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. 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. 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/) ====================================================================== 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: [ for in ] 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: [ for in if ] 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. Changed Modules --------------- ftplib - ntransfercmd(), transfercmd(), and retrbinary() all now optionally support the RFC 959 REST command. socket - new function getfqdn() readline - new functions to read, write and truncate history files. The readline section of the library reference manual contains an example. XXX: I'm sure there are others Obsolete Modules ---------------- None. However note that 1.6 made a whole slew of modules obsolete: stdwin, soundex, cml, cmpcache, dircache, dump, find, grep, packmail, poly, zmod, strop, util, whatsound. Changed, New, Obsolete Tools ---------------------------- XXX: are there any? If not, say "None" here. 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.) 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. XXX: Fill this out. ======================================================================