**************************** What's New In Python 3.4 **************************** .. :Author: Someone (uncomment if there is a principal author) .. Rules for maintenance: * Anyone can add text to this document, but the maintainer reserves the right to rewrite any additions. In particular, for obscure or esoteric features, the maintainer may reduce any addition to a simple reference to the new documentation rather than explaining the feature inline. * While the maintainer will periodically go through Misc/NEWS and add changes, it's best not to rely on this. We know from experience that any changes that aren't in the What's New documentation around the time of the original release will remain largely unknown to the community for years, even if they're added later. We also know from experience that other priorities can arise, and the maintainer will run out of time to do updates - in such cases, end users will be much better served by partial notifications that at least give a hint about new features to investigate. * This is not a complete list of every single change; completeness is the purpose of Misc/NEWS. The What's New should focus on changes that are visible to Python *users* and that *require* a feature release (i.e. most bug fixes should only be recorded in Misc/NEWS) * PEPs should not be marked Final until they have an entry in What's New. A placeholder entry that is just a section header and a link to the PEP (e.g ":pep:`397` has been implemented") is acceptable. If a PEP has been implemented and noted in What's New, don't forget to mark it as Final! * If you want to draw your new text to the attention of the maintainer, add 'XXX' to the beginning of the paragraph or section. * It's OK to add just a very brief note about a change. For example: "The :ref:`~socket.transmogrify()` function was added to the :mod:`socket` module." The maintainer will research the change and write the necessary text (if appropriate). The advantage of doing this is that even if no more descriptive text is ever added, readers will at least have a notification that the new feature exists and a link to the relevant documentation. * You can comment out your additions if you like, but it's not necessary (especially when a final release is some months away). * Credit the author of a patch or bugfix. Just the name is sufficient; the e-mail address isn't necessary. * It's helpful to add the bug/patch number as a comment: The :ref:`~socket.transmogrify()` function was added to the :mod:`socket` module. (Contributed by P.Y. Developer in :issue:`12345`.) This saves the maintainer the effort of going through the Mercurial log when researching a change. * Cross referencing tip: :ref:`mod.attr` will display as ``mod.attr``, while :ref:`~mod.attr` will display as ``attr``. This article explains the new features in Python 3.4, compared to 3.3. .. Python 3.4 was released on TBD. 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. .. seealso:: :pep:`429` - Python 3.4 Release Schedule Summary -- Release highlights ============================= .. This section singles out the most important changes in Python 3.4. Brevity is key. New syntax features: * No new syntax features are planned for Python 3.4. New library modules: * :mod:`asyncio`: New provisonal API for asynchronous IO (:pep:`3156`). * :mod:`enum`: Support for enumeration types (:pep:`435`). * :mod:`selectors`: High-level and efficient I/O multiplexing, built upon the :mod:`select` module primitives. * :mod:`statistics`: A basic numerically stable statistics library (:pep:`450`). New expected features for Python implementations: * :ref:`PEP 446: Make newly created file descriptors non-inheritable `. * command line option for :ref:`isolated mode `, (:issue:`16499`). Significantly Improved Library Modules: * Single-dispatch generic functions in :mod:`functoools` (:pep:`443`) * SHA-3 (Keccak) support for :mod:`hashlib`. * TLSv1.1 and TLSv1.2 support for :mod:`ssl`. * :mod:`multiprocessing` now has option to avoid using :func:`os.fork` on Unix (:issue:`8713`). CPython implementation improvements: * :ref:`PEP 442: Safe object finalization ` * :ref:`PEP 445: Configurable memory allocators ` * Improve finalization of Python modules to avoid setting their globals to None, in most cases (:issue:`18214`). * A more efficient :mod:`marshal` format (:issue:`16475`). * "Argument Clinic", an initial step towards providing improved introspection support for builtin and standard library extension types implemented in C (:pep:`436`) Please read on for a comprehensive list of user-facing changes. .. _pep-446: PEP 446: Make newly created file descriptors non-inheritable ============================================================ :pep:`446` makes newly created file descriptors :ref:`non-inheritable `. New functions and methods: * :func:`os.get_inheritable`, :func:`os.set_inheritable` * :func:`os.get_handle_inheritable`, :func:`os.set_handle_inheritable` * :meth:`socket.socket.get_inheritable`, :meth:`socket.socket.set_inheritable` .. seealso:: :pep:`446` - Make newly created file descriptors non-inheritable PEP written and implemented by Victor Stinner. Other Language Changes ====================== Some smaller changes made to the core Python language are: * Unicode database updated to UCD version 6.3. * :func:`min` and :func:`max` now accept a *default* argument that can be used to specify the value they return if the iterable they are evaluating has no elements. Contributed by Julian Berman in :issue:`18111`. * Module objects are now :mod:`weakref`'able. * Module ``__file__`` attributes (and related values) should now always contain absolute paths by default, with the sole exception of ``__main__.__file__`` when a script has been executed directly using a relative path (Contributed by Brett Cannon in :issue:`18416`). New Modules =========== asyncio ------- The new :mod:`asyncio` module (defined in :pep:`3156`) provides a standard pluggable event loop model for Python, providing solid asynchronous IO support in the standard library, and making it easier for other event loop implementations to interoperate with the standard library and each other. For Python 3.4, this module is considered a :term:`provisional API`. .. seealso:: :pep:`3156` - Asynchronous IO Support Rebooted: the "asyncio" Module PEP written and implementation led by Guido van Rossum. enum ---- The new :mod:`enum` module (defined in :pep:`435`) provides a standard implementation of enumeration types, allowing other modules (such as :mod:`socket`) to provide more informative error messages and better debugging support by replacing opaque integer constants with backwards compatible enumeration values. .. seealso:: :pep:`435` - Adding an Enum type to the Python standard library PEP written by Barry Warsaw, Eli Bendersky and Ethan Furman, implemented by Ethan Furman. selectors --------- The new :mod:`selectors` module (created as part of implementing :pep:`3156`) allows high-level and efficient I/O multiplexing, built upon the :mod:`select` module primitives. statistics ---------- The new :mod:`statistics` module (defined in :pep:`450`) offers some core statistics functionality directly in the standard library. This module supports calculation of the mean, median, mode, variance and standard deviation of a data series. .. seealso:: :pep:`450` - Adding A Statistics Module To The Standard Library PEP written and implemented by Steven D'Aprano Improved Modules ================ aifc ---- The :meth:`~aifc.getparams` method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in :issue:`17818`.) audioop ------- Added support for 24-bit samples (:issue:`12866`). codecs ------ The :meth:`codecs.encode` and :meth:`codecs.decode` convenience functions are now properly documented. These functions have existed in the :mod:`codecs` module since ~2004, but were previously only discoverable through runtime introspection. Unlike the convenience methods on :class:`str`, :class:`bytes` and :class:`bytearray`, these convenience functions support arbitrary codecs, rather than being limited to Unicode text encodings. colorsys -------- 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. contextlib ---------- The new :class:`contextlib.suppress` context manager helps to clarify the intent of code that deliberately suppresses exceptions from a single statement. (Contributed by Raymond Hettinger in :issue:`15806` and Zero Piraeus in :issue:`19266`) The new :func:`contextlib.redirect_stdout` context manager makes it easier for utility scripts to handle inflexible APIs that don't provide any options to retrieve their output as a string or direct it to somewhere other than :data:`sys.stdout`. In conjunction with :class:`io.StringIO`, this context manager is also useful for checking expected output from command line utilities. (Contribute by Raymond Hettinger in :issue:`15805`) The :mod:`contextlib` documentation has also been updated to include a :ref:`discussion ` of the differences between single use, reusable and reentrant context managers. dis --- The :mod:`dis` module is now built around an :class:`~dis.Instruction` class that provides details of individual bytecode operations and a :func:`~dis.get_instructions` iterator that emits the Instruction stream for a given piece of Python code. The various display tools in the :mod:`dis` module have been updated to be based on these new components. The new :class:`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`) doctest ------- Added :data:`~doctest.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 :mod:`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 :mod:`unittest` CLI). (Contributed by R. David Murray in :issue:`11390`.) email ----- :meth:`~email.message.Message.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 :mod:`~email.generator` in order to pass formatting parameters to its ``flatten`` method. New method :meth:`~email.message.Message.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 :class:`~email.message.Message` :meth:`~email.message.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`.) A pair of new subclasses of :class:`~email.message.Message` have been added, along with a new sub-module, :mod:`~email.contentmanager`. All documentation is currently in the new module, which is being added as part of the new :term:`provisional ` email API. These classes provide a number of new methods that make extracting content from and inserting content into email messages much easier. See the :mod:`~email.contentmanager` documentation for details. These API additions complete the bulk of the work that was planned as part of the email6 project. The currently provisional API is scheduled to become final in Python 3.5 (possibly with a few minor additions in the area of error handling). (Contributed by R. David Murray in :issue:`18891`.) functools --------- New :func:`functools.singledispatch` decorator: see the :pep:`443`. hashlib ------- New :func:`hashlib.pbkdf2_hmac` function. (Contributed by Christian Heimes in :issue:`18582`) inspect ------- The inspect module now offers a basic :ref:`command line interface ` to quickly display source code and other information for modules, classes and functions. (Contributed by Claudiu Popa and Nick Coghlan in :issue:`18626`) :func:`~inspect.unwrap` makes it easy to unravel wrapper function chains created by :func:`functools.wraps` (and any other API that sets the ``__wrapped__`` attribute on a wrapper function). (Contributed by Daniel Urban, Aaron Iles and Nick Coghlan in :issue:`13266`) As part of the implementation of the new :mod:`enum` module, the :mod:`inspect` module now has substantially better support for custom ``__dir__`` methods and dynamic class attributes provided through metaclasses (Contributed by Ethan Furman in :issue:`18929` and :issue:`19030`) mmap ---- mmap objects can now be weakref'ed. (Contributed by Valerie Lambert in :issue:`4885`.) multiprocessing --------------- On Unix two new *start methods* have been added for starting processes using :mod:`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. os -- New functions to get and set the :ref:`inheritable flag ` of a file descriptors or a Windows handle: * :func:`os.get_inheritable`, :func:`os.set_inheritable` * :func:`os.get_handle_inheritable`, :func:`os.set_handle_inheritable` pdb --- The ``print`` command has been removed from :mod:`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 :pdbcmd:`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`.) poplib ------ New :meth:`~poplib.POP3.stls` method to switch a clear-text POP3 session into an encrypted POP3 session. New :meth:`~poplib.POP3.capa` method to query the capabilities advertised by the POP3 server. (Contributed by Lorenzo Catucci in :issue:`4473`.) pprint ------ The :mod:`pprint` module now supports *compact* mode for formatting long sequences (:issue:`19132`). pydoc ----- While significant changes have not been made to :mod:`pydoc` directly, its handling of custom ``__dir__`` methods and various descriptor behaviours has been improved substantially by the underlying changes in the :mod:`inspect` module. resource -------- New :func:`resource.prlimit` function and Linux specific constants. (Contributed by Christian Heimes in :issue:`16595` and :issue:`19324`.) smtplib ------- :exc:`~smtplib.SMTPException` is now a subclass of :exc:`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 ------ Socket objects have new methods to get or set their :ref:`inheritable flag `: * :meth:`socket.socket.get_inheritable`, :meth:`socket.socket.set_inheritable` The ``socket.AF_*`` and ``socket.SOCK_*`` constants are enumeration values, using the new :mod:`enum` module. This allows descriptive reporting during debugging, instead of seeing integer "magic numbers". ssl --- TLSv1.1 and TLSv1.2 support. (Contributed by Michele Orrù and Antoine Pitrou in :issue:`16692`) * New diagnostic functions :func:`~ssl.get_default_verify_paths`, :meth:`~ssl.SSLContext.cert_store_stats` and :meth:`~ssl.SSLContext.get_ca_certs` * Add :func:`ssl.enum_cert_store` to retrieve certificates and CRL from Windows' cert store. (Contributed by Christian Heimes in :issue:`18143`, :issue:`18147` and :issue:`17134`.) Support for server-side SNI using the new :meth:`ssl.SSLContext.set_servername_callback` method. (Contributed by Daniel Black in :issue:`8109`.) stat ---- The :mod:`stat` module is now backed by a C implementation in :mod:`_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. struct ------ Streaming struct unpacking using :func:`struct.iter_unpack`. (Contributed by Antoine Pitrou in :issue:`17804`.) sunau ----- The :meth:`~sunau.getparams` method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in :issue:`18901`.) :meth:`sunau.open` now supports the context manager protocol (:issue:`18878`). traceback --------- A new :func:`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`). urllib ------ Add support.for ``data:`` URLs in :mod:`urllib.request`. (Contributed by Mathias Panzenböck in :issue:`16423`.) unittest -------- Support for easy dynamically-generated subtests using the :meth:`~unittest.TestCase.subTest` context manager. (Contributed by Antoine Pitrou in :issue:`16997`.) wave ---- The :meth:`~wave.getparams` method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in :issue:`17487`.) :meth:`wave.open` now supports the context manager protocol. (Contributed by Claudiu Popa in :issue:`17616`.) weakref ------- New :class:`~weakref.WeakMethod` class simulates weak references to bound methods. (Contributed by Antoine Pitrou in :issue:`14631`.) New :class:`~weakref.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`) xml.etree --------- Add an event-driven parser for non-blocking applications, :class:`~xml.etree.ElementTree.XMLPullParser`. (Contributed by Antoine Pitrou in :issue:`17741`.) zipfile.PyZipfile ----------------- Add a filter function to ignore some packages (tests for instance), :meth:`~zipfile.PyZipFile.writepy`. (Contributed by Christian Tismer in :issue:`19274`.) Other improvements ================== Tab-completion is now enabled by default in the interactive interpreter. (Contributed by Antoine Pitrou and Éric Araujo in :issue:`5845`.) Python invocation changes ========================= Invoking the Python interpreter with ``--version`` now outputs the version to standard output instead of standard error (:issue:`18338`). Similar changes were made to :mod:`argparse` (:issue:`18920`) and other modules that have script-like invocation capabilities (:issue:`18922`). Optimizations ============= 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`.) * The interpreter starts about 30% faster. A couple of measures lead to the speedup. The interpreter loads fewer modules on startup, e.g. the :mod:`re`, :mod:`collections` and :mod:`locale` modules and their dependencies are no longer imported by default. The marshal module has been improved to load compiled Python code faster. (Contributed by Antoine Pitrou, Christian Heimes and Victor Stinner in :issue:`19219`, :issue:`19218`, :issue:`19209`, :issue:`19205` and :issue:`9548`) CPython Implementation Changes ============================== .. _pep-445: PEP 445: Customization of CPython memory allocators --------------------------------------------------- :pep:`445` adds new C level interfaces to customize memory allocation in the CPython interpreter. .. seealso:: :pep:`445` - Add new APIs to customize Python memory allocators PEP written and implemented by Victor Stinner. .. _pep-442: PEP 442: Safe object finalization --------------------------------- :pep:`442` removes the current limitations and quirks of object finalization in CPython. With it, objects with :meth:`__del__` methods, as well as generators with :keyword:`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 :const:`None` during interpreter shutdown in most cases, instead relying on the normal operation of the cyclic garbage collector. .. seealso:: :pep:`442` - Safe object finalization PEP written and implemented by Antoine Pitrou. Other build and C API changes ----------------------------- Changes to Python's build process and to the C API include: * The new :c:func:`Py_SetStandardStreamEncoding` pre-initialization API allows applications embedding the CPython interpreter to reliably force a particular encoding and error handler for the standard streams (Contributed by Bastien Montagne and Nick Coghlan in :issue:`16129`) * Most Python C APIs that don't mutate string arguments are now correctly marked as accepting ``const char *`` rather than ``char *`` (Contributed by Serhiy Storchaka in :issue:`1772673`). * "Argument Clinic" (:pep:`436`) is now part of the CPython build process and can be used to simplify the process of defining and maintaining accurate signatures for builtins and standard library extension modules implemented in C. .. note:: The Argument Clinic PEP is not fully up to date with the state of the implementation. This has been deemed acceptable by the release manager and core development team in this case, as Argument Clinic will not be made available as a public API for third party use in Python 3.4. Deprecated ========== Unsupported Operating Systems ----------------------------- * OS/2 * Windows 2000 Deprecated Python modules, functions and methods ------------------------------------------------ * :meth:`difflib.SequenceMatcher.isbjunk` and :meth:`difflib.SequenceMatcher.isbpopular` were removed: use ``x in sm.bjunk`` and ``x in sm.bpopular``, where *sm* is a :class:`~difflib.SequenceMatcher` object. * :func:`importlib.util.module_for_loader` is pending deprecation. Using :func:`importlib.util.module_to_load` and :meth:`importlib.abc.Loader.init_module_attrs` allows subclasses of a loader to more easily customize module loading. * The :mod:`imp` module is pending deprecation. To keep compatibility with Python 2/3 code bases, the module's removal is currently not scheduled. * The :mod:`formatter` module is pending deprecation and is slated for removal in Python 3.6. Deprecated functions and types of the C API ------------------------------------------- * The ``PyThreadState.tick_counter`` field has been removed: its value was meaningless since Python 3.2 ("new GIL"). Deprecated features ------------------- * The site module adding a "site-python" directory to sys.path, if it exists, is deprecated (:issue:`19375`). Porting to Python 3.4 ===================== This section lists previously described changes and other bugfixes that may require changes to your code. * The ABCs defined in :mod:`importlib.abc` now either raise the appropriate exception or return a default value instead of raising :exc:`NotImplementedError` blindly. This will only affect code calling :func:`super` and falling through all the way to the ABCs. For compatibility, catch both :exc:`NotImplementedError` or the appropriate exception as needed. * The module type now initializes the :attr:`__package__` and :attr:`__loader__` attributes to ``None`` by default. To determine if these attributes were set in a backwards-compatible fashion, use e.g. ``getattr(module, '__loader__', None) is not None``. * :meth:`importlib.util.module_for_loader` now sets ``__loader__`` and ``__package__`` unconditionally to properly support reloading. If this is not desired then you will need to set these attributes manually. You can use :func:`importlib.util.module_to_load` for module management. * Import now resets relevant attributes (e.g. ``__name__``, ``__loader__``, ``__package__``, ``__file__``, ``__cached__``) unconditionally when reloading. * Frozen packages no longer set ``__path__`` to a list containing the package name but an empty list instead. Determing if a module is a package should be done using ``hasattr(module, '__path__')``. * :c:func:`PyErr_SetImportError` now sets :exc:`TypeError` when its **msg** argument is not set. Previously only ``NULL`` was returned with no exception set. * :func:`py_compile.compile` now raises :exc:`FileExistsError` if the file path it would write to is a symlink or a non-regular file. This is to act as a warning that import will overwrite those files with a regular file regardless of what type of file path they were originally. * :meth:`importlib.abc.SourceLoader.get_source` no longer raises :exc:`ImportError` when the source code being loaded triggers a :exc:`SyntaxError` or :exc:`UnicodeDecodeError`. As :exc:`ImportError` is meant to be raised only when source code cannot be found but it should, it was felt to be over-reaching/overloading of that meaning when the source code is found but improperly structured. If you were catching ImportError before and wish to continue to ignore syntax or decoding issues, catch all three exceptions now. * :func:`functools.update_wrapper` and :func:`functools.wraps` now correctly set the ``__wrapped__`` attribute to the function being wrapper, even if that function also had its ``__wrapped__`` attribute set. This means ``__wrapped__`` attributes now correctly link a stack of decorated functions rather than every ``__wrapped__`` attribute in the chain referring to the innermost function. Introspection libraries that assumed the previous behaviour was intentional can use :func:`inspect.unwrap` to access the first function in the chain that has no ``__wrapped__`` attribute. * (C API) The result of the :c:data:`PyOS_ReadlineFunctionPointer` callback must now be a string allocated by :c:func:`PyMem_RawMalloc` or :c:func:`PyMem_RawRealloc`, or *NULL* if an error occurred, instead of a string allocated by :c:func:`PyMem_Malloc` or :c:func:`PyMem_Realloc`. * :class:`importlib.machinery.PathFinder` now passes on the current working directory to objects in :data:`sys.path_hooks` for the empty string. This results in :data:`sys.path_importer_cache` never containing ``''``, thus iterating through :data:`sys.path_importer_cache` based on :data:`sys.path` will not find all keys. A module's ``__file__`` when imported in the current working directory will also now have an absolute path, including when using ``-m`` with the interpreter (this does not influence when the path to a file is specified on the command-line). * :class:`importlib.machinery.FileFinder` no longer special-cases the empty string to be changed to ``'.'``.