| Subject: FAQ: Python -- an object-oriented language |
| Newsgroups: comp.lang.python,comp.answers,news.answers |
| Followup-to: comp.lang.python |
| From: guido@cwi.nl (Guido van Rossum) |
| Reply-to: guido@cwi.nl (Guido van Rossum) |
| Approved: news-answers-request@MIT.Edu |
| |
| Archive-name: python-faq/part1 |
| Version: 1.11 |
| Last-modified: 25 July 1994 |
| |
| 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, 1994 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 |
| CWI, dept. CST |
| Kruislaan 413 |
| P.O. Box 94079 |
| 1090 GB Amsterdam |
| The Netherlands |
| Email: guido@cwi.nl |
| |
| The latest version of this FAQ is available by anonymous ftp from |
| ftp.cwi.nl [192.16.191.128] in the directory /pub/python, with |
| filename python-FAQ. It will also be posted regularly to the |
| newsgroups comp.answers and comp.lang.python. |
| |
| Many FAQs, including this one, are available by anonymous ftp from |
| rtfm.mit.edu [18.181.0.24] in the directory 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. |
| |
| 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 |
| 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 book on Python, or will there be one out soon? |
| 1.8. Q. Are there any published articles about Python that I can quote? |
| 1.9. Q. How does the Python version numbering scheme work? |
| 1.10. Q. Are there other ftp sites that carry Python related material? |
| 1.11. Q. Are there copyright restrictions on the use of Python? |
| |
| 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? |
| |
| 3. Building Python |
| 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 building Python with STDWIN on SGI IRIX. |
| 3.4. Q. Link errors after rerunning the configure script. |
| 3.5. Q. The python interpreter complains about options passed to a |
| script (after the script name). |
| 3.6. Q. When building on the SGI, make tries to run python to create |
| glmodule.c, but python hasn't been built or installed yet. |
| 3.7. Q. Python built with gcc for the DEC Alpha doesn't work. |
| 3.8. Q. Other trouble building Python on platform X. |
| |
| 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. 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? |
| 4.10. Q. I try to run a program from the Demo directory but it fails with |
| ImportError: No module named ...; what gives? |
| 4.11. Q. What GUI toolkits exist for Python? |
| 4.12. Q. Are there any interfaces to commercial database in Python? |
| |
| 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? |
| |
| 6. Python's design |
| 6.1. Q. Why isn't there a generic copying operation for objects in |
| Python? |
| 6.2. Q. Why isn't there a generic way to implement persistent objects |
| in Python? (Persistent == automatically saved to and restored from |
| disk.) |
| 6.3. Q. Why isn't there a switch or case statement in Python? |
| 6.4. Q. Why does Python use indentation for grouping of statements? |
| 6.5. Q. Why are Python strings immutable? |
| 6.6. Q. Why don't strings have methods like index() or sort(), like |
| lists? |
| 6.7. Q. Why does Python use methods for some functionality |
| (e.g. list.index()) but functions for other (e.g. len(list))? |
| 6.8. Q. Why can't I derive a class from built-in types (e.g. lists or |
| files)? |
| 6.9. Q. Why must 'self' be declared and used explicitly in method |
| definitions and calls? |
| |
| 7. Using Python on non-UNIX platforms |
| 7.1. Q. Is there a Mac version of Python? |
| 7.2. Q. Is there a DOS version of Python? |
| 7.3. Q. Is there a Windows version of Python? |
| 7.4. Q. Is there a Windows NT version of Python? |
| 7.5. Q. Is there an OS/2 version of Python? |
| 7.6. Q. I have the PC version but it appears to be only a binary. |
| Where's the library? |
| 7.7. Q. Where's the documentation for the Mac or PC version? |
| 7.8. 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 |
| MS-DOS. |
| |
| 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 wizard, 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! |
| |
| 1.3. Q. How do I obtain a copy of the Python source? |
| |
| A. The latest Python source distribution is always available by |
| anonymous ftp from ftp.cwi.nl [192.16.191.128] in the directory |
| /pub/python, with filename python<version>.tar.gz. (Old versions may |
| have an extension of .Z, indicating use of "compress" compression.) |
| It is a gzip'ed 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.) |
| |
| 1.4. Q. How do I get documentation on Python? |
| |
| A. The latest Python documentation set is always available by |
| anonymous ftp from ftp.cwi.nl [192.16.191.128] in the directory |
| /pub/python, with filename pythondoc-ps<version>.tar.gz. It is a |
| gzip'ed 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. |
| |
| 1.5. Q. Are there other ftp sites that mirror the Python distribution? |
| |
| A. The following sites keep mirrors of the Python distribution: |
| |
| Site IP address Directory |
| |
| gatekeeper.dec.com 16.1.0.2 /pub/plan/python |
| ftp.uu.net 192.48.96.9 /languages/python |
| ftp.wustl.edu 128.252.135.4 /graphics/graphics/sgi-stuff/python |
| ftp.funet.fi 128.214.6.100 /pub/languages/python |
| ftp.fu-berlin.de 130.133.4.50 /pub/unix/languages/python (*python* only) |
| ftp.sunet.se 130.238.127.3 /pub/lang/python |
| unix.hensa.ac.uk 129.12.43.16 /uunet/languages/python |
| |
| Or try archie on e.g. python1.0 to locate the nearest copy of that |
| version... |
| |
| 1.6. Q. Is there a newsgroup or mailing list devoted to Python? |
| |
| A. There is a newsgroup, comp.lang.python, and a mailing list. The |
| newsgroup and mailing list are gatewayed into each other -- if you can |
| read news it is not necessary to subscribe to the mailing list. Send |
| e-mail to python-list-request@cwi.nl to (un)subscribe to the mailing |
| list. Once you're on, send e-mail tp python-list@cwi.nl to send mail |
| to the entire mailing list and newsgroup. |
| |
| 1.7. Q. Is there a book on Python, or will there be one out soon? |
| |
| A. Unfortunately, not yet. Mark Lutz and I are planning to write one, |
| but we are still a in very preliminary stage. If you would like to |
| beat us at it and get rich from book royalties, go ahead! |
| |
| 1.8. 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. |
| |
| A more recent high-level description of Python 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 can be found in the ftp directory mentioned a few |
| questions earlier, with filenames nluug-paper.ps and nluug-slides.ps, |
| respectively. |
| |
| 1.9. Q. How does the Python version numbering scheme work? |
| |
| A. Python versions are numbered A.B.C. 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 release. 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! |
| |
| 1.10. Q. Are there other ftp sites that carry Python related material? |
| |
| A. An interesting ftp site for Python users is ftp.markv.com |
| (192.122.251.1); the directory pub/python contains a growing |
| collection of interesting Python scripts. To submit a script for |
| inclusion, place it together with a readme file (with extension |
| .readme) in the publicly writable directory /incoming/python. This |
| service is maintained by Lance Ellinghouse <lance@markv.com>. |
| |
| 1.11. 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, display those copyrights in any |
| documentation about Python that you produce, 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). |
| |
| |
| 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. |
| |
| 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>. |
| |
| See also the next question. |
| |
| 2.3. Q. Are there any commercial projects going on using Python? |
| |
| A. Several companies have revealed to me that they are planning or |
| considering to use Python in a future product. The furthest is |
| Sunrise Software, who already have a product out using Python -- they |
| use Python for a GUI management application and an SNMP network |
| manangement application. Contact: <info@sunrise.com>. |
| |
| Individuals at many other companies are using Python for |
| internal development (witness their contributions to the Python |
| mailing list or newsgroup). |
| |
| Python has also been elected as an extension language by MADE, a |
| consortium supported by the European Committee's ESPRIT program and |
| consisting of Bull, CWI and some other European companies. Contact: |
| Ivan Herman <ivan@cwi.nl>. |
| |
| 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 and 1.0.x) have been coming out roughly every 3 to 6 |
| months for the past four years. |
| |
| 2.5. Q. What new developments are expected for Python in the future? |
| |
| A. A proposal is being worked out to change the semantics of operator |
| overloading (__add__, __mul__ etc.) to make them more useful for |
| implementing types that don't resemble numbers. Additions will be |
| __call__ (to call an instance as if it were a function) and __eq__, |
| _lt__ etc. (to override individual comparisons). A (new) pthreads |
| interface has been submitted which will be integrated in the next |
| release. The X11/Motif interface will be improved. There are ideas |
| about built-in help using strings placed into function objects, and |
| possibly a switch statement. |
| |
| There will be better ports to the Mac, DOS, Windows, Windows NT, and |
| OS/2. The Windows ports will support dynalically loaded modules using |
| DLLs. The Mac port may also support dynamic loading of modules. |
| There may be a Windows version of STDWIN. |
| |
| Also planned is improved support for embedding Python in other |
| applications, e.g. by renaming most global symbols to have a "Py" |
| prefix and providing more documentation and threading support. |
| |
| 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. |
| |
| |
| 3. Building Python |
| ================== |
| |
| 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. |
| |
| 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 building Python with STDWIN on SGI IRIX. |
| |
| A. Rebuild STDWIN, specifying "CC=cc -cckr" in the Makefile. |
| |
| 3.4. Q. Link errors after rerunning the configure script. |
| |
| A. It is generally necessary to run "make clean" after a configuration |
| change. |
| |
| 3.5. 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. (If you are using this because you link with -lreadline, use |
| the readline distributed with Python instead.) |
| |
| 3.6. 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). |
| |
| 3.7. Q. Python built with gcc for the DEC Alpha doesn't work. |
| |
| People have reported problems with both gcc 2.5.8 and 2.6.0. The DEC |
| OSF/1 cc compiler does not have these problems so it's probably gcc's |
| fault. One person reported that the problem went away when using -g |
| instead of -O so this may be an option if you insist on using gcc. If |
| someone tracks it down more completely I'd like to hear about it! |
| |
| 3.8. Q. Other trouble building Python on platform X. |
| |
| A. Please email the details to <guido@cwi.nl> and I'll look into it. |
| |
| |
| 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) |
| |
| 4.3. Q. Is there a curses/termcap package for Python? |
| |
| A. No, but you can use the "alfa" (== character cell) version of |
| STDWIN. (STDWIN == Standard Windows, a portable windowing system |
| interface by the same author, 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 |
| |
| 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 the 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 efficiency reasons, 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 a changed module |
| being read again, 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 objects imported |
| thus. |
| |
| 4.9. 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. A module can find out its own module name by alooking at the |
| (predefined) global variable __name__. If this has the value |
| '__main__' you are running as a script. 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() |
| |
| 4.10. 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 which hasn't been configured on |
| your system. This especially happens with modules like "stdwin", |
| "gl", "Xt" or "Xm". For 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). 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 file "extensions.tar.gz" file from a Python ftp repository |
| (e.g. ftp.cwi.nl) and follow the instructions there. Note: the X |
| related modules are still somewhat flakey, so don't try this unless |
| you know a bit or two about building X applications on your platform. |
| |
| 4.11. Q. What GUI toolkits exist for Python? |
| |
| A. Depending on what platform(s) you are aiming at, there are several. |
| |
| - The standard Python distribution comes with an interface to STDWIN, |
| a platform-independent low-level windowing interface (you have to ftp |
| the source for stdwin separately, e.g. from ftp.cwi.nl in pub/stdwin |
| or gatekeeper.dec.com in pub/misc/stdwin). STDWIN runs under X11 or |
| the Mac; a Windows port is about to be released. |
| |
| - For SGI only, there's an interface 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 ftp.cs.ruu.nl in pub/SGI/FORMS). |
| |
| - 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) in the Extensions set, which is separately ftp'able |
| from ftp.cwi.nl as pub/python/extensions.tar.gz. |
| |
| - There's an interface to SUIT, the U of Virginia's Simple User |
| Interface Toolkit; it can be ftp'ed from uvacs.cs.virginia.edu as |
| pub/suit/python/SUIT_python.tar.Z. A PC binary of Python 1.0.2 |
| compiled with DJGPP and with SUIT support built-in has been made |
| available by Antonio Costa on ftp site asterix.inescn.pt, directory |
| pub/PC/python, file pyt102su.exe (a self-extracting archive). |
| |
| - There's a neat interface to the Tcl/Tk widget set; ftp it from |
| ftp.cwi.nl as pub/python/tkinter.tar.gz. |
| |
| - There's an interface to WAFE, a Tcl interface to the X11 Motif and |
| Athena widget sets. Last I heard about it it was included in the |
| WAFE 1.0 prerelease, ftp'able from ftp.wu-wien.ac.at as |
| pub/src/X11/wafe/wafe-1.0.tar.gz-prerelease. |
| |
| 4.12. Q. Are there any interfaces to commercial database in Python? |
| |
| A. There's an interface to SYBASE by John Redford |
| <jredford@lehman.com>. |
| |
| There's also an interface to metalbase by Lance Ellinghouse |
| <lance@markv.com>. |
| |
| |
| 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). |
| |
| |
| 6. Python's design |
| ================== |
| |
| 6.1. Q. Why isn't there a generic copying operation for objects in |
| Python? |
| |
| A. Hmm. Maybe there should be one, but it's difficult to assign a |
| useful meaning to copying of open files, sockets and windows, or |
| recursive data structures. As long as you design all your classes |
| yourself you are of course free to define a standard base class that |
| defines an overridable copying operation for all the objects you care |
| about. (One practical point: it would have to be a built-in function, |
| not a standard method name, since not all built-in object types have |
| methods; e.g. strings, integers and tuples don't.) |
| |
| 6.2. Q. Why isn't there a generic way to implement persistent objects |
| in Python? (Persistent == automatically saved to and restored from |
| disk.) |
| |
| A. Hmm, hmm. Basically for the same reasons as why there is no |
| generic copying operation. |
| |
| 6.3. 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 concensus (yet) on whether and how |
| to do range tests. |
| |
| 6.4. 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 there can be no conflicting |
| coding styles. 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! |
| |
| 6.5. 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 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) |
| |
| 6.6. 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 perferred 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). |
| |
| 6.7. 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. |
| |
| 6.8. 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. |
| |
| 6.9. 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 reasily 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. |
| |
| |
| 7. Using Python on non-UNIX platforms |
| ===================================== |
| |
| 7.1. Q. Is there a Mac version of Python? |
| |
| A. Yes. It is on most ftp sites carrying Python as python.sea.hqx -- |
| this is a self-extracting archive containing the application binary as |
| well as the Lib modules. |
| |
| 7.2. Q. Is there a DOS version of Python? |
| |
| A. Yes. More than one, actually: 16python.exe runs in standard DOS |
| mode on 186 CPUs or higher; 32python.exe uses a DOS extender and only |
| runs on a 386 or higher CPUs. Although 16python.exe does not pass the |
| test set because test_grammar is too big for the parser, it actually |
| has about 270 kbyte of allocatable heap space, which is sufficient for |
| fairly large programs. 32python.exe is distributed as a tar file |
| containing the required DOS extended and 387 emulator. Both are on |
| most ftp sites carrying Python. |
| |
| 7.3. Q. Is there a Windows version of Python? |
| |
| A. Yes. Use qwpython.exe. The only problem with it: ^C |
| unconditionally kills the entire program -- it does not raise |
| KeyboardInterrupt. You can also run 16python.exe or 32python.exe in a |
| "DOS box", but qwpython.exe appears to be slightly faster. |
| |
| 7.4. Q. Is there a Windows NT version of Python? |
| |
| A. Yes. Use ntpython.exe. This is for Intel CPUs. If you want a |
| Windows user interface, use qwpython.exe. |
| |
| 7.5. Q. Is there an OS/2 version of Python? |
| |
| A. Yes. You can ftp it (from ftp.cwi.nl in pub/python, or from the |
| mirror sites) as pyth_os2.zip. This contains both an executable and |
| Makefiles for those fortunate enough to have a C compiler. |
| |
| 7.6. Q. I have the 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 pythonlib<version>.tar.gz from most ftp sites |
| carrying Python; this is a subset of the distribution containing just |
| those file. |
| |
| 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 |
| |
| 7.7. Q. Where's the documentation for the Mac or PC version? |
| |
| A. There isn't any. The documentation for the Unix version also |
| applies to the Mac and PC versions. Where applicable, differences |
| are indicated in the text. |
| |
| 7.8. 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, I am quite happy with the Desk |
| Accessory called Sigma Edit; this doesn't require Multifinder or |
| System 7. I work like this: start the interpreter; edit a module file |
| using Sigma Edit; import and test it in the interpreter; edit again in |
| Sigma Edit; 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." |