What’s New In Python 3.5

Date:September 06, 2015

This article explains the new features in Python 3.5, compared to 3.4.

For full details, see the Misc/NEWS file.


Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.5 moves towards release, so it’s worth checking back even after reading earlier versions.

See also

PEP 478 - Python 3.5 Release Schedule

Summary – Release highlights

New syntax features:

  • PEP 465, a new matrix multiplication operator: a @ b.
  • PEP 492, coroutines with async and await syntax.
  • PEP 448, additional unpacking generalizations.

New library modules:

New built-in features:

  • bytes % args, bytearray % args: PEP 461 - Adding % formatting to bytes and bytearray
  • b'\xf0\x9f\x90\x8d'.hex(), bytearray(b'\xf0\x9f\x90\x8d').hex(), memoryview(b'\xf0\x9f\x90\x8d').hex(): issue 9951 - A hex method has been added to bytes, bytearray, and memoryview.
  • Generators have new gi_yieldfrom attribute, which returns the object being iterated by yield from expressions. (Contributed by Benno Leslie and Yury Selivanov in issue 24450.)
  • New RecursionError exception. (Contributed by Georg Brandl in issue 19235.)

Implementation improvements:

  • When the LC_TYPE locale is the POSIX locale (C locale), sys.stdin and sys.stdout are now using the surrogateescape error handler, instead of the strict error handler (issue 19977).
  • PEP 488, the elimination of .pyo files.
  • PEP 489, multi-phase initialization of extension modules.

Significantly Improved Library Modules:

  • collections.OrderedDict is now implemented in C, which improves its performance between 4x to 100x times. Contributed by Eric Snow in issue 16991.
  • You may now pass bytes to the tempfile module’s APIs and it will return the temporary pathname as bytes instead of str. It also accepts a value of None on parameters where only str was accepted in the past to do the right thing based on the types of the other inputs. Two functions, gettempdirb() and gettempprefixb(), have been added to go along with this. This behavior matches that of the os APIs.
  • ssl module gained support for Memory BIO, which decouples SSL protocol handling from network IO. (Contributed by Geert Jansen in issue 21965.)

Security improvements:

  • None yet.

Windows improvements:

  • A new installer for Windows has replaced the old MSI. See Using Python on Windows for more information.
  • Windows builds now use Microsoft Visual C++ 14.0, and extension modules should use the same.

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

PEP 492 - Coroutines with async and await syntax

The PEP added dedicated syntax for declaring coroutines, await expressions, new asynchronous async for and async with statements.


async def read_data(db):
    async with db.transaction():
        data = await db.fetch('SELECT ...')

PEP written and implemented by Yury Selivanov.

See also

PEP 492 – Coroutines with async and await syntax

PEP 461 - Formatting support for bytes and bytearray

This PEP proposes adding % formatting operations similar to Python 2’s str type to bytes and bytearray.


>>> b'Hello %s!' % b'World'
b'Hello World!'
>>> b'x=%i y=%f' % (1, 2.5)
b'x=1 y=2.500000'

Unicode is not allowed for %s, but it is accepted by %a (equivalent of repr(obj).encode('ascii', 'backslashreplace')):

>>> b'Hello %s!' % 'World'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: %b requires bytes, or an object that implements __bytes__, not 'str'
>>> b'price: %a' % '10€'
b"price: '10\\u20ac'"

See also

PEP 461 – Adding % formatting to bytes and bytearray

PEP 465 - A dedicated infix operator for matrix multiplication

This PEP proposes a new binary operator to be used for matrix multiplication, called @. (Mnemonic: @ is * for mATrices.)

See also

PEP 465 – A dedicated infix operator for matrix multiplication

PEP 448 - Additional Unpacking Generalizations

This PEP proposes extended usages of the * iterable unpacking operator and ** dictionary unpacking operators to allow unpacking in more positions, an arbitrary number of times, and in additional circumstances. Specifically, in function calls, in comprehensions and generator expressions, and in displays.

Function calls are proposed to support an arbitrary number of unpackings rather than just one:

>>> print(*[1], *[2], 3)
1 2 3
>>> dict(**{'x': 1}, y=2, **{'z': 3})
{'x': 1, 'y': 2, 'z': 3}

Unpacking is proposed to be allowed inside tuple, list, set, and dictionary displays:

>>> *range(4), 4
(0, 1, 2, 3, 4)
>>> [*range(4), 4]
[0, 1, 2, 3, 4]
>>> {*range(4), 4}
{0, 1, 2, 3, 4}
>>> {'x': 1, **{'y': 2}}
{'x': 1, 'y': 2}

In dictionaries, later values will always override earlier ones:

>>> {'x': 1, **{'x': 2}}
{'x': 2}

>>> {**{'x': 2}, 'x': 1}
{'x': 1}

See also

PEP 448 – Additional Unpacking Generalizations

PEP 484 - Type Hints

This PEP introduces a provisional module to provide these standard definitions and tools, along with some conventions for situations where annotations are not available.

For example, here is a simple function whose argument and return type are declared in the annotations:

def greeting(name: str) -> str:
    return 'Hello ' + name

The type system supports unions, generic types, and a special type named Any which is consistent with (i.e. assignable to and from) all types.

See also

PEP 471 - os.scandir() function – a better and faster directory iterator

PEP 471 adds a new directory iteration function, os.scandir(), to the standard library. Additionally, os.walk() is now implemented using os.scandir(), which speeds it up by 3-5 times on POSIX systems and by 7-20 times on Windows systems.

PEP and implementation written by Ben Hoyt with the help of Victor Stinner.

See also

PEP 471 – os.scandir() function – a better and faster directory iterator

PEP 475: Retry system calls failing with EINTR

PEP 475 adds support for automatic retry of system calls failing with EINTR: this means that user code doesn’t have to deal with EINTR or InterruptedError manually, and should make it more robust against asynchronous signal reception.

Examples of functions which are now retried when interrupted by a signal instead of raising InterruptedError if the Python signal handler does not raise an exception:

PEP and implementation written by Charles-François Natali and Victor Stinner, with the help of Antoine Pitrou (the french connection).

See also

PEP 475 – Retry system calls failing with EINTR

PEP 479: Change StopIteration handling inside generators

PEP 479 changes the behavior of generators: when a StopIteration exception is raised inside a generator, it is replaced with a RuntimeError. To enable the feature a __future__ import should be used:

from __future__ import generator_stop

Without a __future__ import, a PendingDeprecationWarning will be raised.

PEP written by Chris Angelico and Guido van Rossum. Implemented by Chris Angelico, Yury Selivanov and Nick Coghlan.

See also

PEP 479 – Change StopIteration handling inside generators

PEP 486: Make the Python Launcher aware of virtual environments

PEP 486 makes the Windows launcher (see PEP 397) aware of an active virtual environment. When the default interpreter would be used and the VIRTUAL_ENV environment variable is set, the interpreter in the virtual environment will be used.

See also

PEP 486 – Make the Python Launcher aware of virtual environments

PEP 488: Elimination of PYO files

PEP 488 does away with the concept of .pyo files. This means that .pyc files represent both unoptimized and optimized bytecode. To prevent the need to constantly regenerate bytecode files, .pyc files now have an optional opt- tag in their name when the bytecode is optimized. This has the side-effect of no more bytecode file name clashes when running under either -O or -OO. Consequently, bytecode files generated from -O, and -OO may now exist simultaneously. importlib.util.cache_from_source() has an updated API to help with this change.

See also

PEP 488 – Elimination of PYO files

PEP 489: Multi-phase extension module initialization

PEP 489 updates extension module initialization to take advantage of the two step module loading mechanism introduced by PEP 451 in Python 3.4.

This change brings the import semantics of extension modules that opt-in to using the new mechanism much closer to those of Python source and bytecode modules, including the ability to use any valid identifier as a module name, rather than being restricted to ASCII.

See also

PEP 488 – Multi-phase extension module initialization

PEP 485: A function for testing approximate equality

PEP 485 adds the math.isclose() and cmath.isclose() functions which tell whether two values are approximately equal or “close” to each other. Whether or not two values are considered close is determined according to given absolute and relative tolerances.

See also

PEP 485 – A function for testing approximate equality

Other Language Changes

Some smaller changes made to the core Python language are:

  • Added the 'namereplace' error handlers. The 'backslashreplace' error handlers now works with decoding and translating. (Contributed by Serhiy Storchaka in issue 19676 and issue 22286.)
  • The -b option now affects comparisons of bytes with int. (Contributed by Serhiy Storchaka in issue 23681)
  • New Kazakh codec kz1048. (Contributed by Serhiy Storchaka in issue 22682.)
  • Property docstrings are now writable. This is especially useful for collections.namedtuple() docstrings. (Contributed by Berker Peksag in issue 24064.)
  • New Tajik codec koi8_t. (Contributed by Serhiy Storchaka in issue 22681.)

New Modules


The new zipapp module (specified in PEP 441) provides an API and command line tool for creating executable Python Zip Applications, which were introduced in Python 2.6 in issue 1739468 but which were not well publicised, either at the time or since.

With the new module, bundling your application is as simple as putting all the files, including a __main__.py file, into a directory myapp and running:

$ python -m zipapp myapp
$ python myapp.pyz

Improved Modules



  • FieldStorage now supports the context management protocol. (Contributed by Berker Peksag in issue 20289.)




  • You can now update docstrings produced by collections.namedtuple():

    Point = namedtuple('Point', ['x', 'y'])
    Point.__doc__ = 'ordered pair'
    Point.x.__doc__ = 'abscissa'
    Point.y.__doc__ = 'ordinate'

    (Contributed by Berker Peksag in issue 24064.)





  • The charset of the HTML document generated by difflib.HtmlDiff.make_file() can now be customized by using charset keyword-only parameter. The default charset of HTML document changed from 'ISO-8859-1' to 'utf-8'. (Contributed by Berker Peksag in issue 2052.)
  • It’s now possible to compare lists of byte strings with difflib.diff_bytes() (fixes a regression from Python 2).


  • The build and build_ext commands now accept a -j option to enable parallel building of extension modules. (Contributed by Antoine Pitrou in issue 5309.)
  • Added support for the LZMA compression. (Contributed by Serhiy Storchaka in issue 16314.)



  • A new policy option mangle_from_ controls whether or not lines that start with “From ” in email bodies are prefixed with a ‘>’ character by generators. The default is True for compat32 and False for all other policies. (Contributed by Milan Oberkirch in issue 20098.)
  • A new method get_content_disposition() provides easy access to a canonical value for the Content-Disposition header (None if there is no such header). (Contributed by Abhilash Raj in issue 21083.)
  • A new policy option utf8 can be set True to encode email headers using the utf8 charset instead of using encoded words. This allows Messages to be formatted according to RFC 6532 and used with an SMTP server that supports the RFC 6531 SMTPUTF8 extension. (Contributed by R. David Murray in issue 24211.)


  • iglob() and glob() now support recursive search in subdirectories using the “**” pattern. (Contributed by Serhiy Storchaka in issue 13968.)

idlelib and IDLE

Since idlelib implements the IDLE shell and editor and is not intended for import by other programs, it gets improvements with every release. See Lib/idlelib/NEWS.txt for a cumulative list of changes since 3.4.0, as well as changes made in future 3.5.x releases. This file is also available from the IDLE Help -> About Idle dialog.


  • IMAP4 now supports the context management protocol. When used in a with statement, the IMAP4 LOGOUT command will be called automatically at the end of the block. (Contributed by Tarek Ziadé and Serhiy Storchaka in issue 4972.)
  • imaplib now supports RFC 5161: the enable() extension), and RFC 6855: utf-8 support (internationalized email, via the UTF8=ACCEPT argument to enable()). A new attribute, utf8_enabled, tracks whether or not RFC 6855 support is enabled. Milan Oberkirch, R. David Murray, and Maciej Szulik in issue 21800.)
  • imaplib now automatically encodes non-ASCII string usernames and passwords using UTF8, as recommended by the RFCs. (Contributed by Milan Oberkirch in issue 21800.)








  • New os.scandir() function that exposes file information from the operating system when listing a directory. os.scandir() returns an iterator of os.DirEntry objects corresponding to the entries in the directory given by path. (Contributed by Ben Hoyt with the help of Victor Stinner in issue 22524.)
  • os.stat_result now has a st_file_attributes attribute on Windows. (Contributed by Ben Hoyt in issue 21719.)
  • os.urandom(): On Linux 3.17 and newer, the getrandom() syscall is now used when available. On OpenBSD 5.6 and newer, the C getentropy() function is now used. These functions avoid the usage of an internal file descriptor.


  • New commonpath() function that extracts common path prefix. Unlike the commonprefix() function, it always returns a valid path. (Contributed by Rafik Draoui and Serhiy Storchaka in issue 10395.)


  • Serializing more “lookupable” objects (such as unbound methods or nested classes) now are supported with pickle protocols < 4. (Contributed by Serhiy Storchaka in issue 23611.)


  • A new command utf8() enables RFC 6856 (internationalized email) support if the POP server supports it. (Contributed by Milan OberKirch in issue 21804.)


  • Number of capturing groups in regular expression is no longer limited by 100. (Contributed by Serhiy Storchaka in issue 22437.)
  • Now unmatched groups are replaced with empty strings in re.sub() and re.subn(). (Contributed by Serhiy Storchaka in issue 1519638.)


  • move() now accepts a copy_function argument, allowing, for example, copy() to be used instead of the default copy2() if there is a need to ignore metadata. (Contributed by Claudiu Popa in issue 19840.)


  • On Windows, signal.set_wakeup_fd() now also supports socket handles. (Contributed by Victor Stinner in issue 22018.)
  • Different constants of signal module are now enumeration values using the enum module. This allows meaningful names to be printed during debugging, instead of integer “magic numbers”. (Contributed by Giampaolo Rodola’ in issue 21076.)


  • Both SMTPServer and smtpd.SMTPChannel now accept a decode_data keyword to determine if the DATA portion of the SMTP transaction is decoded using the utf-8 codec or is instead provided to process_message() as a byte string. The default is True for backward compatibility reasons, but will change to False in Python 3.6. If decode_data is set to False, the process_message() method must be prepared to accept keyword arguments. (Contributed by Maciej Szulik in issue 19662.)
  • SMTPServer now advertises the 8BITMIME extension (RFC 6152) if if decode_data has been set True. If the client specifies BODY=8BITMIME on the MAIL command, it is passed to process_message() via the mail_options keyword. (Contributed by Milan Oberkirch and R. David Murray in issue 21795.)
  • SMTPServer now supports the SMTPUTF8 extension (RFC 6531: Internationalized Email). If the client specified SMTPUTF8 BODY=8BITMIME on the MAIL command, they are passed to process_message() via the mail_options keyword. It is the responsibility of the process_message() method to correctly handle the SMTPUTF8 data. (Contributed by Milan Oberkirch in issue 21725.)
  • It is now possible to provide, directly or via name resolution, IPv6 addresses in the SMTPServer constructor, and have it successfully connect. (Contributed by Milan Oberkirch in issue 14758.)


  • A new auth() method provides a convenient way to implement custom authentication mechanisms. (Contributed by Milan Oberkirch in issue 15014.)
  • Additional debuglevel (2) shows timestamps for debug messages in smtplib.SMTP. (Contributed by Gavin Chappell and Maciej Szulik in issue 16914.)
  • smtplib now supports RFC 6531 (SMTPUTF8) in both the sendmail() and send_message() commands. (Contributed by Milan Oberkirch and R. David Murray in issue 22027.)




  • New socket.socket.sendfile() method allows to send a file over a socket by using high-performance os.sendfile() function on UNIX resulting in uploads being from 2x to 3x faster than when using plain socket.socket.send(). (Contributed by Giampaolo Rodola’ in issue 17552.)
  • The socket.socket.sendall() method don’t reset the socket timeout anymore each time bytes are received or sent. The socket timeout is now the maximum total duration to send all data.




  • The user scripts directory on Windows is now versioned. (Contributed by Paul Moore in issue 23437.)


  • The tarfile.open() function now supports 'x' (exclusive creation) mode. (Contributed by Berker Peksag in issue 21717.)
  • The extractall() and extract() methods now take a keyword parameter numeric_only. If set to True, the extracted files and directories will be owned by the numeric uid and gid from the tarfile. If set to False (the default, and the behavior in versions prior to 3.5), they will be owned bythe named user and group in the tarfile. (Contributed by Michael Vogt and Eric Smith in issue 23193.)



  • The tkinter._fix module used for setting up the Tcl/Tk environment on Windows has been replaced by a private function in the _tkinter module which makes no permanent changes to environment variables. (Contributed by Zachary Ware in issue 20035.)



  • A new HTTPPasswordMgrWithPriorAuth allows HTTP Basic Authentication credentials to be managed so as to eliminate unnecessary 401 response handling, or to unconditionally send credentials on the first request in order to communicate with servers that return a 404 response instead of a 401 if the Authorization header is not sent. (Contributed by Matej Cepl in issue 19494 and Akshit Khurana in issue 7159.)
  • A new urlencode() parameter quote_via provides a way to control the encoding of query parts if needed. (Contributed by Samwyse and Arnon Yaari in issue 13866.)





  • SAX parsers now support a character stream of InputSource object. (Contributed by Serhiy Storchaka in issue 2175.)



  • Added support for writing ZIP files to unseekable streams. (Contributed by Serhiy Storchaka in issue 23252.)
  • The zipfile.ZipFile.open() function now supports 'x' (exclusive creation) mode. (Contributed by Serhiy Storchaka in issue 21717.)


The following performance enhancements have been added:

Build and C API Changes

Changes to Python’s build process and to the C API include:


New Keywords

async and await are not recommended to be used as variable, class, function or module names. Introduced by PEP 492 in Python 3.5, they will become proper keywords in Python 3.7.

Unsupported Operating Systems

  • Windows XP - Per PEP 11, Microsoft support of Windows XP has ended.

Deprecated Python modules, functions and methods

Deprecated functions and types of the C API

  • None yet.

Deprecated features

  • None yet.


API and Feature Removals

The following obsolete and previously deprecated APIs and features have been removed:

  • The __version__ attribute has been dropped from the email package. The email code hasn’t been shipped separately from the stdlib for a long time, and the __version__ string was not updated in the last few releases.
  • The internal Netrc class in the ftplib module was deprecated in 3.4, and has now been removed. (Contributed by Matt Chaput in issue 6623.)
  • The concept of .pyo files has been removed.
  • The JoinableQueue class in the provisional asyncio module was deprecated in 3.4.4 and is now removed (issue 23464).

Porting to Python 3.5

This section lists previously described changes and other bugfixes that may require changes to your code.

Changes in the Python API

  • PEP 475: System calls are now retried when interrupted by a signal instead of raising InterruptedError if the Python signal handler does not raise an exception.
  • Before Python 3.5, a datetime.time object was considered to be false if it represented midnight in UTC. This behavior was considered obscure and error-prone and has been removed in Python 3.5. See issue 13936 for full details.
  • ssl.SSLSocket.send() now raises either ssl.SSLWantReadError or ssl.SSLWantWriteError on a non-blocking socket if the operation would block. Previously, it would return 0. See issue 20951.
  • The __name__ attribute of generator is now set from the function name, instead of being set from the code name. Use gen.gi_code.co_name to retrieve the code name. Generators also have a new __qualname__ attribute, the qualified name, which is now used for the representation of a generator (repr(gen)). See issue 21205.
  • The deprecated “strict” mode and argument of HTMLParser, HTMLParser.error(), and the HTMLParserError exception have been removed. (Contributed by Ezio Melotti in issue 15114.) The convert_charrefs argument of HTMLParser is now True by default. (Contributed by Berker Peksag in issue 21047.)
  • Although it is not formally part of the API, it is worth noting for porting purposes (ie: fixing tests) that error messages that were previously of the form “‘sometype’ does not support the buffer protocol” are now of the form “a bytes-like object is required, not ‘sometype’”. (Contributed by Ezio Melotti in issue 16518.)
  • If the current directory is set to a directory that no longer exists then FileNotFoundError will no longer be raised and instead find_spec() will return None without caching None in sys.path_importer_cache which is different than the typical case (issue 22834).
  • HTTP status code and messages from http.client and http.server were refactored into a common HTTPStatus enum. The values in http.client and http.server remain available for backwards compatibility. (Contributed by Demian Brecht in issue 21793.)
  • When an import loader defines exec_module() it is now expected to also define create_module() (raises a DeprecationWarning now, will be an error in Python 3.6). If the loader inherits from importlib.abc.Loader then there is nothing to do, else simply define create_module() to return None (issue 23014).
  • re.split() always ignored empty pattern matches, so the 'x*' pattern worked the same as 'x+', and the '\b' pattern never worked. Now re.split() raises a warning if the pattern could match an empty string. For compatibility use patterns that never match an empty string (e.g. 'x+' instead of 'x*'). Patterns that could only match an empty string (such as '\b') now raise an error.
  • The Morsel dict-like interface has been made self consistent: morsel comparison now takes the key and value into account, copy() now results in a Morsel instance rather than a dict, and update() will now raise an exception if any of the keys in the update dictionary are invalid. In addition, the undocumented LegalChars parameter of set() is deprecated and is now ignored. (issue 2211)
  • PEP 488 has removed .pyo files from Python and introduced the optional opt- tag in .pyc file names. The importlib.util.cache_from_source() has gained an optimization parameter to help control the opt- tag. Because of this, the debug_override parameter of the function is now deprecated. .pyo files are also no longer supported as a file argument to the Python interpreter and thus serve no purpose when distributed on their own (i.e. sourcless code distribution). Due to the fact that the magic number for bytecode has changed in Python 3.5, all old .pyo files from previous versions of Python are invalid regardless of this PEP.
  • The socket module now exports the CAN_RAW_FD_FRAMES constant on linux 3.6 and greater.
  • The pygettext.py Tool now uses the standard +NNNN format for timezones in the POT-Creation-Date header.
  • The smtplib module now uses sys.stderr instead of previous module level stderr variable for debug output. If your (test) program depends on patching the module level variable to capture the debug output, you will need to update it to capture sys.stderr instead.
  • The str.startswith() and str.endswith() methods no longer return True when finding the empty string and the indexes are completely out of range. See issue 24284.
  • The inspect.getdoc() function now returns documentation strings inherited from base classes. Documentation strings no longer need to be duplicated if the inherited documentation is appropriate. To suppress an inherited string, an empty string must be specified (or the documentation may be filled in). This change affects the output of the pydoc module and the help() function. See issue 15582.

Changes in the C API

  • The undocumented format member of the (non-public) PyMemoryViewObject structure has been removed.

    All extensions relying on the relevant parts in memoryobject.h must be rebuilt.

  • The PyMemAllocator structure was renamed to PyMemAllocatorEx and a new calloc field was added.

  • Removed non-documented macro PyObject_REPR which leaked references. Use format character %R in PyUnicode_FromFormat()-like functions to format the repr() of the object.

  • Because the lack of the __module__ attribute breaks pickling and introspection, a deprecation warning now is raised for builtin type without the __module__ attribute. Would be an AttributeError in future. (issue 20204)

  • As part of PEP 492 implementation, tp_reserved slot of PyTypeObject was replaced with a tp_as_async slot. Refer to Coroutine Objects for new types, structures and functions.