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@@ -1,18 +1,274 @@ -What's new in this release? +What's New in Python 2.0b1? =========================== Below is a list of all relevant changes since release 1.6. Older -changes are in the file HISTORY. The most recent changes are listed -first. +changes are in the file HISTORY. If you are making the jump directly +from Python 1.5.2 to 2.0, make sure to read the section for 1.6 in the +HISTORY file! Many important changes listed there. -A note on attributions: while I have sprinkled some names throughout -here, I'm grateful to many more people who remain unnamed. You may -find your name in the ACKS file. If you believe you deserve more -credit, let me know and I'll add you to the list! +Alternatively, a good overview of the changes between 1.5.2 and 2.0 is +the document "What's New in Python 2.0" by Kuchling and Moshe Zadka: +http://starship.python.net/crew/amk/python/writing/new-python/. +--Guido van Rossum (home page: http://www.pythonlabs.com/~guido/) ====================================================================== -<To be done> +Source Incompatibilities +------------------------ + +None. Note that 1.6 introduced several incompatibilities with 1.5.2, +such as single-argument append(), connect() and bind(), and changes to +str(long) and repr(float). + + +Binary Incompatibilities +------------------------ + +- Third party extensions built for Python 1.5.x or 1.6 cannot be used +with Python 2.0; these extensions will have to be rebuilt for Python +2.0. + +- On Windows, attempting to import a third party extension built for +Python 1.5.x or 1.6 results in an immediate crash; there's not much we +can do about this. Check your PYTHONPATH environment variable! + +- Python bytecode files (*.pyc and *.pyo) are not compatible between +releases. + + +Overview of Changes Since 1.6 +----------------------------- + +There are many new modules (including brand new XML support through +the xml package, and i18n support through the gettext module); a list +of all new modules is included below. Lots of bugs have been fixed. + +There are several important syntax enhancements, described in more +detail below: + + - Augmented assignment, e.g. x += 1 + + - List comprehensions, e.g. [x**2 for x in range(10)] + + - Extended import statement, e.g. import Module as Name + + - Extended print statement, e.g. print >> file, "Hello" + +Other important changes: + + - Optional collection of cyclical garbage + + +Augmented Assignment +-------------------- + +This must have been the most-requested feature of the past years! +Eleven new assignment operators were added: + + += -+ *= /= %= **= <<= >>= &= ^= |= + +For example, + + A += B + +is similar to + + A = A + B + +except that A is evaluated only once (relevant when A is something +like dict[index].attr). + +However, if A is a mutable object, A may be modified in place. Thus, +if A is a number or a string, A += B has the same effect as A = A+B +(except A is only evaluated once); but if a is a list, A += B has the +same effect as A.extend(B)! + +Classes and built-in object types can override the new operators in +order to implement the in-place behavior; the not-in-place behavior is +used automatically as a fallback when an object doesn't implement the +in-place behavior. For classes, the method name is derived from the +method name for the corresponding not-in-place operator by inserting +an 'i' in front of the name, e.g. __iadd__ implements in-place +__add__. + +Augmented assignment was implemented by Thomas Wouters. + + +List Comprehensions +------------------- + +This is a flexible new notation for lists whose elements are computed +from another list (or lists). The simplest form is: + + [<expression> for <variable> in <sequence>] + +For example, [x**2 for i in range(4)] yields the list [0, 1, 4, 9]. +This is more efficient than map() with a lambda. + +You can also add a condition: + + [<expression> for <variable> in <sequence> if <condition>] + +For example, [w for w in words if w == w.lower()] would yield the list +of words that contain no uppercase characters. This is more efficient +than filter() with a lambda. + +You can also have nested for loops and more than one 'if' clause. For +example, here's a function that flattens a sequence of sequences:: + + def flatten(seq): + return [x for subseq in seq for x in subseq] + + flatten([[0], [1,2,3], [4,5], [6,7,8,9], []]) + +This prints + + [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] + +List comprehensions originated as a patch set from Greg Ewing; Skip +Montanaro and Thomas Wouters also contributed. + + +Extended Import Statement +------------------------- + +Many people have asked for a way to import a module under a different +name. This can be accomplished like this: + + import foo + bar = foo + del foo + +but this common idiom gets old quickly. A simple extension of the +import statement now allows this to be written as follows: + + import foo as bar + +There's also a variant for 'from ... import': + + from foo import bar as spam + +This also works with packages; e.g. you can write this: + + import test.regrtest as regrtest + +Note that 'as' is not a new keyword -- it is recognized only in this +context (this is only possible because the syntax for the import +statement doesn't involve expressions). + +Implemented by Thomas Wouters. + + +Extended Print Statement +------------------------ + +Easily the most controversial new feature, this extension to the print +statement adds an option to make the output go to a different file +than the default sys.stdout. + +For example, to write an error message to sys.stderr, you can now +write: + + print >> sys.stderr, "Error: bad dog!" + +As a special feature, if the expression used to indicate the file +evaluates to None, the current value of sys.stdout used. Thus: + + print >> None, "Hello world" + +is equivalent to + + print "Hello world" + +Design and implementation by Barry Warsaw. + + +Optional Collection of Cyclical Garbage +--------------------------------------- + +Python is now equipped with a garbage collector that can hunt down +cyclical references between Python objects. It's no replacement for +reference counting; in fact, it depends on the reference counts being +correct, and decides that a set of objects belong to a cycle if all +their reference counts can be accounted for from their references to +each other. This devious scheme was first proposed by Eric Tiedemann, +and brought to implementation by Neil Schemenauer. + +There's a module "gc" that lets you control some parameters of the +garbage collection. There's also an option to the configure script +that lets you enable or disable the garbage collection. In 2.0b1, +it's on by default, so that we (hopefully) can collect decent user +experience with this new feature. There are some questions about its +performance. if it proves to be too much of a problem, we'll turn it +off by default in the final 2.0 release. + + +Smaller Changes +--------------- + +A new function zip() was added. zip(seq1, seq2, ...) is equivalent to +map(None, seq1, seq2, ...) when the sequences have the same length; +i.e. zip([1,2,3], [10,20,30]) returns [(1,10), (2,20), (3,30)]. When +the lists are not all the same length, the shortest list wins: +zip([1,2,3], [10,20]) returns [(1,10), (2,20)]. + +sys.version_info is a tuple (major, minor, micro, level, serial). + +Dictionaries have an odd new method, setdefault(key, default). +dict.setdefault(key, default) returns dict[key] if it exists; if not, +it sets dict[key] to default and returns that value. Thus: + + dict.setdefault(key, []).append(item) + +does the same work as this common idiom: + + if not dict.has_key(key): + dict[key] = [] + dict[key].append(item) + + +New Modules and Packages +------------------------ + +atexit - for registering functions to be called when Python exits. + +imputil - Greg Stein's alternative API for writing custom import +hooks. + +pyexpat - an interface to the Expat XML parser, contributed by Paul +Prescod. + +xml - a new package with XML support code organized (so far) in three +subpackages: xml.dom, xml.sax, and xml.parsers. Describing these +would fill a volume. There's a special feature whereby a +user-installed package named _xmlplus overrides the standard +xmlpackage; this is intended to give the XML SIG a hook to distribute +backwards-compatible updates to the standard xml package. + +webbrowser - a platform-independent API to launch a web browser. + + +C-level Changes +--------------- + +Several cleanup jobs were carried out throughout the source code. + +All C code was converted to ANSI C; we got rid of all uses of the +Py_PROTO() macro, which makes the header files a lot more readable. + +Most of the portability hacks were moved to a new header file, +pyport.h; several other new header files were added and some old +header files were removed, in an attempt to create a more rational set +of header files. (Few of these ever need to be included explicitly; +they are all included by Python.h.) + +Vladimir Marangozov redesigned more rational APIs for allocating +memory. See pymem.h. + +Trent Mick ensured portability to 64-bit platforms, under both Linux +and Win64, especially for the new Intel Itanium processor. + +Numerous new APIs were added, e.g. ====================================================================== |