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- What’s New in Python »
This article explains the new features in Python 3.4, compared to 3.3.
For full details, see the changelog.
Note
Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.4 moves towards release, so it’s worth checking back even after reading earlier versions.
See also
New syntax features:
New library modules:
New built-in features:
Implementation improvements:
Significantly Improved Library Modules:
Security improvements:
Please read on for a comprehensive list of user-facing changes.
The PEP 446 makes newly created file descriptors non-inheritable. New functions and methods:
The PEP 445 adds new Application Programming Interfaces (API) to customize Python memory allocators.
This PEP removes the current limitations and quirks of object finalization. With it, objects with __del__() methods, as well as generators with finally clauses, can be finalized when they are part of a reference cycle.
As part of this change, module globals are no longer forcibly set to None during interpreter shutdown, instead relying on the normal operation of the cyclic garbage collector.
See also
Some smaller changes made to the core Python language are:
The getparams() method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in issue 17818.)
The number of digits in the coefficients for the RGB — YIQ conversions have been expanded so that they match the FCC NTSC versions. The change in results should be less than 1% and may better match results found elsewhere.
The dis module is now built around an Instruction class that provides details of individual bytecode operations and a get_instructions() iterator that emits the Instruction stream for a given piece of Python code. The various display tools in the dis module have been updated to be based on these new components.
The new dis.Bytecode class provides an object-oriented API for inspecting bytecode, both in human-readable form and for iterating over instructions.
(Contributed by Nick Coghlan, Ryan Kelly and Thomas Kluyver in issue 11816)
Added FAIL_FAST flag to halt test running as soon as the first failure is detected. (Contributed by R. David Murray and Daniel Urban in issue 16522.)
Updated the doctest command line interface to use argparse, and added -o and -f options to the interface. -o allows doctest options to be specified on the command line, and -f is a shorthand for -o FAIL_FAST (to parallel the similar option supported by the unittest CLI). (Contributed by R. David Murray in issue 11390.)
as_string() now accepts a policy argument to override the default policy of the message when generating a string representation of it. This means that as_string can now be used in more circumstances, instead of having to create and use a generator in order to pass formatting parameters to its flatten method.
New method as_bytes() added to produce a bytes representation of the message in a fashion similar to how as_string produces a string representation. It does not accept the maxheaderlen argument, but does accept the unixfrom and policy arguments. The Message __bytes__() method calls it, meaning that bytes(mymsg) will now produce the intuitive result: a bytes object containing the fully formatted message.
(Contributed by R. David Murray in issue 18600.)
New functools.singledispatch() decorator: see the PEP 443.
The inspect module now offers a basic command line interface to quickly display source code and other information for modules, classes and functions.
unwrap() makes it easy to unravel wrapper function chains created by functools.wraps() (and any other API that sets the __wrapped__ attribute on a wrapper function).
On Unix two new start methods have been added for starting processes using multiprocessing. These make the mixing of processes with threads more robust. See issue 8713.
Also, except when using the old fork start method, child processes will no longer inherit unneeded handles/file descriptors from their parents.
New functions to get and set the inheritable flag of a file descriptors or a Windows handle:
The print command has been removed from pdb, restoring access to the print function.
Rationale: Python2’s pdb did not have a print command; instead, entering print executed the print statement. In Python3 print was mistakenly made an alias for the pdb p command. p, however, prints the repr of its argument, not the str like the Python2 print command did. Worse, the Python3 pdb print command shadowed the Python3 print function, making it inaccessible at the pdb prompt.
(Contributed by Connor Osborn in issue 18764.)
New stls() method to switch a clear-text POP3 session into an encrypted POP3 session.
New capa() method to query the capabilities advertised by the POP3 server.
(Contributed by Lorenzo Catucci in issue 4473.)
The :mod::pprint module now supports compact mode for formatting long sequences (issue 19132).
SMTPException is now a subclass of OSError, which allows both socket level errors and SMTP protocol level errors to be caught in one try/except statement by code that only cares whether or not an error occurred. (issue 2118).
Socket objects have new methods to get or set their inheritable flag:
The socket.AF_* and socket.SOCK_* constants are enumeration values, using the new enum module. This allows descriptive reporting during debugging, instead of seeing integer “magic numbers”.
TLSv1.1 and TLSv1.2 support.
(Contributed by Michele Orrù and Antoine Pitrou in issue 16692)
Support for server-side SNI using the new ssl.SSLContext.set_servername_callback() method.
(Contributed by Daniel Black in issue 8109.)
The stat module is now backed by a C implementation in _stat. A C implementation is required as most of the values aren’t standardized and platform-dependent. (Contributed by Christian Heimes in issue 11016.)
The module supports new file types: door, event port and whiteout.
Streaming struct unpacking using struct.iter_unpack().
(Contributed by Antoine Pitrou in issue 17804.)
The getparams() method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in issue 18901.)
sunau.open() now supports the context manager protocol (issue 18878).
A new traceback.clear_frames() function takes a traceback object and clears the local variables in all of the frames it references, reducing the amount of memory consumed (issue 1565525).
Add support.for data: URLs in urllib.request.
(Contributed by Mathias Panzenböck in issue 16423.)
Support for easy dynamically-generated subtests using the subTest() context manager.
(Contributed by Antoine Pitrou in issue 16997.)
The getparams() method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in issue 17487.)
wave.open() now supports the context manager protocol. (Contributed by Claudiu Popa in issue 17616.)
New WeakMethod class simulates weak references to bound methods. (Contributed by Antoine Pitrou in issue 14631.)
New finalize class makes it possible to register a callback to be invoked when an object is garbage collected, without needing to carefully manage the lifecycle of the weak reference itself. (Contributed by Richard Oudkerk in issue 15528)
Add an event-driven parser for non-blocking applications, XMLPullParser.
(Contributed by Antoine Pitrou in issue 17741.)
Tab-completion is now enabled by default in the interactive interpreter.
(Contributed by Antoine Pitrou and Éric Araujo in issue 5845.)
Invoking the Python interpreter with --version now outputs the version to standard output instead of standard error (issue 18338). Similar changes were made to argparse (issue 18920) and other modules that have script-like invocation capabilities (issue 18922).
Major performance enhancements have been added:
The UTF-32 decoder is now 3x to 4x faster.
The cost of hash collisions for sets is now reduced. Each hash table probe now checks a series of consecutive, adjacent key/hash pairs before continuing to make random probes through the hash table. This exploits cache locality to make collision resolution less expensive.
The collision resolution scheme can be described as a hybrid of linear probing and open addressing. The number of additional linear probes defaults to nine. This can be changed at compile-time by defining LINEAR_PROBES to be any value. Set LINEAR_PROBES=0 to turn-off linear probing entirely.
(Contributed by Raymond Hettinger in :issue”18771.)
This section lists previously described changes and other bugfixes that may require changes to your code.