What’s New In Python 3.4

This article explains the new features in Python 3.4, compared to 3.3.

For full details, see the changelog.


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

PEP 429 - Python 3.4 Release Schedule

Summary – Release highlights

New syntax features:

  • No new syntax features are planned for Python 3.4.

New library modules:

New expected features for Python implementations:

Significantly Improved Library Modules:

CPython implementation improvements:

Please read on for a comprehensive list of user-facing changes.

PEP 453: Explicit bootstrapping of pip in Python installations

The new ensurepip module (defined in PEP 453) provides a standard cross-platform mechanism to boostrap the pip installer into Python installations and virtual environments.

The venv module and the pyvenv utility make use of this module to make pip readily available in virtual environments. When using the command line interface, pip is installed by default, while for the module API installation of pip must be requested explicitly.

For CPython source builds on POSIX systems, the make install and make altinstall commands bootstrap pip by default. This behaviour can be controlled through configure options, and overridden through Makefile options.

On Windows and Mac OS X, the CPython installers now offer the option to install pip along with CPython itself.


The implementation of PEP 453 is still a work in progress. Refer to issue 19347 for the progress on additional steps:

  • Having the binary installers install pip by default
  • Recommending the use of pip in the “Installing Python Module” documentation.

See also

PEP 453 - Explicit bootstrapping of pip in Python installations
PEP written by Donald Stufft and Nick Coghlan, implemented by Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily.

PEP 446: Make newly created file descriptors non-inheritable

PEP 446 makes newly created file descriptors non-inheritable. New functions and methods:

See also

PEP 446 - Make newly created file descriptors non-inheritable
PEP written and implemented by Victor Stinner.

Improvements to codec handling

Since it was first introduced, the codecs module has always been intended to operate as a type-neutral dynamic encoding and decoding system. However, its close coupling with the Python text model, especially the type restricted convenience methods on the builtin str, bytes and bytearray types, has historically obscured that fact.

As a key step in clarifying the situation, the codecs.encode() and codecs.decode() convenience functions are now properly documented in Python 2.7, 3.3 and 3.4. These functions have existed in the codecs module (and have been covered by the regression test suite) since Python 2.4, but were previously only discoverable through runtime introspection.

Unlike the convenience methods on str, bytes and bytearray, these convenience functions support arbitrary codecs in both Python 2 and Python 3, rather than being limited to Unicode text encodings (in Python 3) or basestring <-> basestring conversions (in Python 2).

In Python 3.4, the interpreter is able to identify the known non-text encodings provided in the standard library and direct users towards these general purpose convenience functions when appropriate:

>>> b"abcdef".decode("hex")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs

>>> "hello".encode("rot13")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
LookupError: 'rot13' is not a text encoding; use codecs.encode() to handle arbitrary codecs

In a related change, whenever it is feasible without breaking backwards compatibility, exceptions raised during encoding and decoding operations will be wrapped in a chained exception of the same type that mentions the name of the codec responsible for producing the error:

>>> import codecs

>>> codecs.decode(b"abcdefgh", "hex")
binascii.Error: Non-hexadecimal digit found

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: decoding with 'hex' codec failed (Error: Non-hexadecimal digit found)

>>> codecs.encode("hello", "bz2")
TypeError: 'str' does not support the buffer interface

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: encoding with 'bz2' codec failed (TypeError: 'str' does not support the buffer interface)

Finally, as the examples above show, these improvements have permitted the restoration of the convenience aliases for the non-Unicode codecs that were themselves restored in Python 3.2. This means that encoding binary data to and from its hexadecimal representation (for example) can now be written as:

>>> from codecs import encode, decode
>>> encode(b"hello", "hex")
>>> decode(b"68656c6c6f", "hex")

The binary and text transforms provided in the standard library are detailed in Binary Transforms and Text Transforms.

(Contributed by Nick Coghlan in issue 7475, , issue 17827, issue 17828 and issue 19619)

PEP 451: A ModuleSpec Type for the Import System

PEP 451 provides an encapsulation of the information about a module that the import machinery will use to load it, (i.e. a module spec). This helps simplify both the import implementation and several import-related APIs. The change is also a stepping stone for several future import-related improvements.


The public-facing changes from the PEP are entirely backward-compatible. Furthermore, they should be transparent to everyone but importer authors. Key finder and loader methods have been deprecated, but they will continue working. New importers should use the new methods described in the PEP. Existing importers should be updated to implement the new methods.

Pickle protocol 4

The new pickle protocol addresses a number of issues that were present in previous protocols, such as the serialization of nested classes, very large strings and containers, or classes whose __new__() method takes keyword-only arguments. It also brings a couple efficiency improvements.

See also

PEP 3154 - Pickle protocol 4
PEP written by Antoine Pitrou and implemented by Alexandre Vassalotti.

Other Language Changes

Some smaller changes made to the core Python language are:

  • Unicode database updated to UCD version 6.3.
  • min() and 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 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).
  • Now all the UTF-* codecs (except UTF-7) reject surrogates during both encoding and decoding unless the surrogatepass error handler is used, with the exception of the UTF-16 decoder that accepts valid surrogate pairs, and the UTF-16 encoder that produces them while encoding non-BMP characters. Contributed by Victor Stinner, Kang-Hao (Kenny) Lu and Serhiy Storchaka in issue 12892.

New Modules


The new 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 provisional API.

See also

PEP 3156 - Asynchronous IO Support Rebooted: the “asyncio” Module
PEP written and implementation led by Guido van Rossum.


The new enum module (defined in PEP 435) provides a standard implementation of enumeration types, allowing other modules (such as socket) to provide more informative error messages and better debugging support by replacing opaque integer constants with backwards compatible enumeration values.

See also

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.


The new pathlib module offers classes representing filesystem paths with semantics appropriate for different operating systems. Path classes are divided between pure paths, which provide purely computational operations without I/O, and concrete paths, which inherit from pure paths but also provide I/O operations.

For Python 3.4, this module is considered a provisional API.

See also

PEP 428 - The pathlib module – object-oriented filesystem paths
PEP written and implemented by Antoine Pitrou.


The new selectors module (created as part of implementing PEP 3156) allows high-level and efficient I/O multiplexing, built upon the select module primitives.


The new 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.

See also

PEP 450 - Adding A Statistics Module To The Standard Library
PEP written and implemented by Steven D’Aprano


The new tracemalloc module (defined in PEP 454) is a debug tool to trace memory blocks allocated by Python. It provides the following information:

  • Traceback where an object was allocated
  • Statistics on allocated memory blocks per filename and per line number: total size, number and average size of allocated memory blocks
  • Compute the differences between two snapshots to detect memory leaks

See also

PEP 454 - Add a new tracemalloc module to trace Python memory allocations
PEP written and implemented by Victor Stinner

Improved Modules


The getparams() method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in issue 17818.)


Added support for 24-bit samples (issue 12866).

Added the byteswap() function to convert big-endian samples to little-endian and vice versa (issue 19641).


The encoding and decoding functions in base64 now accept any bytes-like object in cases where it previously required a bytes or bytearray instance (issue 17839).


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 new 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 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 sys.stdout. In conjunction with io.StringIO, this context manager is also useful for checking expected output from command line utilities. (Contribute by Raymond Hettinger in issue 15805)

The contextlib documentation has also been updated to include a discussion of the differences between single use, reusable and reentrant context managers.


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 and Claudiu Popa in issue 17916)


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.)

A pair of new subclasses of Message have been added, along with a new sub-module, contentmanager. All documentation is currently in the new module, which is being added as part of the new 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 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.)


The new partialmethod() descriptor bring partial argument application to descriptors, just as partial() provides for normal callables. The new descriptor also makes it easier to get arbitrary callables (including partial() instances) to behave like normal instance methods when included in a class definition.

(Contributed by Alon Horev and Nick Coghlan in issue 4331)

The new singledispatch() decorator brings support for single-dispatch generic functions to the Python standard library. Where object oriented programming focuses on grouping multiple operations on a common set of data into a class, a generic function focuses on grouping multiple implementations of an operation that allows it to work with different kinds of data.

See also

PEP 443 - Single-dispatch generic functions
PEP written and implemented by Łukasz Langa.


New hashlib.pbkdf2_hmac() function.

(Contributed by Christian Heimes in issue 18582)


Added a new html.unescape() function that converts HTML5 character references to the corresponding Unicode characters.

(Contributed by Ezio Melotti in issue 2927)

Added a new convert_charrefs keyword argument to HTMLParser that, when True, automatically converts all character references. For backward-compatibility, its value defaults to False, but it will change to True in future versions, so you are invited to set it explicitly and update your code to use this new feature.

(Contributed by Ezio Melotti in issue 13633)

The strict argument of HTMLParser is now deprecated.

(Contributed by Ezio Melotti in issue 15114)


The inspect module now offers a basic 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)

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). (Contributed by Daniel Urban, Aaron Iles and Nick Coghlan in issue 13266)

As part of the implementation of the new enum module, the 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 objects can now be weakref’ed.

(Contributed by Valerie Lambert in issue 4885.)


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 pprint module now supports compact mode for formatting long sequences (issue 19132).


While significant changes have not been made to pydoc directly, its handling of custom __dir__ methods and various descriptor behaviours has been improved substantially by the underlying changes in the inspect module.


Added re.fullmatch() function and regex.fullmatch() method, which anchor the pattern at both ends of the string to match. (Contributed by Matthew Barnett in issue 16203.)

The repr of regex objects now includes the pattern and the flags; the repr of match objects now includes the start, end, and the part of the string that matched.

(Contributed by Serhiy Storchaka in issue 13592 and issue 17087.)


New resource.prlimit() function and Linux specific constants. (Contributed by Christian Heimes in issue 16595 and issue 19324.)


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)

(Contributed by Christian Heimes in issue 18143, issue 18147 and issue 17134.)

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.)


Add a filter function to ignore some packages (tests for instance), 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 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.)

  • The interpreter starts about 30% faster. A couple of measures lead to the speedup. The interpreter loads fewer modules on startup, e.g. the re, collections and 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: Customization of CPython memory allocators

PEP 445 adds new C level interfaces to customize memory allocation in the CPython interpreter.

See also

PEP 445 - Add new APIs to customize Python memory allocators
PEP written and implemented by Victor Stinner.

PEP 442: Safe object finalization

PEP 442 removes the current limitations and quirks of object finalization in CPython. 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 in most cases, instead relying on the normal operation of the cyclic garbage collector.

See also

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 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.


    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.


Unsupported Operating Systems

  • OS/2
  • Windows 2000

Deprecated Python modules, functions and methods

  • difflib.SequenceMatcher.isbjunk() and difflib.SequenceMatcher.isbpopular() were removed: use x in sm.bjunk and x in sm.bpopular, where sm is a SequenceMatcher object.
  • importlib.util.module_for_loader() is pending deprecation. Using importlib.util.module_to_load() and importlib.abc.Loader.init_module_attrs() allows subclasses of a loader to more easily customize module loading.
  • The imp module is pending deprecation. To keep compatibility with Python 2/3 code bases, the module’s removal is currently not scheduled.
  • The formatter module is pending deprecation and is slated for removal in Python 3.6.
  • MD5 as default digestmod for hmac is deprecated. Python 3.6 will require an explicit digest name or constructor as digestmod argument.

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 importlib.abc now either raise the appropriate exception or return a default value instead of raising NotImplementedError blindly. This will only affect code calling super() and falling through all the way to the ABCs. For compatibility, catch both NotImplementedError or the appropriate exception as needed.
  • The module type now initializes the __package__ and __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.
  • 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 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__').
  • PyErr_SetImportError() now sets TypeError when its msg argument is not set. Previously only NULL was returned with no exception set.
  • py_compile.compile() now raises 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.
  • importlib.abc.SourceLoader.get_source() no longer raises ImportError when the source code being loaded triggers a SyntaxError or UnicodeDecodeError. As 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.
  • functools.update_wrapper() and 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 inspect.unwrap() to access the first function in the chain that has no __wrapped__ attribute.
  • (C API) The result of the PyOS_ReadlineFunctionPointer callback must now be a string allocated by PyMem_RawMalloc() or PyMem_RawRealloc(), or NULL if an error occurred, instead of a string allocated by PyMem_Malloc() or PyMem_Realloc().
  • importlib.machinery.PathFinder now passes on the current working directory to objects in sys.path_hooks for the empty string. This results in sys.path_importer_cache never containing '', thus iterating through sys.path_importer_cache based on 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).