| **************************** |
| What's New in Python 2.1 |
| **************************** |
| |
| :Author: A.M. Kuchling |
| |
| .. |release| replace:: 1.01 |
| |
| .. $Id: whatsnew21.tex 50964 2006-07-30 03:03:43Z fred.drake $ |
| |
| |
| Introduction |
| ============ |
| |
| This article explains the new features in Python 2.1. While there aren't as |
| many changes in 2.1 as there were in Python 2.0, there are still some pleasant |
| surprises in store. 2.1 is the first release to be steered through the use of |
| Python Enhancement Proposals, or PEPs, so most of the sizable changes have |
| accompanying PEPs that provide more complete documentation and a design |
| rationale for the change. This article doesn't attempt to document the new |
| features completely, but simply provides an overview of the new features for |
| Python programmers. Refer to the Python 2.1 documentation, or to the specific |
| PEP, for more details about any new feature that particularly interests you. |
| |
| One recent goal of the Python development team has been to accelerate the pace |
| of new releases, with a new release coming every 6 to 9 months. 2.1 is the first |
| release to come out at this faster pace, with the first alpha appearing in |
| January, 3 months after the final version of 2.0 was released. |
| |
| The final release of Python 2.1 was made on April 17, 2001. |
| |
| .. ====================================================================== |
| |
| |
| PEP 227: Nested Scopes |
| ====================== |
| |
| The largest change in Python 2.1 is to Python's scoping rules. In Python 2.0, |
| at any given time there are at most three namespaces used to look up variable |
| names: local, module-level, and the built-in namespace. This often surprised |
| people because it didn't match their intuitive expectations. For example, a |
| nested recursive function definition doesn't work:: |
| |
| def f(): |
| ... |
| def g(value): |
| ... |
| return g(value-1) + 1 |
| ... |
| |
| The function :func:`g` will always raise a :exc:`NameError` exception, because |
| the binding of the name ``g`` isn't in either its local namespace or in the |
| module-level namespace. This isn't much of a problem in practice (how often do |
| you recursively define interior functions like this?), but this also made using |
| the :keyword:`lambda` statement clumsier, and this was a problem in practice. |
| In code which uses :keyword:`lambda` you can often find local variables being |
| copied by passing them as the default values of arguments. :: |
| |
| def find(self, name): |
| "Return list of any entries equal to 'name'" |
| L = filter(lambda x, name=name: x == name, |
| self.list_attribute) |
| return L |
| |
| The readability of Python code written in a strongly functional style suffers |
| greatly as a result. |
| |
| The most significant change to Python 2.1 is that static scoping has been added |
| to the language to fix this problem. As a first effect, the ``name=name`` |
| default argument is now unnecessary in the above example. Put simply, when a |
| given variable name is not assigned a value within a function (by an assignment, |
| or the :keyword:`def`, :keyword:`class`, or :keyword:`import` statements), |
| references to the variable will be looked up in the local namespace of the |
| enclosing scope. A more detailed explanation of the rules, and a dissection of |
| the implementation, can be found in the PEP. |
| |
| This change may cause some compatibility problems for code where the same |
| variable name is used both at the module level and as a local variable within a |
| function that contains further function definitions. This seems rather unlikely |
| though, since such code would have been pretty confusing to read in the first |
| place. |
| |
| One side effect of the change is that the ``from module import *`` and |
| ``exec`` statements have been made illegal inside a function scope under |
| certain conditions. The Python reference manual has said all along that ``from |
| module import *`` is only legal at the top level of a module, but the CPython |
| interpreter has never enforced this before. As part of the implementation of |
| nested scopes, the compiler which turns Python source into bytecodes has to |
| generate different code to access variables in a containing scope. ``from |
| module import *`` and ``exec`` make it impossible for the compiler to |
| figure this out, because they add names to the local namespace that are |
| unknowable at compile time. Therefore, if a function contains function |
| definitions or :keyword:`lambda` expressions with free variables, the compiler |
| will flag this by raising a :exc:`SyntaxError` exception. |
| |
| To make the preceding explanation a bit clearer, here's an example:: |
| |
| x = 1 |
| def f(): |
| # The next line is a syntax error |
| exec 'x=2' |
| def g(): |
| return x |
| |
| Line 4 containing the ``exec`` statement is a syntax error, since |
| ``exec`` would define a new local variable named ``x`` whose value should |
| be accessed by :func:`g`. |
| |
| This shouldn't be much of a limitation, since ``exec`` is rarely used in |
| most Python code (and when it is used, it's often a sign of a poor design |
| anyway). |
| |
| Compatibility concerns have led to nested scopes being introduced gradually; in |
| Python 2.1, they aren't enabled by default, but can be turned on within a module |
| by using a future statement as described in PEP 236. (See the following section |
| for further discussion of PEP 236.) In Python 2.2, nested scopes will become |
| the default and there will be no way to turn them off, but users will have had |
| all of 2.1's lifetime to fix any breakage resulting from their introduction. |
| |
| |
| .. seealso:: |
| |
| :pep:`227` - Statically Nested Scopes |
| Written and implemented by Jeremy Hylton. |
| |
| .. ====================================================================== |
| |
| |
| PEP 236: __future__ Directives |
| ============================== |
| |
| The reaction to nested scopes was widespread concern about the dangers of |
| breaking code with the 2.1 release, and it was strong enough to make the |
| Pythoneers take a more conservative approach. This approach consists of |
| introducing a convention for enabling optional functionality in release N that |
| will become compulsory in release N+1. |
| |
| The syntax uses a ``from...import`` statement using the reserved module name |
| :mod:`__future__`. Nested scopes can be enabled by the following statement:: |
| |
| from __future__ import nested_scopes |
| |
| While it looks like a normal :keyword:`import` statement, it's not; there are |
| strict rules on where such a future statement can be put. They can only be at |
| the top of a module, and must precede any Python code or regular |
| :keyword:`import` statements. This is because such statements can affect how |
| the Python bytecode compiler parses code and generates bytecode, so they must |
| precede any statement that will result in bytecodes being produced. |
| |
| |
| .. seealso:: |
| |
| :pep:`236` - Back to the :mod:`__future__` |
| Written by Tim Peters, and primarily implemented by Jeremy Hylton. |
| |
| .. ====================================================================== |
| |
| |
| PEP 207: Rich Comparisons |
| ========================= |
| |
| In earlier versions, Python's support for implementing comparisons on user- |
| defined classes and extension types was quite simple. Classes could implement a |
| :meth:`__cmp__` method that was given two instances of a class, and could only |
| return 0 if they were equal or +1 or -1 if they weren't; the method couldn't |
| raise an exception or return anything other than a Boolean value. Users of |
| Numeric Python often found this model too weak and restrictive, because in the |
| number-crunching programs that numeric Python is used for, it would be more |
| useful to be able to perform elementwise comparisons of two matrices, returning |
| a matrix containing the results of a given comparison for each element. If the |
| two matrices are of different sizes, then the compare has to be able to raise an |
| exception to signal the error. |
| |
| In Python 2.1, rich comparisons were added in order to support this need. |
| Python classes can now individually overload each of the ``<``, ``<=``, ``>``, |
| ``>=``, ``==``, and ``!=`` operations. The new magic method names are: |
| |
| +-----------+----------------+ |
| | Operation | Method name | |
| +===========+================+ |
| | ``<`` | :meth:`__lt__` | |
| +-----------+----------------+ |
| | ``<=`` | :meth:`__le__` | |
| +-----------+----------------+ |
| | ``>`` | :meth:`__gt__` | |
| +-----------+----------------+ |
| | ``>=`` | :meth:`__ge__` | |
| +-----------+----------------+ |
| | ``==`` | :meth:`__eq__` | |
| +-----------+----------------+ |
| | ``!=`` | :meth:`__ne__` | |
| +-----------+----------------+ |
| |
| (The magic methods are named after the corresponding Fortran operators ``.LT.``. |
| ``.LE.``, &c. Numeric programmers are almost certainly quite familiar with |
| these names and will find them easy to remember.) |
| |
| Each of these magic methods is of the form ``method(self, other)``, where |
| ``self`` will be the object on the left-hand side of the operator, while |
| ``other`` will be the object on the right-hand side. For example, the |
| expression ``A < B`` will cause ``A.__lt__(B)`` to be called. |
| |
| Each of these magic methods can return anything at all: a Boolean, a matrix, a |
| list, or any other Python object. Alternatively they can raise an exception if |
| the comparison is impossible, inconsistent, or otherwise meaningless. |
| |
| The built-in ``cmp(A,B)`` function can use the rich comparison machinery, |
| and now accepts an optional argument specifying which comparison operation to |
| use; this is given as one of the strings ``"<"``, ``"<="``, ``">"``, ``">="``, |
| ``"=="``, or ``"!="``. If called without the optional third argument, |
| :func:`cmp` will only return -1, 0, or +1 as in previous versions of Python; |
| otherwise it will call the appropriate method and can return any Python object. |
| |
| There are also corresponding changes of interest to C programmers; there's a new |
| slot ``tp_richcmp`` in type objects and an API for performing a given rich |
| comparison. I won't cover the C API here, but will refer you to PEP 207, or to |
| 2.1's C API documentation, for the full list of related functions. |
| |
| |
| .. seealso:: |
| |
| :pep:`207` - Rich Comparisions |
| Written by Guido van Rossum, heavily based on earlier work by David Ascher, and |
| implemented by Guido van Rossum. |
| |
| .. ====================================================================== |
| |
| |
| PEP 230: Warning Framework |
| ========================== |
| |
| Over its 10 years of existence, Python has accumulated a certain number of |
| obsolete modules and features along the way. It's difficult to know when a |
| feature is safe to remove, since there's no way of knowing how much code uses it |
| --- perhaps no programs depend on the feature, or perhaps many do. To enable |
| removing old features in a more structured way, a warning framework was added. |
| When the Python developers want to get rid of a feature, it will first trigger a |
| warning in the next version of Python. The following Python version can then |
| drop the feature, and users will have had a full release cycle to remove uses of |
| the old feature. |
| |
| Python 2.1 adds the warning framework to be used in this scheme. It adds a |
| :mod:`warnings` module that provide functions to issue warnings, and to filter |
| out warnings that you don't want to be displayed. Third-party modules can also |
| use this framework to deprecate old features that they no longer wish to |
| support. |
| |
| For example, in Python 2.1 the :mod:`regex` module is deprecated, so importing |
| it causes a warning to be printed:: |
| |
| >>> import regex |
| __main__:1: DeprecationWarning: the regex module |
| is deprecated; please use the re module |
| >>> |
| |
| Warnings can be issued by calling the :func:`warnings.warn` function:: |
| |
| warnings.warn("feature X no longer supported") |
| |
| The first parameter is the warning message; an additional optional parameters |
| can be used to specify a particular warning category. |
| |
| Filters can be added to disable certain warnings; a regular expression pattern |
| can be applied to the message or to the module name in order to suppress a |
| warning. For example, you may have a program that uses the :mod:`regex` module |
| and not want to spare the time to convert it to use the :mod:`re` module right |
| now. The warning can be suppressed by calling :: |
| |
| import warnings |
| warnings.filterwarnings(action = 'ignore', |
| message='.*regex module is deprecated', |
| category=DeprecationWarning, |
| module = '__main__') |
| |
| This adds a filter that will apply only to warnings of the class |
| :class:`DeprecationWarning` triggered in the :mod:`__main__` module, and applies |
| a regular expression to only match the message about the :mod:`regex` module |
| being deprecated, and will cause such warnings to be ignored. Warnings can also |
| be printed only once, printed every time the offending code is executed, or |
| turned into exceptions that will cause the program to stop (unless the |
| exceptions are caught in the usual way, of course). |
| |
| Functions were also added to Python's C API for issuing warnings; refer to PEP |
| 230 or to Python's API documentation for the details. |
| |
| |
| .. seealso:: |
| |
| :pep:`5` - Guidelines for Language Evolution |
| Written by Paul Prescod, to specify procedures to be followed when removing old |
| features from Python. The policy described in this PEP hasn't been officially |
| adopted, but the eventual policy probably won't be too different from Prescod's |
| proposal. |
| |
| :pep:`230` - Warning Framework |
| Written and implemented by Guido van Rossum. |
| |
| .. ====================================================================== |
| |
| |
| PEP 229: New Build System |
| ========================= |
| |
| When compiling Python, the user had to go in and edit the :file:`Modules/Setup` |
| file in order to enable various additional modules; the default set is |
| relatively small and limited to modules that compile on most Unix platforms. |
| This means that on Unix platforms with many more features, most notably Linux, |
| Python installations often don't contain all useful modules they could. |
| |
| Python 2.0 added the Distutils, a set of modules for distributing and installing |
| extensions. In Python 2.1, the Distutils are used to compile much of the |
| standard library of extension modules, autodetecting which ones are supported on |
| the current machine. It's hoped that this will make Python installations easier |
| and more featureful. |
| |
| Instead of having to edit the :file:`Modules/Setup` file in order to enable |
| modules, a :file:`setup.py` script in the top directory of the Python source |
| distribution is run at build time, and attempts to discover which modules can be |
| enabled by examining the modules and header files on the system. If a module is |
| configured in :file:`Modules/Setup`, the :file:`setup.py` script won't attempt |
| to compile that module and will defer to the :file:`Modules/Setup` file's |
| contents. This provides a way to specific any strange command-line flags or |
| libraries that are required for a specific platform. |
| |
| In another far-reaching change to the build mechanism, Neil Schemenauer |
| restructured things so Python now uses a single makefile that isn't recursive, |
| instead of makefiles in the top directory and in each of the :file:`Python/`, |
| :file:`Parser/`, :file:`Objects/`, and :file:`Modules/` subdirectories. This |
| makes building Python faster and also makes hacking the Makefiles clearer and |
| simpler. |
| |
| |
| .. seealso:: |
| |
| :pep:`229` - Using Distutils to Build Python |
| Written and implemented by A.M. Kuchling. |
| |
| .. ====================================================================== |
| |
| |
| PEP 205: Weak References |
| ======================== |
| |
| Weak references, available through the :mod:`weakref` module, are a minor but |
| useful new data type in the Python programmer's toolbox. |
| |
| Storing a reference to an object (say, in a dictionary or a list) has the side |
| effect of keeping that object alive forever. There are a few specific cases |
| where this behaviour is undesirable, object caches being the most common one, |
| and another being circular references in data structures such as trees. |
| |
| For example, consider a memoizing function that caches the results of another |
| function ``f(x)`` by storing the function's argument and its result in a |
| dictionary:: |
| |
| _cache = {} |
| def memoize(x): |
| if _cache.has_key(x): |
| return _cache[x] |
| |
| retval = f(x) |
| |
| # Cache the returned object |
| _cache[x] = retval |
| |
| return retval |
| |
| This version works for simple things such as integers, but it has a side effect; |
| the ``_cache`` dictionary holds a reference to the return values, so they'll |
| never be deallocated until the Python process exits and cleans up This isn't |
| very noticeable for integers, but if :func:`f` returns an object, or a data |
| structure that takes up a lot of memory, this can be a problem. |
| |
| Weak references provide a way to implement a cache that won't keep objects alive |
| beyond their time. If an object is only accessible through weak references, the |
| object will be deallocated and the weak references will now indicate that the |
| object it referred to no longer exists. A weak reference to an object *obj* is |
| created by calling ``wr = weakref.ref(obj)``. The object being referred to is |
| returned by calling the weak reference as if it were a function: ``wr()``. It |
| will return the referenced object, or ``None`` if the object no longer exists. |
| |
| This makes it possible to write a :func:`memoize` function whose cache doesn't |
| keep objects alive, by storing weak references in the cache. :: |
| |
| _cache = {} |
| def memoize(x): |
| if _cache.has_key(x): |
| obj = _cache[x]() |
| # If weak reference object still exists, |
| # return it |
| if obj is not None: return obj |
| |
| retval = f(x) |
| |
| # Cache a weak reference |
| _cache[x] = weakref.ref(retval) |
| |
| return retval |
| |
| The :mod:`weakref` module also allows creating proxy objects which behave like |
| weak references --- an object referenced only by proxy objects is deallocated -- |
| but instead of requiring an explicit call to retrieve the object, the proxy |
| transparently forwards all operations to the object as long as the object still |
| exists. If the object is deallocated, attempting to use a proxy will cause a |
| :exc:`weakref.ReferenceError` exception to be raised. :: |
| |
| proxy = weakref.proxy(obj) |
| proxy.attr # Equivalent to obj.attr |
| proxy.meth() # Equivalent to obj.meth() |
| del obj |
| proxy.attr # raises weakref.ReferenceError |
| |
| |
| .. seealso:: |
| |
| :pep:`205` - Weak References |
| Written and implemented by Fred L. Drake, Jr. |
| |
| .. ====================================================================== |
| |
| |
| PEP 232: Function Attributes |
| ============================ |
| |
| In Python 2.1, functions can now have arbitrary information attached to them. |
| People were often using docstrings to hold information about functions and |
| methods, because the ``__doc__`` attribute was the only way of attaching any |
| information to a function. For example, in the Zope Web application server, |
| functions are marked as safe for public access by having a docstring, and in |
| John Aycock's SPARK parsing framework, docstrings hold parts of the BNF grammar |
| to be parsed. This overloading is unfortunate, since docstrings are really |
| intended to hold a function's documentation; for example, it means you can't |
| properly document functions intended for private use in Zope. |
| |
| Arbitrary attributes can now be set and retrieved on functions using the regular |
| Python syntax:: |
| |
| def f(): pass |
| |
| f.publish = 1 |
| f.secure = 1 |
| f.grammar = "A ::= B (C D)*" |
| |
| The dictionary containing attributes can be accessed as the function's |
| :attr:`__dict__`. Unlike the :attr:`__dict__` attribute of class instances, in |
| functions you can actually assign a new dictionary to :attr:`__dict__`, though |
| the new value is restricted to a regular Python dictionary; you *can't* be |
| tricky and set it to a :class:`UserDict` instance, or any other random object |
| that behaves like a mapping. |
| |
| |
| .. seealso:: |
| |
| :pep:`232` - Function Attributes |
| Written and implemented by Barry Warsaw. |
| |
| .. ====================================================================== |
| |
| |
| PEP 235: Importing Modules on Case-Insensitive Platforms |
| ======================================================== |
| |
| Some operating systems have filesystems that are case-insensitive, MacOS and |
| Windows being the primary examples; on these systems, it's impossible to |
| distinguish the filenames ``FILE.PY`` and ``file.py``, even though they do store |
| the file's name in its original case (they're case-preserving, too). |
| |
| In Python 2.1, the :keyword:`import` statement will work to simulate case- |
| sensitivity on case-insensitive platforms. Python will now search for the first |
| case-sensitive match by default, raising an :exc:`ImportError` if no such file |
| is found, so ``import file`` will not import a module named ``FILE.PY``. Case- |
| insensitive matching can be requested by setting the :envvar:`PYTHONCASEOK` |
| environment variable before starting the Python interpreter. |
| |
| .. ====================================================================== |
| |
| |
| PEP 217: Interactive Display Hook |
| ================================= |
| |
| When using the Python interpreter interactively, the output of commands is |
| displayed using the built-in :func:`repr` function. In Python 2.1, the variable |
| :func:`sys.displayhook` can be set to a callable object which will be called |
| instead of :func:`repr`. For example, you can set it to a special pretty- |
| printing function:: |
| |
| >>> # Create a recursive data structure |
| ... L = [1,2,3] |
| >>> L.append(L) |
| >>> L # Show Python's default output |
| [1, 2, 3, [...]] |
| >>> # Use pprint.pprint() as the display function |
| ... import sys, pprint |
| >>> sys.displayhook = pprint.pprint |
| >>> L |
| [1, 2, 3, <Recursion on list with id=135143996>] |
| >>> |
| |
| |
| .. seealso:: |
| |
| :pep:`217` - Display Hook for Interactive Use |
| Written and implemented by Moshe Zadka. |
| |
| .. ====================================================================== |
| |
| |
| PEP 208: New Coercion Model |
| =========================== |
| |
| How numeric coercion is done at the C level was significantly modified. This |
| will only affect the authors of C extensions to Python, allowing them more |
| flexibility in writing extension types that support numeric operations. |
| |
| Extension types can now set the type flag ``Py_TPFLAGS_CHECKTYPES`` in their |
| ``PyTypeObject`` structure to indicate that they support the new coercion model. |
| In such extension types, the numeric slot functions can no longer assume that |
| they'll be passed two arguments of the same type; instead they may be passed two |
| arguments of differing types, and can then perform their own internal coercion. |
| If the slot function is passed a type it can't handle, it can indicate the |
| failure by returning a reference to the ``Py_NotImplemented`` singleton value. |
| The numeric functions of the other type will then be tried, and perhaps they can |
| handle the operation; if the other type also returns ``Py_NotImplemented``, then |
| a :exc:`TypeError` will be raised. Numeric methods written in Python can also |
| return ``Py_NotImplemented``, causing the interpreter to act as if the method |
| did not exist (perhaps raising a :exc:`TypeError`, perhaps trying another |
| object's numeric methods). |
| |
| |
| .. seealso:: |
| |
| :pep:`208` - Reworking the Coercion Model |
| Written and implemented by Neil Schemenauer, heavily based upon earlier work by |
| Marc-André Lemburg. Read this to understand the fine points of how numeric |
| operations will now be processed at the C level. |
| |
| .. ====================================================================== |
| |
| |
| PEP 241: Metadata in Python Packages |
| ==================================== |
| |
| A common complaint from Python users is that there's no single catalog of all |
| the Python modules in existence. T. Middleton's Vaults of Parnassus at |
| http://www.vex.net/parnassus/ are the largest catalog of Python modules, but |
| registering software at the Vaults is optional, and many people don't bother. |
| |
| As a first small step toward fixing the problem, Python software packaged using |
| the Distutils :command:`sdist` command will include a file named |
| :file:`PKG-INFO` containing information about the package such as its name, |
| version, and author (metadata, in cataloguing terminology). PEP 241 contains |
| the full list of fields that can be present in the :file:`PKG-INFO` file. As |
| people began to package their software using Python 2.1, more and more packages |
| will include metadata, making it possible to build automated cataloguing systems |
| and experiment with them. With the result experience, perhaps it'll be possible |
| to design a really good catalog and then build support for it into Python 2.2. |
| For example, the Distutils :command:`sdist` and :command:`bdist_\*` commands |
| could support a ``upload`` option that would automatically upload your |
| package to a catalog server. |
| |
| You can start creating packages containing :file:`PKG-INFO` even if you're not |
| using Python 2.1, since a new release of the Distutils will be made for users of |
| earlier Python versions. Version 1.0.2 of the Distutils includes the changes |
| described in PEP 241, as well as various bugfixes and enhancements. It will be |
| available from the Distutils SIG at https://www.python.org/sigs/distutils-sig/. |
| |
| |
| .. seealso:: |
| |
| :pep:`241` - Metadata for Python Software Packages |
| Written and implemented by A.M. Kuchling. |
| |
| :pep:`243` - Module Repository Upload Mechanism |
| Written by Sean Reifschneider, this draft PEP describes a proposed mechanism for |
| uploading Python packages to a central server. |
| |
| .. ====================================================================== |
| |
| |
| New and Improved Modules |
| ======================== |
| |
| * Ka-Ping Yee contributed two new modules: :mod:`inspect.py`, a module for |
| getting information about live Python code, and :mod:`pydoc.py`, a module for |
| interactively converting docstrings to HTML or text. As a bonus, |
| :file:`Tools/scripts/pydoc`, which is now automatically installed, uses |
| :mod:`pydoc.py` to display documentation given a Python module, package, or |
| class name. For example, ``pydoc xml.dom`` displays the following:: |
| |
| Python Library Documentation: package xml.dom in xml |
| |
| NAME |
| xml.dom - W3C Document Object Model implementation for Python. |
| |
| FILE |
| /usr/local/lib/python2.1/xml/dom/__init__.pyc |
| |
| DESCRIPTION |
| The Python mapping of the Document Object Model is documented in the |
| Python Library Reference in the section on the xml.dom package. |
| |
| This package contains the following modules: |
| ... |
| |
| :file:`pydoc` also includes a Tk-based interactive help browser. :file:`pydoc` |
| quickly becomes addictive; try it out! |
| |
| * Two different modules for unit testing were added to the standard library. |
| The :mod:`doctest` module, contributed by Tim Peters, provides a testing |
| framework based on running embedded examples in docstrings and comparing the |
| results against the expected output. PyUnit, contributed by Steve Purcell, is a |
| unit testing framework inspired by JUnit, which was in turn an adaptation of |
| Kent Beck's Smalltalk testing framework. See http://pyunit.sourceforge.net/ for |
| more information about PyUnit. |
| |
| * The :mod:`difflib` module contains a class, :class:`SequenceMatcher`, which |
| compares two sequences and computes the changes required to transform one |
| sequence into the other. For example, this module can be used to write a tool |
| similar to the Unix :program:`diff` program, and in fact the sample program |
| :file:`Tools/scripts/ndiff.py` demonstrates how to write such a script. |
| |
| * :mod:`curses.panel`, a wrapper for the panel library, part of ncurses and of |
| SYSV curses, was contributed by Thomas Gellekum. The panel library provides |
| windows with the additional feature of depth. Windows can be moved higher or |
| lower in the depth ordering, and the panel library figures out where panels |
| overlap and which sections are visible. |
| |
| * The PyXML package has gone through a few releases since Python 2.0, and Python |
| 2.1 includes an updated version of the :mod:`xml` package. Some of the |
| noteworthy changes include support for Expat 1.2 and later versions, the ability |
| for Expat parsers to handle files in any encoding supported by Python, and |
| various bugfixes for SAX, DOM, and the :mod:`minidom` module. |
| |
| * Ping also contributed another hook for handling uncaught exceptions. |
| :func:`sys.excepthook` can be set to a callable object. When an exception isn't |
| caught by any :keyword:`try`...\ :keyword:`except` blocks, the exception will be |
| passed to :func:`sys.excepthook`, which can then do whatever it likes. At the |
| Ninth Python Conference, Ping demonstrated an application for this hook: |
| printing an extended traceback that not only lists the stack frames, but also |
| lists the function arguments and the local variables for each frame. |
| |
| * Various functions in the :mod:`time` module, such as :func:`asctime` and |
| :func:`localtime`, require a floating point argument containing the time in |
| seconds since the epoch. The most common use of these functions is to work with |
| the current time, so the floating point argument has been made optional; when a |
| value isn't provided, the current time will be used. For example, log file |
| entries usually need a string containing the current time; in Python 2.1, |
| ``time.asctime()`` can be used, instead of the lengthier |
| ``time.asctime(time.localtime(time.time()))`` that was previously required. |
| |
| This change was proposed and implemented by Thomas Wouters. |
| |
| * The :mod:`ftplib` module now defaults to retrieving files in passive mode, |
| because passive mode is more likely to work from behind a firewall. This |
| request came from the Debian bug tracking system, since other Debian packages |
| use :mod:`ftplib` to retrieve files and then don't work from behind a firewall. |
| It's deemed unlikely that this will cause problems for anyone, because Netscape |
| defaults to passive mode and few people complain, but if passive mode is |
| unsuitable for your application or network setup, call ``set_pasv(0)`` on |
| FTP objects to disable passive mode. |
| |
| * Support for raw socket access has been added to the :mod:`socket` module, |
| contributed by Grant Edwards. |
| |
| * The :mod:`pstats` module now contains a simple interactive statistics browser |
| for displaying timing profiles for Python programs, invoked when the module is |
| run as a script. Contributed by Eric S. Raymond. |
| |
| * A new implementation-dependent function, ``sys._getframe([depth])``, has |
| been added to return a given frame object from the current call stack. |
| :func:`sys._getframe` returns the frame at the top of the call stack; if the |
| optional integer argument *depth* is supplied, the function returns the frame |
| that is *depth* calls below the top of the stack. For example, |
| ``sys._getframe(1)`` returns the caller's frame object. |
| |
| This function is only present in CPython, not in Jython or the .NET |
| implementation. Use it for debugging, and resist the temptation to put it into |
| production code. |
| |
| .. ====================================================================== |
| |
| |
| Other Changes and Fixes |
| ======================= |
| |
| There were relatively few smaller changes made in Python 2.1 due to the shorter |
| release cycle. A search through the CVS change logs turns up 117 patches |
| applied, and 136 bugs fixed; both figures are likely to be underestimates. Some |
| of the more notable changes are: |
| |
| * A specialized object allocator is now optionally available, that should be |
| faster than the system :func:`malloc` and have less memory overhead. The |
| allocator uses C's :func:`malloc` function to get large pools of memory, and |
| then fulfills smaller memory requests from these pools. It can be enabled by |
| providing the :option:`--with-pymalloc` option to the :program:`configure` |
| script; see :file:`Objects/obmalloc.c` for the implementation details. |
| |
| Authors of C extension modules should test their code with the object allocator |
| enabled, because some incorrect code may break, causing core dumps at runtime. |
| There are a bunch of memory allocation functions in Python's C API that have |
| previously been just aliases for the C library's :func:`malloc` and |
| :func:`free`, meaning that if you accidentally called mismatched functions, the |
| error wouldn't be noticeable. When the object allocator is enabled, these |
| functions aren't aliases of :func:`malloc` and :func:`free` any more, and |
| calling the wrong function to free memory will get you a core dump. For |
| example, if memory was allocated using :func:`PyMem_New`, it has to be freed |
| using :func:`PyMem_Del`, not :func:`free`. A few modules included with Python |
| fell afoul of this and had to be fixed; doubtless there are more third-party |
| modules that will have the same problem. |
| |
| The object allocator was contributed by Vladimir Marangozov. |
| |
| * The speed of line-oriented file I/O has been improved because people often |
| complain about its lack of speed, and because it's often been used as a naïve |
| benchmark. The :meth:`readline` method of file objects has therefore been |
| rewritten to be much faster. The exact amount of the speedup will vary from |
| platform to platform depending on how slow the C library's :func:`getc` was, but |
| is around 66%, and potentially much faster on some particular operating systems. |
| Tim Peters did much of the benchmarking and coding for this change, motivated by |
| a discussion in comp.lang.python. |
| |
| A new module and method for file objects was also added, contributed by Jeff |
| Epler. The new method, :meth:`xreadlines`, is similar to the existing |
| :func:`xrange` built-in. :func:`xreadlines` returns an opaque sequence object |
| that only supports being iterated over, reading a line on every iteration but |
| not reading the entire file into memory as the existing :meth:`readlines` method |
| does. You'd use it like this:: |
| |
| for line in sys.stdin.xreadlines(): |
| # ... do something for each line ... |
| ... |
| |
| For a fuller discussion of the line I/O changes, see the python-dev summary for |
| January 1-15, 2001 at https://www.python.org/dev/summary/2001-01-1/. |
| |
| * A new method, :meth:`popitem`, was added to dictionaries to enable |
| destructively iterating through the contents of a dictionary; this can be faster |
| for large dictionaries because there's no need to construct a list containing |
| all the keys or values. ``D.popitem()`` removes a random ``(key, value)`` pair |
| from the dictionary ``D`` and returns it as a 2-tuple. This was implemented |
| mostly by Tim Peters and Guido van Rossum, after a suggestion and preliminary |
| patch by Moshe Zadka. |
| |
| * Modules can now control which names are imported when ``from module import *`` |
| is used, by defining an ``__all__`` attribute containing a list of names that |
| will be imported. One common complaint is that if the module imports other |
| modules such as :mod:`sys` or :mod:`string`, ``from module import *`` will add |
| them to the importing module's namespace. To fix this, simply list the public |
| names in ``__all__``:: |
| |
| # List public names |
| __all__ = ['Database', 'open'] |
| |
| A stricter version of this patch was first suggested and implemented by Ben |
| Wolfson, but after some python-dev discussion, a weaker final version was |
| checked in. |
| |
| * Applying :func:`repr` to strings previously used octal escapes for |
| non-printable characters; for example, a newline was ``'\012'``. This was a |
| vestigial trace of Python's C ancestry, but today octal is of very little |
| practical use. Ka-Ping Yee suggested using hex escapes instead of octal ones, |
| and using the ``\n``, ``\t``, ``\r`` escapes for the appropriate characters, |
| and implemented this new formatting. |
| |
| * Syntax errors detected at compile-time can now raise exceptions containing the |
| filename and line number of the error, a pleasant side effect of the compiler |
| reorganization done by Jeremy Hylton. |
| |
| * C extensions which import other modules have been changed to use |
| :func:`PyImport_ImportModule`, which means that they will use any import hooks |
| that have been installed. This is also encouraged for third-party extensions |
| that need to import some other module from C code. |
| |
| * The size of the Unicode character database was shrunk by another 340K thanks |
| to Fredrik Lundh. |
| |
| * Some new ports were contributed: MacOS X (by Steven Majewski), Cygwin (by |
| Jason Tishler); RISCOS (by Dietmar Schwertberger); Unixware 7 (by Billy G. |
| Allie). |
| |
| And there's the usual list of minor bugfixes, minor memory leaks, docstring |
| edits, and other tweaks, too lengthy to be worth itemizing; see the CVS logs for |
| the full details if you want them. |
| |
| .. ====================================================================== |
| |
| |
| Acknowledgements |
| ================ |
| |
| The author would like to thank the following people for offering suggestions on |
| various drafts of this article: Graeme Cross, David Goodger, Jay Graves, Michael |
| Hudson, Marc-André Lemburg, Fredrik Lundh, Neil Schemenauer, Thomas Wouters. |
| |