C only as a last
-resort. One thing to notice is that function and (especially) method
-calls are rather expensive; if you have designed a purely OO interface
-with lots of tiny functions that don't do much more than get or set an
-instance variable or call another method, you may consider using a
-more direct way, e.g. directly accessing instance variables. Also see
-the standard module "profile" (described in the file
-"python/lib/profile.doc") which makes it possible to find out where
-your program is spending most of its time (if you have some patience
--- the profiling itself can slow your program down by an order of
-magnitude).
-
-4.8. Q. When I have imported a module, then edit it, and import it
-again (into the same Python process), the changes don't seem to take
-place. What is going on?
-
-A. For reasons of efficiency as well as consistency, Python only reads
-the module file on the first time a module is imported. (Otherwise a
-program consisting of many modules, each of which imports the same
-basic module, would read the basic module over and over again.) To
-force rereading of a changed module, do this:
-
- import modname
- reload(modname)
-
-Warning: this technique is not 100% fool-proof. In particular,
-modules containing statements like
-
- from modname import some_objects
-
-will continue to work with the old version of the imported objects.
-
-4.9. Q. How do I find the current module name?
-
-A. A module can find out its own module name by looking at the
-(predefined) global variable __name__. If this has the value
-'__main__' you are running as a script.
-
-4.10. Q. I have a module in which I want to execute some extra code
-when it is run as a script. How do I find out whether I am running as
-a script?
-
-A. See the previous question. E.g. if you put the following on the
-last line of your module, main() is called only when your module is
-running as a script:
-
- if __name__ == '__main__': main()
-
-4.11. Q. I try to run a program from the Demo directory but it fails
-with ImportError: No module named ...; what gives?
-
-A. This is probably an optional module (written in C!) which hasn't
-been configured on your system. This especially happens with modules
-like "Tkinter", "stdwin", "gl", "Xt" or "Xm". For Tkinter, STDWIN and
-many other modules, see Modules/Setup.in for info on how to add these
-modules to your Python, if it is possible at all. Sometimes you will
-have to ftp and build another package first (e.g. STDWIN). Sometimes
-the module only works on specific platforms (e.g. gl only works on SGI
-machines).
-
-NOTE: if the complaint is about "Tkinter" (upper case T) and you have
-already configured module "tkinter" (lower case t), the solution is
-*not* to rename tkinter to Tkinter or vice versa. There is probably
-something wrong with your module search path. Check out the value of
-sys.path.
-
-For X-related modules (Xt and Xm) you will have to do more work: they
-are currently not part of the standard Python distribution. You will
-have to ftp the Extensions tar file, e.g.
-<URL:ftp://ftp.python.org/pub/python/src/extensions.tar.gz> and follow
-the instructions there.
-
-See also the next question.
-
-4.12. Q. I have successfully built Python with STDWIN but it can't
-find some modules (e.g. stdwinevents).
-
-A. There's a subdirectory of the library directory named 'stdwin'
-which should be in the default module search path. There's a line in
-Modules/Setup(.in) that you have to enable for this purpose --
-unfortunately in the latest release it's not near the other
-STDWIN-related lines so it's easy to miss it.
-
-4.13. Q. What GUI toolkits exist for Python?
-
-A. Depending on what platform(s) you are aiming at, there are several.
-
-Currently supported solutions:
-
-- There's a neat object-oriented interface to the Tcl/Tk widget set,
-called Tkinter. It is part of the standard Python distribution and
-well-supported -- all you need to do is build and install Tcl/Tk and
-enable the _tkinter module and the TKPATH definition in Modules/Setup
-when building Python. This is probably the easiest to install and
-use, and the most complete widget set. It is also very likely that in
-the future the standard Python GUI API will be based on or at least
-look very much like the Tkinter interface. For more info about Tk,
-including pointers to the source, see the Tcl/Tk home page
-<URL:http://www.sunlabs.com/research/tcl/>. Tcl/Tk is now fully
-portable to the Mac and Windows platforms (NT and 95 only); you need
-Python 1.4beta3 or later and Tk 4.1patch1 or later.
-
-- There's an interface to X11, including the Athena and Motif widget
-sets (and a few individual widgets, like Mosaic's HTML widget and
-SGI's GL widget) available from
-<URL:ftp://ftp.python.org/pub/python/src/X-extension.tar.gz>.
-Support by Sjoerd Mullender <sjoerd@cwi.nl>.
-
-- On top of the X11 interface there's the (recently revived) vpApp
-toolkit by Per Spilling, now also maintained by Sjoerd Mullender
-<sjoerd@cwi.nl>. See <URL:ftp://ftp.cwi.nl/pub/sjoerd/vpApp.tar.gz>.
-
-- The Mac port has a rich and ever-growing set of modules that support
-the native Mac toolbox calls. See the documentation that comes with
-the Mac port. See <URL:ftp://ftp.python.org/pub/python/mac>. Support
-by Jack Jansen <jack@cwi.nl>.
-
-- The NT port supported by Mark Hammond <MHammond@skippinet.com.au>
-(see question 7.2) includes an interface to the Microsoft Foundation
-Classes and a Python programming environment using it that's written
-mostly in Python. See
-<URL:ftp://ftp.python.org/pub/python/pythonwin/>.
-
-- There's an object-oriented GUI based on the Microsoft Foundation
-Classes model called WPY, supported by Jim Ahlstrom <jim@interet.com>.
-Programs written in WPY run unchanged and with native look and feel on
-Windows NT/95, Windows 3.1 (using win32s), and on Unix (using Tk).
-Source and binaries for Windows and Linux are available in
-<URL:ftp://ftp.python.org/pub/python/wpy/>.
-
-Obsolete or minority solutions:
-
-- There's an interface to wxWindows. wxWindows is a portable GUI
-class library written in C++. It supports XView, Motif, MS-Windows as
-targets. There is some support for Macs and CURSES as well.
-wxWindows preserves the look and feel of the underlying graphics
-toolkit. See the wxPython WWW page at
-<URL:http://www.aiai.ed.ac.uk/~jacs/wx/wxpython/wxpython.html>.
-Support for wxPython (by Harri Pasanen <pa@tekla.fi>) appears
-to have a low priority.
-
-- For SGI IRIX only, there are unsupported interfaces to the complete
-GL (Graphics Library -- low level but very good 3D capabilities) as
-well as to FORMS (a buttons-and-sliders-etc package built on top of GL
-by Mark Overmars -- ftp'able from
-<URL:ftp://ftp.cs.ruu.nl/pub/SGI/FORMS/>). This is probably also
-becoming obsolete, as OpenGL takes over.
-
-- There's an interface to STDWIN, a platform-independent low-level
-windowing interface for Mac and X11. This is totally unsupported and
-rapidly becoming obsolete. The STDWIN sources are at
-<URL:ftp://ftp.cwi.nl/pub/stdwin/>. (For info about STDWIN 2.0,
-please refer to Steven Pemberton <steven@cwi.nl> -- I believe it is
-also dead.)
-
-- There once was an interface to WAFE, a Tcl interface to the X11
-Motif and Athena widget sets. WAFE is at
-<URL:ftp://ftp.wu-wien.ac.at/pub/src/X11/wafe/>. It's not clear what
-the status of the Python support is.
-
-- (The Fresco port that was mentioned in earlier versions of this FAQ
-no longer seems to exist. Inquire with Mark Linton.)
-
-4.14. Q. Are there any interfaces to database packages in Python?
-
-A. There's a whole collection of them in the contrib area of the ftp
-server, see <URL:http://www.python.org/ftp/python/contrib/Database/>.
-
-4.15. Q. Is it possible to write obfuscated one-liners in Python?
-
-A. Yes. See the following three examples, due to Ulf Bartelt:
-
- # Primes < 1000
- print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0,
- map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000)))
-
- # First 10 Fibonacci numbers
- print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f),
- range(10))
-
- # Mandelbrot set
- print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y,
- Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM,
- Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro,
- i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y
- >=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr(
- 64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy
- ))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24)
- # \___ ___/ \___ ___/ | | |__ lines on screen
- # V V | |______ columns on screen
- # | | |__________ maximum of "iterations"
- # | |_________________ range on y axis
- # |____________________________ range on x axis
-
-Don't try this at home, kids!
-
-4.16. Q. Is there an equivalent of C's "?:" ternary operator?
-
-A. Not directly. In many cases you can mimic a?b:c with "a and b or
-c", but there's a flaw: if b is zero (or empty, or None -- anything
-that tests false) then c will be selected instead. In many cases you
-can prove by looking at the code that this can't happen (e.g. because
-b is a constant or has a type that can never be false), but in general
-this can be a problem.
-
-Tim Peters (who wishes it was Steve Majewski) suggested the following
-solution: (a and [b] or [c])[0]. Because [b] is a singleton list it
-is never false, so the wrong path is never taken; then applying [0] to
-the whole thing gets the b or c that you really wanted. Ugly, but it
-gets you there in the rare cases where it is really inconvenient to
-rewrite your code using 'if'.
-
-4.17. Q. My class defines __del__ but it is not called when I delete the
-object.
-
-A. There are several possible reasons for this.
-
-- The del statement does not necessarily call __del__ -- it simply
-decrements the object's reference count, and if this reaches zero
-__del__ is called.
-
-- If your data structures contain circular links (e.g. a tree where
-each child has a parent pointer and each parent has a list of
-children) the reference counts will never go back to zero. You'll
-have to define an explicit close() method which removes those
-pointers. Please don't ever call __del__ directly -- __del__ should
-call close() and close() should make sure that it can be called more
-than once for the same object.
-
-- If the object has ever been a local variable (or argument, which is
-really the same thing) to a function that caught an expression in an
-except clause, chances are that a reference to the object still exists
-in that function's stack frame as contained in the stack trace.
-Normally, deleting (better: assigning None to) sys.exc_traceback will
-take care of this. If you a stack was printed for an unhandled
-exception in an interactive interpreter, delete sys.last_traceback
-instead.
-
-- There is code that deletes all objects when the interpreter exits,
-but if your Python has been configured to support threads, it is not
-called (because other threads may still be active). You can define
-your own cleanup function using sys.exitfunc (see question 4.4).
-
-- Finally, if your __del__ method raises an exception, this will be
-ignored. Starting with Python 1.4beta3, a warning message is printed
-to sys.stderr when this happens.
-
-4.18. Q. How do I change the shell environment for programs called
-using os.popen() or os.system()? Changing os.environ doesn't work.
-
-A. You must be using either a version of python before 1.4, or on a
-(rare) system that doesn't have the putenv() library function.
-
-Before Python 1.4, modifying the environment passed to subshells was
-left out of the interpreter because there seemed to be no
-well-established portable way to do it (in particular, some systems,
-have putenv(), others have setenv(), and some have none at all). As
-of Python 1.4, almost all Unix systems *do* have putenv(), and so does
-the Win32 API, and thus the os module was modified so that changes to
-os.environ are trapped and the corresponding putenv() call is made.
-
-4.19. Q. What is a class?
-
-A. A class is the particular object type that is created by executing
-a class statement. Class objects are used as templates, to create
-class instance objects, which embody both the data structure and
-program routines specific to a datatype.
-
-4.20. Q. What is a method?
-
-A. A method is a function that you normally call as
-x.name(arguments...) for some object x. The term is used for methods
-of classes and class instances as well as for methods of built-in
-objects. (The latter have a completely different implementation and
-only share the way their calls look in Python code.) Methods of
-classes (and class instances) are defined as functions inside the
-class definition.
-
-4.21. Q. What is self?
-
-A. Self is merely a conventional name for the first argument of a
-method -- i.e. a function defined inside a class definition. A method
-defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for
-some instance x of the class in which the definition occurs;
-the called method will think it is called as meth(x, a, b, c).
-
-4.22. Q. What is a unbound method?
-
-A. An unbound method is a method defined in a class that is not yet
-bound to an instance. You get an unbound method if you ask for a
-class attribute that happens to be a function. You get a bound method
-if you ask for an instance attribute. A bound method knows which
-instance it belongs to and calling it supplies the instance automatically;
-an unbound method only knows which class it wants for its first
-argument (a derived class is also OK). Calling an unbound method
-doesn't "magically" derive the first argument from the context -- you
-have to provide it explicitly.
-
-4.23. Q. How do I call a method defined in a base class from a derived
-class that overrides it?
-
-A. If your class definition starts with "class Derived(Base): ..."
-then you can call method meth defined in Base (or one of Base's base
-classes) as Base.meth(self, arguments...). Here, Base.meth is an
-unbound method (see previous question).
-
-4.24. Q. How do I call a method from a base class without using the
-name of the base class?
-
-A. DON'T DO THIS. REALLY. I MEAN IT. It appears that you could call
-self.__class__.__bases__[0].meth(self, arguments...) but this fails when
-a doubly-derived method is derived from your class: for its instances,
-self.__class__.__bases__[0] is your class, not its base class -- so
-(assuming you are doing this from within Derived.meth) you would start
-a recursive call.
-
-4.25. Q. How can I organize my code to make it easier to change the base
-class?
-
-A. You could define an alias for the base class, assign the real base
-class to it before your class definition, and use the alias throughout
-your class. Then all you have to change is the value assigned to the
-alias. Incidentally, this trick is also handy if you want to decide
-dynamically (e.g. depending on availability of resources) which base
-class to use. Example:
-
- BaseAlias = <real base class>
- class Derived(BaseAlias):
- def meth(self):
- BaseAlias.meth(self)
- ...
-
-4.26. Q. How can I find the methods or attributes of an object?
-
-A. This depends on the object type.
-
-For an instance x of a user-defined class, instance attributes are
-found in the dictionary x.__dict__, and methods and attributes defined
-by its class are found in x.__class__.__bases__[i].__dict__ (for i in
-range(len(x.__class__.__bases__))). You'll have to walk the tree of
-base classes to find *all* class methods and attributes.
-
-Many, but not all built-in types define a list of their method names
-in x.__methods__, and if they have data attributes, their names may be
-found in x.__members__. However this is only a convention.
-
-For more information, read the source of the standard (but
-undocumented) module newdir.
-
-4.27. Q. I can't seem to use os.read() on a pipe created with os.popen().
-
-A. os.read() is a low-level function which takes a file descriptor (a
-small integer). os.popen() creates a high-level file object -- the
-same type used for sys.std{in,out,err} and returned by the builtin
-open() function. Thus, to read n bytes from a pipe p created with
-os.popen(), you need to use p.read(n).
-
-4.28. Q. How can I create a stand-alone binary from a Python script?
-
-The demo script "Demo/scripts/freeze.py" does what you want. (It's
-actually not a demo but a support tool -- there is some extra code in
-the interpreter to accommodate it.) It requires that you have the
-Python build tree handy, complete with all the lib*.a files.
-
-This works by scanning your source recursively for import statements
-(both forms) and looking for the modules on the standard Python path
-as well as in the source directory (for built-in modules). It then
-"compiles" the modules written in Python to C code (array initializers
-that can be turned into code objects using the marshal module) and
-creates a custom-made config file that only contains those built-in
-modules which are actually used in the program. It then compiles the
-generated C code and links it with the rest of the Python interpreter
-to form a self-contained binary which acts exactly like your script.
-
-Hint: the freeze program only works if your script's filename ends in
-".py".
-
-4.29. Q. What WWW tools are there for Python?
-
-A. See the chapter titled "Internet and WWW" in the Library Reference
-Manual. There's also a web browser written in Python, called Grail --
-see <URL:http://grail.cnri.reston.va.us/grail/>.
-
-Steve Miale <smiale@cs.indiana.edu> has written a modular WWW browser
-called Dancer. An alpha version can be FTP'ed from
-<URL:ftp://ftp.cs.indiana.edu/pub/smiale/dancer.tar.gz>. (There are a
-few articles about Dancer in the (hyper)mail archive
-<URL:http://www.cwi.nl/~guido/hypermail/python-1994q3/index.html>.)
-
-4.30. Q. How do I run a subprocess with pipes connected to both input
-and output?
-
-A. This is really a UNIX question. Also, in general, it is unwise to
-do so, because you can easily cause a deadlock where the parent
-process is blocked waiting for output from the child, while the child
-is blocked waiting for input from the child. This can be caused
-because the parent expects the child to output more text than it does,
-or it can be caused by data being stuck in stdio buffers due to lack
-of flushing. The Python parent can of course explicitly flush the data
-it sends to the child before it reads any output, but if the child is
-a naive C program it can easily have been written to never explicitly
-flush its output, even if it is interactive, since flushing is
-normally automatic.
-
-In many cases, all you really need is to run some data through a
-command and get the result back. Unless the data is infinite in size,
-the easiest (and often the most efficient!) way to do this is to write
-it to a temporary file and run the command with that temporary file as
-input. The standard module tempfile exports a function mktemp() which
-generates unique temporary file names.
-
-If after reading all of the above you still want to connect two pipes
-to a subprocess's standard input and output, here's a simple solution,
-due to Jack Jansen:
-
- import os
- import sys
- import string
-
- MAXFD = 100 # Max number of file descriptors in this system
-
- def popen2(cmd):
- cmd = string.split(cmd)
- p2cread, p2cwrite = os.pipe()
- c2pread, c2pwrite = os.pipe()
- pid = os.fork()
- if pid == 0:
- # Child
- os.close(0)
- os.close(1)
- if os.dup(p2cread) != 0:
- sys.stderr.write('popen2: bad read dup\n')
- if os.dup(c2pwrite) != 1:
- sys.stderr.write('popen2: bad write dup\n')
- for i in range(3, MAXFD):
- try:
- os.close(i)
- except:
- pass
- try:
- os.execv(cmd[0], cmd)
- finally:
- os._exit(1)
- os.close(p2cread)
- tochild = os.fdopen(p2cwrite, 'w')
- os.close(c2pwrite)
- fromchild = os.fdopen(c2pread, 'r')
- return fromchild, tochild
-
-Note that many interactive programs (e.g. vi) don't work well with
-pipes substituted for standard input and output. You will have to use
-pseudo ttys ("ptys") instead of pipes. There is some undocumented
-code to use these in the library module pty.py -- I'm afraid you're on
-your own here.
-
-A different answer is a Python interface to Don Libes' "expect"
-library. A prerelease of this is available on the Python ftp mirror
-sites in the contrib subdirectory as expy-0.3.tar.gz, e.g.
-<URL:ftp://ftp.python.org/pub/python/contrib/expy-0.3.tar.gz>.
-
-4.31. Q. How do I call a function if I have the arguments in a tuple?
-
-A. Use the built-in function apply(). For instance,
-
- func(1, 2, 3)
-
-is equivalent to
-
- args = (1, 2, 3)
- apply(func, args)
-
-Note that func(args) is not the same -- it calls func() with exactly
-one argument, the tuple args, instead of three arguments, the integers
-1, 2 and 3.
-
-4.32. Q. How do I enable font-lock-mode for Python in Emacs?
-
-A. Assuming you're already using python-mode and font-lock-mode
-separately, all you need to do is put this in your .emacs file:
-
- (defun my-python-mode-hook ()
- (setq font-lock-keywords python-font-lock-keywords)
- (font-lock-mode 1))
- (add-hook 'python-mode-hook 'my-python-mode-hook)
-
-4.33. Q. Is there an inverse to the format operator (a la C's scanf())?
-
-A. Not as such.
-
-For simple input parsing, the easiest approach is usually to split
-the line into whitespace-delimited words using string.split(), and to
-convert decimal strings to numeric values using string.atoi(),
-string.atol() or string.atof(). (Python's atoi() is 32-bit and its
-atol() is arbitrary precision.) If you want to use another delimiter
-than whitespace, use string.splitfield() (possibly combining it with
-string.strip() which removes surrounding whitespace from a string).
-
-For more complicated input parsing, regular expressions (see module
-regex) are better suited and more powerful than C's scanf().
-
-4.34. Q. Can I have Tk events handled while waiting for I/O?
-
-A. Yes, and you don't even need threads! But you'll have to
-restructure your I/O code a bit. Tk has the equivalent of Xt's
-XtAddInput() call, which allows you to register a callback function
-which will be called from the Tk mainloop when I/O is possible on a
-file descriptor. Here's what you need:
-
- from Tkinter import tkinter
- tkinter.createfilehandler(file, mask, callback)
-
-The file may be a Python file or socket object (actually, anything
-with a fileno() method), or an integer file descriptor. The mask is
-one of the constants tkinter.READABLE or tkinter.WRITABLE. The
-callback is called as follows:
-
- callback(file, mask)
-
-You must unregister the callback when you're done, using
-
- tkinter.deletefilehandler(file)
-
-Note: since you don't know *how many bytes* are available for reading,
-you can't use the Python file object's read or readline methods, since
-these will insist on reading a predefined number of bytes. For
-sockets, the recv() or recvfrom() methods will work fine; for other
-files, use os.read(file.fileno(), maxbytecount).
-
-4.35. Q. How do I write a function with output parameters (call by reference)?
-
-A. [Mark Lutz] The thing to remember is that arguments are passed by
-assignment in Python. Since assignment just creates references to
-objects, there's no alias between an argument name in the caller and
-callee, and so no call-by-reference per se. But you can simulate it
-in a number of ways:
-
-1) By using global variables; but you probably shouldn't :-)
-
-2) By passing a mutable (changeable in-place) object:
-
- def func1(a):
- a[0] = 'new-value' # 'a' references a mutable list
- a[1] = a[1] + 1 # changes a shared object
-
- args = ['old-value', 99]
- func1(args)
- print args[0], args[1] # output: new-value 100
-
-3) By return a tuple, holding the final values of arguments:
-
- def func2(a, b):
- a = 'new-value' # a and b are local names
- b = b + 1 # assigned to new objects
- return a, b # return new values
-
- x, y = 'old-value', 99
- x, y = func2(x, y)
- print x, y # output: new-value 100
-
-4) And other ideas that fall-out from Python's object model. For
- instance, it might be clearer to pass in a mutable dictionary:
-
- def func3(args):
- args['a'] = 'new-value' # args is a mutable dictionary
- args['b'] = args['b'] + 1 # change it in-place
-
- args = {'a':' old-value', 'b': 99}
- func3(args)
- print args['a'], args['b']
-
-5) Or bundle-up values in a class instance:
-
- class callByRef:
- def __init__(self, **args):
- for (key, value) in args.items():
- setattr(self, key, value)
-
- def func4(args):
- args.a = 'new-value' # args is a mutable callByRef
- args.b = args.b + 1 # change object in-place
-
- args = callByRef(a='old-value', b=99)
- func4(args)
- print args.a, args.b
-
- But there's probably no good reason to get this complicated :-).
-
-[Python' author favors solution 3 in most cases.]
-
-4.36. Q. Please explain the rules for local and global variables in Python.
-
-A. [Ken Manheimer] In Python, procedure variables are implicitly
-global, unless they assigned anywhere within the block. In that case
-they are implicitly local, and you need to explicitly declare them as
-'global'.
-
-Though a bit surprising at first, a moments consideration explains
-this. On one hand, requirement of 'global' for assigned vars provides
-a bar against unintended side-effects. On the other hand, if global
-were required for all global references, you'd be using global all the
-time. Eg, you'd have to declare as global every reference to a
-builtin function, or to a component of an imported module. This
-clutter would defeat the usefulness of the 'global' declaration for
-identifying side-effects.
-
-4.37. Q. How can I have modules that mutually import each other?
-
-A. Jim Roskind recommends the following order in each module:
-
-First: all exports (like globals, functions, and classes that don't
-need imported bases classes).
-
-Then: all import statements.
-
-Finally: all active code (including globals that are initialized from
-imported values).
-
-Python's author doesn't like this approach much because the imports
-appear in a strange place, but has to admit that it works. His
-recommended strategy is to avoid all uses of "from <module> import *"
-(so everything from an imported module is referenced as
-<module>.<name>) and to place all code inside functions.
-Initializations of global variables and class variables should use
-constants or built-in functions only.
-
-4.38. Q. How do I copy an object in Python?
-
-A. There is no generic copying operation built into Python, however
-most object types have some way to create a clone. Here's how for the
-most common objects:
-
-- For immutable objects (numbers, strings, tuples), cloning is
-unnecessary since their value can't change.
-
-- For lists (and generally for mutable sequence types), a clone is
-created by the expression l[:].
-
-- For dictionaries, the following function returns a clone:
-
- def dictclone(o):
- n = {}
- for k in o.keys(): n[k] = o[k]
- return n
-
-- Finally, for generic objects, the "copy" module defines two
-functions for copying objects. copy.copy(x) returns a copy as shown
-by the above rules. copy.deepcopy(x) also copies the elements of
-composite objects. See the section on this module in the Library
-Reference Manual.
-
-4.39. Q. How to implement persistent objects in Python? (Persistent ==
-automatically saved to and restored from disk.)
-
-A. The library module "pickle" now solves this in a very general way
-(though you still can't store things like open files, sockests or
-windows), and the library module "shelve" uses pickle and (g)dbm to
-create presistent mappings containing arbitrary Python objects.
-
-4.40. Q. I try to use __spam and I get an error about _SomeClassName__spam.
-
-A. Variables with double leading underscore are "mangled" to provide a
-simple but effective way to define class private variables. See the
-chapter "New in Release 1.4" in the Python Tutorial.
-
-4.41. Q. How do I delete a file? And other file questions.
-
-A. Use os.remove(filename) or os.unlink(filename); for documentation,
-see the posix section of the library manual. They are the same,
-unlink() is simply the Unix name for this function. In earlier
-versions of Python, only os.unlink() was available.
-
-To remove a directory, use os.rmdir(); use os.mkdir() to create one.
-
-To rename a file, use os.rename().
-
-To truncate a file, open it using f = open(filename, "w+"), and use
-f.truncate(offset); offset defaults to the current seek position.
-There's also os.ftruncate(fd, offset) for files opened with os.open()
--- for advanced Unix hacks only.
-
-4.42. Q. How to modify urllib or httplib to support HTTP/1.1?
-
-A. Apply the following patch to httplib.py:
-
-41c41
-< replypat = regsub.gsub('\\.', '\\\\.', HTTP_VERSION) + \
----
-> replypat = regsub.gsub('\\.', '\\\\.', 'HTTP/1.[0-9]+') + \
-
-4.43. Q. Unexplicable syntax errors in compile() or exec.
-
-A. When a statement suite (as opposed to an expression) is compiled by
-compile(), exec or execfile(), it *must* end in a newline. In some
-cases, when the source ends in an indented block it appears that at
-least two newlines are required.
-
-4.44. Q. How do I convert a string to a number?
-
-A. To convert, e.g., the string '144' to the number 144, import the
-module string and use the string.atoi() function. For floating point
-numbers, use string.atof(); for long integers, use string.atol(). See
-the library reference manual section for the string module for more
-details. While you could use the built-in function eval() instead of
-any of those, this is not recommended, because someone could pass you
-a Python expression that might have unwanted side effects (like
-reformatting your disk).
-
-4.45. Q. How do I convert a number to a string?
-
-A. To convert, e.g., the number 144 to the string '144', use the
-built-in function repr() or the backquote notation (these are
-equivalent). If you want a hexadecimal or octal representation, use
-the built-in functions hex() or oct(), respectively. For fancy
-formatting, use the % operator on strings, just like C printf formats,
-e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'.
-See the library reference manual for details.
-
-
-5. Extending Python
-===================
-
-5.1. Q. Can I create my own functions in C?
-
-A. Yes, you can create built-in modules containing functions,
-variables, exceptions and even new types in C. This is explained in
-the document "Extending and Embedding the Python Interpreter" (the
-LaTeX file Doc/ext.tex). Also read the chapter on dynamic loading.
-
-5.2. Q. Can I create my own functions in C++?
-
-A. Yes, using the C-compatibility features found in C++. Basically
-you place extern "C" { ... } around the Python include files and put
-extern "C" before each function that is going to be called by the
-Python interpreter. Global or static C++ objects with constructors
-are probably not a good idea.
-
-5.3. Q. How can I execute arbitrary Python statements from C?
-
-A. The highest-level function to do this is run_command() which takes
-a single string argument which is executed in the context of module
-__main__ and returns 0 for success and -1 when an exception occurred
-(including SyntaxError). If you want more control, use run_string();
-see the source for run_command() in Python/pythonrun.c.
-
-5.4. Q. How can I evaluate an arbitrary Python expression from C?
-
-A. Call the function run_string() from the previous question with the
-start symbol eval_input; it then parses an expression, evaluates it
-and returns its value. See exec_eval() in Python/bltinmodule.c.
-
-5.5. Q. How do I extract C values from a Python object?
-
-A. That depends on the object's type. If it's a tuple,
-gettuplesize(o) returns its length and gettupleitem(o, i) returns its
-i'th item; similar for lists with getlistsize(o) and getlistitem(o,
-i). For strings, getstringsize(o) returns its length and
-getstringvalue(o) a pointer to its value (note that Python strings may
-contain null bytes so strlen() is not safe). To test which type an
-object is, first make sure it isn't NULL, and then use
-is_stringobject(o), is_tupleobject(o), is_listobject(o) etc.
-
-5.6. Q. How do I use mkvalue() to create a tuple of arbitrary length?
-
-A. You can't. Use t = newtupleobject(n) instead, and fill it with
-objects using settupleitem(t, i, o) -- note that this "eats" a
-reference count of o. Similar for lists with newlistobject(n) and
-setlistitem(l, i, o). Note that you *must* set all the tuple items to
-some value before you pass the tuple to Python code --
-newtupleobject(n) initializes them to NULL, which isn't a valid Python
-value.
-
-5.7. Q. How do I call an object's method from C?
-
-A. Here's a function (untested) that might become part of the next
-release in some form. It uses <stdarg.h> to allow passing the
-argument list on to vmkvalue():
-
- object *call_method(object *inst, char *methodname, char *format, ...)
- {
- object *method;
- object *args;
- object *result;
- va_list va;
- method = getattr(inst, methodname);
- if (method == NULL) return NULL;
- va_start(va, format);
- args = vmkvalue(format, va);
- va_end(va);
- if (args == NULL) {
- DECREF(method);
- return NULL;
- }
- result = call_object(method, args);
- DECREF(method);
- DECREF(args);
- return result;
- }
-
-This works for any instance that has methods -- whether built-in or
-user-defined. You are responsible for eventually DECREF'ing the
-return value.
-
-To call, e.g., a file object's "seek" method with arguments 10, 0
-(assuming the file object pointer is "f"):
-
- res = call_method(f, "seek", "(OO)", 10, 0);
- if (res == NULL) {
- ... an exception occurred ...
- }
- else {
- DECREF(res);
- }
-
-Note that since call_object() *always* wants a tuple for the argument
-list, to call a function without arguments, pass "()" for the format,
-and to call a function with one argument, surround the argument in
-parentheses, e.g. "(i)".
-
-5.8. Q. How do I catch the output from print_error()?
-
-A. (Due to Mark Hammond):
-
-* in Python code, define an object that supports the "write()" method.
-
-* redirect sys.stdout and sys.stderr to this object.
-
-* call print_error, or just allow the standard traceback mechanism to
-work.
-
-Then, the output will go wherever your write() method sends it.
-
-5.9. Q. How do I access a module written in Python from C?
-
-A. You can get a pointer to the module object as follows:
-
- module = import_module("<modulename>");
-
-If the module hasn't been imported yet (i.e. it is not yet present in
-sys.modules), this initializes the module; otherwise it simply returns
-the value of sys.modules["<modulename>"]. Note that it doesn't enter
-the module into any namespace -- it only ensures it has been
-initialized and is stored in sys.modules.
-
-You can then access the module's attributes (i.e. any name defined in
-the module) as follows:
-
- attr = getattr(module, "<attrname>");
-
-Calling setattr(), to assign to variables in the module, also works.
-
-5.10. Q. How do I interface to C++ objects from Python?
-
-A. Depending on your requirements, there are many approaches. To do
-this manually, begin by reading the "Extending and Embedding" document
-(Doc/ext.tex, see also <URL:http://www.python.org/doc/>). Realize
-that for the Python run-time system, there isn't a whole lot of
-difference between C and C++ -- so the strategy to build a new Python
-type around a C structure (pointer) type will also work for C++
-objects.
-
-A useful automated approach (which also works for C) is SWIG:
-<URL:http://www.cs.utah.edu/~beazley/SWIG/>.
-
-
-6. Python's design
-==================
-
-6.1. Q. Why isn't there a switch or case statement in Python?
-
-A. You can do this easily enough with a sequence of
-if... elif... elif... else. There have been some proposals for switch
-statement syntax, but there is no consensus (yet) on whether and how
-to do range tests.
-
-6.2. Q. Why does Python use indentation for grouping of statements?
-
-A. Basically I believe that using indentation for grouping is
-extremely elegant and contributes a lot to the clarity of the average
-Python program. Most people learn to love this feature after a while.
-Some arguments for it:
-
-- Since there are no begin/end brackets there cannot be a disagreement
-between grouping perceived by the parser and the human reader. I
-remember long ago seeing a C fragment like this:
-
- if (x <= y)
- x++;
- y--;
- z++;
-
-and staring a long time at it wondering why y was being decremented
-even for x > y... (And I wasn't a C newbie then either.)
-
-- Since there are no begin/end brackets, Python is much less prone to
-coding-style conflicts. In C there are loads of different ways to
-place the braces (including the choice whether to place braces around
-single statements in certain cases, for consistency). If you're used
-to reading (and writing) code that uses one style, you will feel at
-least slightly uneasy when reading (or being required to write)
-another style.
-
-- Many coding styles place begin/end brackets on a line by themself.
-This makes programs considerably longer and wastes valuable screen
-space, making it harder to get a good overview over a program.
-Ideally, a function should fit on one basic tty screen (say, 20
-lines). 20 lines of Python are worth a LOT more than 20 lines of C.
-This is not solely due to the lack of begin/end brackets (the lack of
-declarations also helps, and the powerful operations of course), but
-it certainly helps!
-
-6.3. Q. Why are Python strings immutable?
-
-A. There are two advantages. One is performance: knowing that a
-string is immutable makes it easy to lay it out at construction time
--- fixed and unchanging storage requirements. (This is also one of
-the reasons for the distinction between tuples and lists.) The
-other is that strings in Python are considered as "elemental" as
-numbers. No amount of activity will change the value 8 to anything
-else, and in Python, no amount of activity will change the string
-"eight" to anything else. (Adapted from Jim Roskind)
-
-6.4. Q. Why don't strings have methods like index() or sort(), like
-lists?
-
-A. Good question. Strings currently don't have methods at all
-(likewise tuples and numbers). Long ago, it seemed unnecessary to
-implement any of these functions in C, so a standard library module
-"string" written in Python was created that performs string related
-operations. Since then, the cry for performance has moved most of
-them into the built-in module strop (this is imported by module
-string, which is still the preferred interface, without loss of
-performance except during initialization). Some of these functions
-(e.g. index()) could easily be implemented as string methods instead,
-but others (e.g. sort()) can't, since their interface prescribes that
-they modify the object, while strings are immutable (see the previous
-question).
-
-6.5. Q. Why does Python use methods for some functionality
-(e.g. list.index()) but functions for other (e.g. len(list))?
-
-A. Functions are used for those operations that are generic for a
-group of types and which should work even for objects that don't have
-methods at all (e.g. numbers, strings, tuples). Also, implementing
-len(), max(), min() as a built-in function is actually less code than
-implementing them as methods for each type. One can quibble about
-individual cases but it's really too late to change such things
-fundamentally now.
-
-6.6. Q. Why can't I derive a class from built-in types (e.g. lists or
-files)?
-
-A. This is caused by the relatively late addition of (user-defined)
-classes to the language -- the implementation framework doesn't easily
-allow it. See the answer to question 4.2 for a work-around. This
-*may* be fixed in the (distant) future.
-
-6.7. Q. Why must 'self' be declared and used explicitly in method
-definitions and calls?
-
-A. By asking this question you reveal your C++ background. :-)
-When I added classes, this was (again) the simplest way of
-implementing methods without too many changes to the interpreter. I
-borrowed the idea from Modula-3. It turns out to be very useful, for
-a variety of reasons.
-
-First, it makes it more obvious that you are using a method or
-instance attribute instead of a local variable. Reading "self.x" or
-"self.meth()" makes it absolutely clear that an instance variable or
-method is used even if you don't know the class definition by heart.
-In C++, you can sort of tell by the lack of a local variable
-declaration (assuming globals are rare or easily recognizable) -- but
-in Python, there are no local variable declarations, so you'd have to
-look up the class definition to be sure.
-
-Second, it means that no special syntax is necessary if you want to
-explicitly reference or call the method from a particular class. In
-C++, if you want to use a method from base class that is overridden in
-a derived class, you have to use the :: operator -- in Python you can
-write baseclass.methodname(self, <argument list>). This is
-particularly useful for __init__() methods, and in general in cases
-where a derived class method wants to extend the base class method of
-the same name and thus has to call the base class method somehow.
-
-Lastly, for instance variables, it solves a syntactic problem with
-assignment: since local variables in Python are (by definition!) those
-variables to which a value assigned in a function body (and that
-aren't explicitly declared global), there has to be some way to tell
-the interpreter that an assignment was meant to assign to an instance
-variable instead of to a local variable, and it should preferably be
-syntactic (for efficiency reasons). C++ does this through
-declarations, but Python doesn't have declarations and it would be a
-pity having to introduce them just for this purpose. Using the
-explicit "self.var" solves this nicely. Similarly, for using instance
-variables, having to write "self.var" means that references to
-unqualified names inside a method don't have to search the instance's
-directories.
-
-6.8. Q. Can't you emulate threads in the interpreter instead of
-relying on an OS-specific thread implementation?
-
-A. Unfortunately, the interpreter pushes at least one C stack frame
-for each Python stack frame. Also, extensions can call back into
-Python at almost random moments. Therefore a complete threads
-implementation requires thread support for C.
-
-6.9. Q. Why can't lambda forms contain statements?
-
-A. Python lambda forms cannot contain statements because Python's
-syntactic framework can't handle statements nested inside expressions.
-
-However, in Python, this is not a serious problem. Unlike lambda
-forms in other languages, where they add functionality, Python lambdas
-are only a shorthand notation if you're too lazy to define a function.
-
-Functions are already first class objects in Python, and can be
-declared in a local scope. Therefore the only advantage of using a
-lambda form instead of a locally-defined function is that you'll have
-to invent a name for the function -- but that's just a local variable
-to which the function object (which is exactly the same type of object
-that a lambda form yields) is assigned!
-
-6.10. Q. Why don't lambdas have access to variables defined in the
-containing scope?
-
-A. Because they are implemented as ordinary functions.
-See question 4.5 above.
-
-6.11. Q. Why can't recursive functions be defined inside other functions?
-
-A. See question 4.5 above.
-
-6.12. Q. Why is there no more efficient way of iterating over a dictionary
-than first constructing the list of keys()?
-
-A. Have you tried it? I bet it's fast enough for your purposes! In
-most cases such a list takes only a few percent of the space occupied
-by the dictionary -- it needs only 4 bytes (the size of a pointer) per
-key -- a dictionary costs 8 bytes per key plus between 30 and 70
-percent hash table overhead, plus the space for the keys and values --
-by necessity all keys are unique objects and a string object (the most
-common key type) costs at least 18 bytes plus the length of the
-string. Add to that the values contained in the dictionary, and you
-see that 4 bytes more per item really isn't that much more memory...
-
-A call to dict.keys() makes one fast scan over the dictionary
-(internally, the iteration function does exist) copying the pointers
-to the key objects into a pre-allocated list object of the right size.
-The iteration time isn't lost (since you'll have to iterate anyway --
-unless in the majority of cases your loop terminates very prematurely
-(which I doubt since you're getting the keys in random order).
-
-I don't expose the dictionary iteration operation to Python
-programmers because the dictionary shouldn't be modified during the
-entire iteration -- if it is, there's a very small chance that the
-dictionary is reorganized because the hash table becomes too full, and
-then the iteration may miss some items and see others twice. Exactly
-because this only occurs rarely, it would lead to hidden bugs in
-programs: it's easy never to have it happen during test runs if you
-only insert or delete a few items per iteration -- but your users will
-surely hit upon it sooner or later.
-
-6.13. Q. Can Python be compiled to machine code, C or some other language?
-
-A. Not easily. Python's high level data types, dynamic typing of
-objects and run-time invocation of the interpreter (using eval() or
-exec) together mean that a "compiled" Python program would probably
-consist mostly of calls into the Python run-time system, even for
-seemingly simple operations like "x+1". Thus, the performance gain
-would probably be minimal.
-
-Internally, Python source code is always translated into a "virtual
-machine code" or "byte code" representation before it is interpreted
-(by the "Python virtual machine" or "bytecode interpreter"). In order
-to avoid the overhead of parsing and translating modules that rarely
-change over and over again, this byte code is written on a file whose
-name ends in ".pyc" whenever a module is parsed (from a file whose
-name ends in ".py"). When the corresponding .py file is changed, it
-is parsed and translated again and the .pyc file is rewritten. There
-is no performance difference once the .pyc file has been loaded (the
-bytecode read from the .pyc file is exactly the same as the bytecode
-created by direct translation). The only difference is that loading
-code from a .pyc file is faster than parsing and translating a .py
-file, so the presence of precompiled .pyc files will generally improve
-start-up time of Python scripts. If desired, the Lib/compileall.py
-module/script can be used to force creation of valid .pyc files for a
-given set of modules.
-
-If you are looking for a way to translate Python programs in order to
-distribute them in binary form, without the need to distribute the
-interpreter and library as well, have a look at the freeze.py script
-in the Tools/freeze directory. This creates a single binary file
-incorporating your program, the Python interpreter, and those parts of
-the Python library that are needed by your program. Of course, the
-resulting binary will only run on the same type of platform as that
-used to create it.
-
-Hints for proper usage of freeze.py:
-
-- the script must be in a file whose name ends in .py
-
-- you must have installed Python fully:
-
- make install
- make libinstall
- make inclinstall
- make libainstall
-
-6.14. Q. Why doesn't Python use proper garbage collection?
-
-A. It's looking less and less likely that Python will ever get
-"automatic" garbage collection (GC). For one thing, unless this were
-added to C as a standard feature, it's a portability pain in the ass.
-And yes, I know about the Xerox library. It has bits of assembler
-code for *most* *common* platforms. Not for all. And although it is
-mostly transparent, it isn't completely transparent (when I once
-linked Python with it, it dumped core).
-
-"Proper" GC also becomes a problem when Python gets embedded into
-other applications. While in a stand-alone Python it may be fine to
-replace the standard malloc() and free() with versions provided by the
-GC library, an application embedding Python may want to have its *own*
-substitute for malloc() and free(), and may not want Python's. Right
-now, Python works with anything that implements malloc() and free()
-properly.
-
-Besides, the predictability of destructor calls in Python is kind of
-attractive. With GC, the following code (which is fine in current
-Python) will run out of file descriptors long before it runs out of
-memory:
-
- for file in <very long list of files>:
- f = open(file)
- c = file.read(1)
-
-Using the current reference counting and destructor scheme, each new
-assignment to f closes the previous file. Using GC, this is not
-guaranteed. Sure, you can think of ways to fix this. But it's not
-off-the-shelf technology.
-
-
-7. Using Python on non-UNIX platforms
-=====================================
-
-7.1. Q. Is there a Mac version of Python?
-
-A. Yes, see the "mac" subdirectory of the distribution sites,
-e.g. <URL:ftp://ftp.python.org/pub/python/mac/>.
-
-7.2. Q. Are there DOS and Windows versions of Python?
-
-A. Yes. There is a plethora of not-always-compatible versions. See
-the "pythonwin", "wpy", "nt" and "pc" subdirectories of the
-distribution sites. A quick comparison:
-
-PythonWin: Extensive support for the 32-bit native Windows API and GUI
-building using MFC. Windows NT and Windows 95 only (and Windows
-3.1(1) using win32s, until Microsoft stops supporting it :-( ).
-<URL:ftp://ftp.python.org/pub/python/pythonwin/>.
-
-WPY: Ports to DOS, Windows 3.1(1), Windows 95, Windows NT and OS/2.
-Also contains a GUI package that offers portability between Windows
-(not DOS) and Unix, and native look and feel on both.
-<URL:ftp://ftp.python.org/pub/python/wpy/>.
-
-NT: Basic ports built straight from the 1.4 distribution for Windows
-95 and Windows NT. This will eventually provide core support for
-both PythonWin and WPY on all 32-bit Microsoft platforms.
-<URL:ftp://ftp.python.org/pub/python/nt/>.
-
-PC: Old, unsupported ports to DOS, Windows 3.1(1) and OS/2.
-<URL:ftp://ftp.python.org/pub/python/pc/>.
-
-7.3. Q. Is there an OS/2 version of Python?
-
-A. Yes, see the "pc" and "wpy" subdirectory of the distribution sites
-(see above).
-
-7.4. Q. Is there a VMS version of Python?
-
-A. Donn Cave <donn@cac.washington.edu> did a partial port. The
-results of his efforts are on public display in
-<<URL:ftp://ftp.python.org/pub/python/contrib/vms.tar.gz/>. Someone
-else is working on a more complete port, for details watch the list.
-
-7.5. Q. What about IBM mainframes, or other non-UNIX platforms?
-
-A. I haven't heard about these, except I remember hearing about an
-OS/9 port and a port to Vxworks (both operating systems for embedded
-systems). If you're interested in any of this, go directly to the
-newsgroup and ask there, you may find exactly what you need. For
-example, a port to MPE/iX 5.0 on HP3000 computers was just announced,
-see <URL:http://www.allegro.com/software/>.
-
-7.6. Q. Where are the source or Makefiles for the non-UNIX versions?
-
-A. The standard sources can (almost) be used. Additional sources can
-be found in the platform-specific subdirectories of the distribution.
-
-7.7. Q. What is the status and support for the non-UNIX versions?
-
-A. I don't have access to most of these platforms, so in general I am
-dependent on material submitted by volunteers(*). However I strive to
-integrate all changes needed to get it to compile on a particular
-platform back into the standard sources, so porting of the next
-version to the various non-UNIX platforms should be easy.
-
-(*) For the Macintosh, that volunteer is me, with help from Jack
-Jansen <jack@cwi.nl>.
-
-7.8. Q. I have a PC version but it appears to be only a binary.
-Where's the library?
-
-A. You still need to copy the files from the distribution directory
-"python/Lib" to your system. If you don't have the full distribution,
-you can get the file lib<version>.tar.gz from most ftp sites carrying
-Python; this is a subset of the distribution containing just those
-files, e.g. <URL:ftp://ftp.python.org/pub/python/src/lib1.1.tar.gz>.
-
-Once you have installed the library, you need to point sys.path to it.
-Assuming the library is in C:\misc\python\lib, the following commands
-will point your Python interpreter to it (note the doubled backslashes
--- you can also use single forward slashes instead):
-
- >>> import sys
- >>> sys.path.insert(0, 'C:\\misc\\python\\lib')
- >>>
-
-For a more permanent effect, set the environment variable PYTHONPATH,
-as follows (talking to a DOS prompt):
-
- C> SET PYTHONPATH=C:\misc\python\lib
-
-7.9. Q. Where's the documentation for the Mac or PC version?
-
-A. The documentation for the Unix version also applies to the Mac and
-PC versions. Where applicable, differences are indicated in the text.
-
-7.10. Q. The Mac (PC) version doesn't seem to have any facilities for
-creating or editing programs apart from entering it interactively, and
-there seems to be no way to save code that was entered interactively.
-How do I create a Python program on the Mac (PC)?
-
-A. Use an external editor. On the Mac, BBEdit seems to be a popular
-no-frills text editor. I work like this: start the interpreter; edit
-a module file using BBedit; import and test it in the interpreter;
-edit again in BBedit; then use the built-in function reload() to
-re-read the imported module; etc.
-
-Regarding the same question for the PC, Kurt Wm. Hemr writes: "While
-anyone with a pulse could certainly figure out how to do the same on
-MS-Windows, I would recommend the NotGNU Emacs clone for MS-Windows.
-Not only can you easily resave and "reload()" from Python after making
-changes, but since WinNot auto-copies to the clipboard any text you
-select, you can simply select the entire procedure (function) which
-you changed in WinNot, switch to QWPython, and shift-ins to reenter
-the changed program unit."
diff --git a/Misc/README b/Misc/README
index 0ad373b..c737c5e 100644
--- a/Misc/README
+++ b/Misc/README
@@ -14,7 +14,7 @@
BLURB A quick description of Python for newcomers
BLURB.LUTZ A testimonial from a converted Tcl/Perl hacker :-)
COPYRIGHT The Python copyright notice
-FAQ Frequently Asked Questions about Python (and answers)
+FAQ (deleted -- see http://www.python.org for the online FAQ)
Fixcprt.py Fix the copyright message (a yearly chore :-)
HISTORY News from previous releases -- oldest last
HPUX-NOTES Notes about dynamic loading under HP-UX