What’s New In Python 3.5

Editors:Elvis Pranskevichus <elvis@magic.io>, Yury Selivanov <yury@magic.io>

This article explains the new features in Python 3.5, compared to 3.4. Python 3.5 was released on September 13, 2015.  See the changelog for a full list of changes.

See also

PEP 478 - Python 3.5 Release Schedule

Summary – Release highlights

New syntax features:

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

New library modules:

New built-in features:

  • bytes % args, bytearray % args: PEP 461 – Adding % formatting to bytes and bytearray.
  • New bytes.hex(), bytearray.hex() and memoryview.hex() methods. (Contributed by Arnon Yaari in bpo-9951.)
  • memoryview now supports tuple indexing (including multi-dimensional). (Contributed by Antoine Pitrou in bpo-23632.)
  • Generators have a new gi_yieldfrom attribute, which returns the object being iterated by yield from expressions. (Contributed by Benno Leslie and Yury Selivanov in bpo-24450.)
  • A new RecursionError exception is now raised when maximum recursion depth is reached. (Contributed by Georg Brandl in bpo-19235.)

CPython implementation improvements:

  • When the LC_TYPE locale is the POSIX locale (C locale), sys.stdin and sys.stdout now use the surrogateescape error handler, instead of the strict error handler. (Contributed by Victor Stinner in bpo-19977.)
  • .pyo files are no longer used and have been replaced by a more flexible scheme that includes the optimization level explicitly in .pyc name. (See PEP 488 overview.)
  • Builtin and extension modules are now initialized in a multi-phase process, which is similar to how Python modules are loaded. (See PEP 489 overview.)

Significant improvements in the standard library:

Security improvements:

  • SSLv3 is now disabled throughout the standard library. It can still be enabled by instantiating a ssl.SSLContext manually. (See bpo-22638 for more details; this change was backported to CPython 3.4 and 2.7.)
  • HTTP cookie parsing is now stricter, in order to protect against potential injection attacks. (Contributed by Antoine Pitrou in bpo-22796.)

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, including many other smaller improvements, CPython optimizations, deprecations, and potential porting issues.

New Features

PEP 492 - Coroutines with async and await syntax

PEP 492 greatly improves support for asynchronous programming in Python by adding awaitable objects, coroutine functions, asynchronous iteration, and asynchronous context managers.

Coroutine functions are declared using the new async def syntax:

>>> async def coro():
...     return 'spam'

Inside a coroutine function, the new await expression can be used to suspend coroutine execution until the result is available. Any object can be awaited, as long as it implements the awaitable protocol by defining the __await__() method.

PEP 492 also adds async for statement for convenient iteration over asynchronous iterables.

An example of a rudimentary HTTP client written using the new syntax:

import asyncio

async def http_get(domain):
    reader, writer = await asyncio.open_connection(domain, 80)

        b'GET / HTTP/1.1',
        b'Host: %b' % domain.encode('latin-1'),
        b'Connection: close',
        b'', b''

    async for line in reader:
        print('>>>', line)


loop = asyncio.get_event_loop()

Similarly to asynchronous iteration, there is a new syntax for asynchronous context managers. The following script:

import asyncio

async def coro(name, lock):
    print('coro {}: waiting for lock'.format(name))
    async with lock:
        print('coro {}: holding the lock'.format(name))
        await asyncio.sleep(1)
        print('coro {}: releasing the lock'.format(name))

loop = asyncio.get_event_loop()
lock = asyncio.Lock()
coros = asyncio.gather(coro(1, lock), coro(2, lock))

will output:

coro 2: waiting for lock
coro 2: holding the lock
coro 1: waiting for lock
coro 2: releasing the lock
coro 1: holding the lock
coro 1: releasing the lock

Note that both async for and async with can only be used inside a coroutine function declared with async def.

Coroutine functions are intended to be run inside a compatible event loop, such as the asyncio loop.


Changed in version 3.5.2: Starting with CPython 3.5.2, __aiter__ can directly return asynchronous iterators. Returning an awaitable object will result in a PendingDeprecationWarning.

See more details in the Asynchronous Iterators documentation section.

See also

PEP 492 – Coroutines with async and await syntax
PEP written and implemented by Yury Selivanov.

PEP 465 - A dedicated infix operator for matrix multiplication

PEP 465 adds the @ infix operator for matrix multiplication. Currently, no builtin Python types implement the new operator, however, it can be implemented by defining __matmul__(), __rmatmul__(), and __imatmul__() for regular, reflected, and in-place matrix multiplication. The semantics of these methods is similar to that of methods defining other infix arithmetic operators.

Matrix multiplication is a notably common operation in many fields of mathematics, science, engineering, and the addition of @ allows writing cleaner code:

S = (H @ beta - r).T @ inv(H @ V @ H.T) @ (H @ beta - r)

instead of:

S = dot((dot(H, beta) - r).T,
        dot(inv(dot(dot(H, V), H.T)), dot(H, beta) - r))

NumPy 1.10 has support for the new operator:

>>> import numpy

>>> x = numpy.ones(3)
>>> x
array([ 1., 1., 1.])

>>> m = numpy.eye(3)
>>> m
array([[ 1., 0., 0.],
       [ 0., 1., 0.],
       [ 0., 0., 1.]])

>>> x @ m
array([ 1., 1., 1.])

See also

PEP 465 – A dedicated infix operator for matrix multiplication
PEP written by Nathaniel J. Smith; implemented by Benjamin Peterson.

PEP 448 - Additional Unpacking Generalizations

PEP 448 extends the allowed uses of the * iterable unpacking operator and ** dictionary unpacking operator. It is now possible to use an arbitrary number of unpackings in function calls:

>>> print(*[1], *[2], 3, *[4, 5])
1 2 3 4 5

>>> def fn(a, b, c, d):
...     print(a, b, c, d)

>>> fn(**{'a': 1, 'c': 3}, **{'b': 2, 'd': 4})
1 2 3 4

Similarly, tuple, list, set, and dictionary displays allow multiple unpackings (see Expression lists and Dictionary displays):

>>> *range(4), 4
(0, 1, 2, 3, 4)

>>> [*range(4), 4]
[0, 1, 2, 3, 4]

>>> {*range(4), 4, *(5, 6, 7)}
{0, 1, 2, 3, 4, 5, 6, 7}

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

See also

PEP 448 – Additional Unpacking Generalizations
PEP written by Joshua Landau; implemented by Neil Girdhar, Thomas Wouters, and Joshua Landau.

PEP 461 - percent formatting support for bytes and bytearray

PEP 461 adds support for the % interpolation operator to bytes and bytearray.

While interpolation is usually thought of as a string operation, there are cases where interpolation on bytes or bytearrays makes sense, and the work needed to make up for this missing functionality detracts from the overall readability of the code. This issue is particularly important when dealing with wire format protocols, which are often a mixture of binary and ASCII compatible text.


>>> b'Hello %b!' % b'World'
b'Hello World!'

>>> b'x=%i y=%f' % (1, 2.5)
b'x=1 y=2.500000'

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

>>> b'Hello %b!' % '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'"

Note that %s and %r conversion types, although supported, should only be used in codebases that need compatibility with Python 2.

See also

PEP 461 – Adding % formatting to bytes and bytearray
PEP written by Ethan Furman; implemented by Neil Schemenauer and Ethan Furman.

PEP 484 - Type Hints

Function annotation syntax has been a Python feature since version 3.0 (PEP 3107), however the semantics of annotations has been left undefined.

Experience has shown that the majority of function annotation uses were to provide type hints to function parameters and return values. It became evident that it would be beneficial for Python users, if the standard library included the base definitions and tools for type annotations.

PEP 484 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

While these annotations are available at runtime through the usual __annotations__ attribute, no automatic type checking happens at runtime. Instead, it is assumed that a separate off-line type checker (e.g. mypy) will be used for on-demand source code analysis.

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

  • typing module documentation

  • PEP 484 – Type Hints

    PEP written by Guido van Rossum, Jukka Lehtosalo, and Łukasz Langa; implemented by Guido van Rossum.

  • PEP 483 – The Theory of Type Hints

    PEP written by Guido van Rossum

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 scandir, which makes it 3 to 5 times faster on POSIX systems and 7 to 20 times faster on Windows systems. This is largely achieved by greatly reducing the number of calls to os.stat() required to walk a directory tree.

Additionally, scandir returns an iterator, as opposed to returning a list of file names, which improves memory efficiency when iterating over very large directories.

The following example shows a simple use of os.scandir() to display all the files (excluding directories) in the given path that don’t start with '.'. The entry.is_file() call will generally not make an additional system call:

for entry in os.scandir(path):
    if not entry.name.startswith('.') and entry.is_file():

See also

PEP 471 – os.scandir() function – a better and faster directory iterator
PEP written and implemented by Ben Hoyt with the help of Victor Stinner.

PEP 475: Retry system calls failing with EINTR

An errno.EINTR error code is returned whenever a system call, that is waiting for I/O, is interrupted by a signal. Previously, Python would raise InterruptedError in such cases. This meant that, when writing a Python application, the developer had two choices:

  1. Ignore the InterruptedError.
  2. Handle the InterruptedError and attempt to restart the interrupted system call at every call site.

The first option makes an application fail intermittently. The second option adds a large amount of boilerplate that makes the code nearly unreadable. Compare:

print("Hello World")


while True:
        print("Hello World")
    except InterruptedError:

PEP 475 implements automatic retry of system calls on EINTR. This removes the burden of dealing with EINTR or InterruptedError in user code in most situations and makes Python programs, including the standard library, more robust. Note that the system call is only retried if the signal handler does not raise an exception.

Below is a list of functions which are now retried when interrupted by a signal:

See also

PEP 475 – Retry system calls failing with EINTR
PEP and implementation written by Charles-François Natali and Victor Stinner, with the help of Antoine Pitrou (the French connection).

PEP 479: Change StopIteration handling inside generators

The interaction of generators and StopIteration in Python 3.4 and earlier was sometimes surprising, and could conceal obscure bugs. Previously, StopIteration raised accidentally inside a generator function was interpreted as the end of the iteration by the loop construct driving the generator.

PEP 479 changes the behavior of generators: when a StopIteration exception is raised inside a generator, it is replaced with a RuntimeError before it exits the generator frame. The main goal of this change is to ease debugging in the situation where an unguarded next() call raises StopIteration and causes the iteration controlled by the generator to terminate silently. This is particularly pernicious in combination with the yield from construct.

This is a backwards incompatible change, so to enable the new behavior, a __future__ import is necessary:

>>> from __future__ import generator_stop

>>> def gen():
...     next(iter([]))
...     yield
>>> next(gen())
Traceback (most recent call last):
  File "<stdin>", line 2, in gen

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

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: generator raised StopIteration

Without a __future__ import, a PendingDeprecationWarning will be raised whenever a StopIteration exception is raised inside a generator.

See also

PEP 479 – Change StopIteration handling inside generators
PEP written by Chris Angelico and Guido van Rossum. Implemented by Chris Angelico, Yury Selivanov and Nick Coghlan.

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. Relative tolerance is the maximum allowed difference between isclose arguments, relative to the larger absolute value:

>>> import math
>>> a = 5.0
>>> b = 4.99998
>>> math.isclose(a, b, rel_tol=1e-5)
>>> math.isclose(a, b, rel_tol=1e-6)

It is also possible to compare two values using absolute tolerance, which must be a non-negative value:

>>> import math
>>> a = 5.0
>>> b = 4.99998
>>> math.isclose(a, b, abs_tol=0.00003)
>>> math.isclose(a, b, abs_tol=0.00001)

See also

PEP 485 – A function for testing approximate equality
PEP written by Christopher Barker; implemented by Chris Barker and Tal Einat.

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 written and implemented by Paul Moore.

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 written and implemented by Brett Cannon.

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 489 – Multi-phase extension module initialization
PEP written by Petr Viktorin, Stefan Behnel, and Nick Coghlan; implemented by Petr Viktorin.

Other Language Changes

Some smaller changes made to the core Python language are:

  • Added the "namereplace" error handlers. The "backslashreplace" error handlers now work with decoding and translating. (Contributed by Serhiy Storchaka in bpo-19676 and bpo-22286.)
  • The -b option now affects comparisons of bytes with int. (Contributed by Serhiy Storchaka in bpo-23681.)
  • New Kazakh kz1048 and Tajik koi8_t codecs. (Contributed by Serhiy Storchaka in bpo-22682 and bpo-22681.)
  • Property docstrings are now writable. This is especially useful for collections.namedtuple() docstrings. (Contributed by Berker Peksag in bpo-24064.)
  • Circular imports involving relative imports are now supported. (Contributed by Brett Cannon and Antoine Pitrou in bpo-17636.)

New Modules


The new typing provisional module provides standard definitions and tools for function type annotations. See Type Hints for more information.


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 bpo-1739468, but which were not well publicized, 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

The module implementation has been contributed by Paul Moore in bpo-23491.

See also

PEP 441 – Improving Python ZIP Application Support

Improved Modules


The ArgumentParser class now allows disabling abbreviated usage of long options by setting allow_abbrev to False. (Contributed by Jonathan Paugh, Steven Bethard, paul j3 and Daniel Eriksson in bpo-14910.)


Since the asyncio module is provisional, all changes introduced in Python 3.5 have also been backported to Python 3.4.x.

Notable changes in the asyncio module since Python 3.4.0:

  • New debugging APIs: loop.set_debug() and loop.get_debug() methods. (Contributed by Victor Stinner.)
  • The proactor event loop now supports SSL. (Contributed by Antoine Pitrou and Victor Stinner in bpo-22560.)
  • A new loop.is_closed() method to check if the event loop is closed. (Contributed by Victor Stinner in bpo-21326.)
  • A new loop.create_task() to conveniently create and schedule a new Task for a coroutine. The create_task method is also used by all asyncio functions that wrap coroutines into tasks, such as asyncio.wait(), asyncio.gather(), etc. (Contributed by Victor Stinner.)
  • A new transport.get_write_buffer_limits() method to inquire for high- and low- water limits of the flow control. (Contributed by Victor Stinner.)
  • The async() function is deprecated in favor of ensure_future(). (Contributed by Yury Selivanov.)
  • New loop.set_task_factory() and loop.get_task_factory() methods to customize the task factory that loop.create_task() method uses. (Contributed by Yury Selivanov.)
  • New Queue.join() and Queue.task_done() queue methods. (Contributed by Victor Stinner.)
  • The JoinableQueue class was removed, in favor of the asyncio.Queue class. (Contributed by Victor Stinner.)

Updates in 3.5.1:

  • The ensure_future() function and all functions that use it, such as loop.run_until_complete(), now accept all kinds of awaitable objects. (Contributed by Yury Selivanov.)
  • New run_coroutine_threadsafe() function to submit coroutines to event loops from other threads. (Contributed by Vincent Michel.)
  • New Transport.is_closing() method to check if the transport is closing or closed. (Contributed by Yury Selivanov.)
  • The loop.create_server() method can now accept a list of hosts. (Contributed by Yann Sionneau.)

Updates in 3.5.2:

  • New loop.create_future() method to create Future objects. This allows alternative event loop implementations, such as uvloop, to provide a faster asyncio.Future implementation. (Contributed by Yury Selivanov.)
  • New loop.get_exception_handler() method to get the current exception handler. (Contributed by Yury Selivanov.)
  • New StreamReader.readuntil() method to read data from the stream until a separator bytes sequence appears. (Contributed by Mark Korenberg.)
  • The loop.create_connection() and loop.create_server() methods are optimized to avoid calling the system getaddrinfo function if the address is already resolved. (Contributed by A. Jesse Jiryu Davis.)
  • The loop.sock_connect(sock, address) no longer requires the address to be resolved prior to the call. (Contributed by A. Jesse Jiryu Davis.)


The BZ2Decompressor.decompress method now accepts an optional max_length argument to limit the maximum size of decompressed data. (Contributed by Nikolaus Rath in bpo-15955.)


The FieldStorage class now supports the context manager protocol. (Contributed by Berker Peksag in bpo-20289.)


A new function isclose() provides a way to test for approximate equality. (Contributed by Chris Barker and Tal Einat in bpo-24270.)


The InteractiveInterpreter.showtraceback() method now prints the full chained traceback, just like the interactive interpreter. (Contributed by Claudiu Popa in bpo-17442.)


The OrderedDict class is now implemented in C, which makes it 4 to 100 times faster. (Contributed by Eric Snow in bpo-16991.)

OrderedDict.items(), OrderedDict.keys(), OrderedDict.values() views now support reversed() iteration. (Contributed by Serhiy Storchaka in bpo-19505.)

The deque class now defines index(), insert(), and copy(), and supports the + and * operators. This allows deques to be recognized as a MutableSequence and improves their substitutability for lists. (Contributed by Raymond Hettinger in bpo-23704.)

Docstrings produced by namedtuple() can now be updated:

Point = namedtuple('Point', ['x', 'y'])
Point.__doc__ += ': Cartesian coodinate'
Point.x.__doc__ = 'abscissa'
Point.y.__doc__ = 'ordinate'

(Contributed by Berker Peksag in bpo-24064.)

The UserString class now implements the __getnewargs__(), __rmod__(), casefold(), format_map(), isprintable(), and maketrans() methods to match the corresponding methods of str. (Contributed by Joe Jevnik in bpo-22189.)


The Sequence.index() method now accepts start and stop arguments to match the corresponding methods of tuple, list, etc. (Contributed by Devin Jeanpierre in bpo-23086.)

A new Generator abstract base class. (Contributed by Stefan Behnel in bpo-24018.)

New Awaitable, Coroutine, AsyncIterator, and AsyncIterable abstract base classes. (Contributed by Yury Selivanov in bpo-24184.)

For earlier Python versions, a backport of the new ABCs is available in an external PyPI package.


A new compileall option, -j N, allows running N workers simultaneously to perform parallel bytecode compilation. The compile_dir() function has a corresponding workers parameter. (Contributed by Claudiu Popa in bpo-16104.)

Another new option, -r, allows controlling the maximum recursion level for subdirectories. (Contributed by Claudiu Popa in bpo-19628.)

The -q command line option can now be specified more than once, in which case all output, including errors, will be suppressed. The corresponding quiet parameter in compile_dir(), compile_file(), and compile_path() can now accept an integer value indicating the level of output suppression. (Contributed by Thomas Kluyver in bpo-21338.)


The Executor.map() method now accepts a chunksize argument to allow batching of tasks to improve performance when ProcessPoolExecutor() is used. (Contributed by Dan O’Reilly in bpo-11271.)

The number of workers in the ThreadPoolExecutor constructor is optional now. The default value is 5 times the number of CPUs. (Contributed by Claudiu Popa in bpo-21527.)


configparser now provides a way to customize the conversion of values by specifying a dictionary of converters in the ConfigParser constructor, or by defining them as methods in ConfigParser subclasses. Converters defined in a parser instance are inherited by its section proxies.


>>> import configparser
>>> conv = {}
>>> conv['list'] = lambda v: [e.strip() for e in v.split() if e.strip()]
>>> cfg = configparser.ConfigParser(converters=conv)
>>> cfg.read_string("""
... [s]
... list = a b c d e f g
... """)
>>> cfg.get('s', 'list')
'a b c d e f g'
>>> cfg.getlist('s', 'list')
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> section = cfg['s']
>>> section.getlist('list')
['a', 'b', 'c', 'd', 'e', 'f', 'g']

(Contributed by Łukasz Langa in bpo-18159.)


The new redirect_stderr() context manager (similar to redirect_stdout()) makes it easier for utility scripts to handle inflexible APIs that write their output to sys.stderr and don’t provide any options to redirect it:

>>> import contextlib, io, logging
>>> f = io.StringIO()
>>> with contextlib.redirect_stderr(f):
...     logging.warning('warning')
>>> f.getvalue()

(Contributed by Berker Peksag in bpo-22389.)


The writerow() method now supports arbitrary iterables, not just sequences. (Contributed by Serhiy Storchaka in bpo-23171.)


The new update_lines_cols() function updates the LINES and COLS environment variables. This is useful for detecting manual screen resizing. (Contributed by Arnon Yaari in bpo-4254.)


dumb.open always creates a new database when the flag has the value "n". (Contributed by Claudiu Popa in bpo-18039.)


The charset of HTML documents generated by HtmlDiff.make_file() can now be customized by using a new charset keyword-only argument. The default charset of HTML document changed from "ISO-8859-1" to "utf-8". (Contributed by Berker Peksag in bpo-2052.)

The diff_bytes() function can now compare lists of byte strings. This fixes a regression from Python 2. (Contributed by Terry J. Reedy and Greg Ward in bpo-17445.)


Both the build and build_ext commands now accept a -j option to enable parallel building of extension modules. (Contributed by Antoine Pitrou in bpo-5309.)

The distutils module now supports xz compression, and can be enabled by passing xztar as an argument to bdist --format. (Contributed by Serhiy Storchaka in bpo-16314.)


The DocTestSuite() function returns an empty unittest.TestSuite if module contains no docstrings, instead of raising ValueError. (Contributed by Glenn Jones in bpo-15916.)


A new policy option Policy.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 bpo-20098.)

A new Message.get_content_disposition() method provides easy access to a canonical value for the Content-Disposition header. (Contributed by Abhilash Raj in bpo-21083.)

A new policy option EmailPolicy.utf8 can be set to True to encode email headers using the UTF-8 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 bpo-24211.)

The mime.text.MIMEText constructor now accepts a charset.Charset instance. (Contributed by Claude Paroz and Berker Peksag in bpo-16324.)


The Enum callable has a new parameter start to specify the initial number of enum values if only names are provided:

>>> Animal = enum.Enum('Animal', 'cat dog', start=10)
>>> Animal.cat
<Animal.cat: 10>
>>> Animal.dog
<Animal.dog: 11>

(Contributed by Ethan Furman in bpo-21706.)


The enable(), register(), dump_traceback() and dump_traceback_later() functions now accept file descriptors in addition to file-like objects. (Contributed by Wei Wu in bpo-23566.)


Most of the lru_cache() machinery is now implemented in C, making it significantly faster. (Contributed by Matt Joiner, Alexey Kachayev, and Serhiy Storchaka in bpo-14373.)


The iglob() and glob() functions now support recursive search in subdirectories, using the "**" pattern. (Contributed by Serhiy Storchaka in bpo-13968.)


The mode argument of the GzipFile constructor now accepts "x" to request exclusive creation. (Contributed by Tim Heaney in bpo-19222.)


Element comparison in merge() can now be customized by passing a key function in a new optional key keyword argument, and a new optional reverse keyword argument can be used to reverse element comparison:

>>> import heapq
>>> a = ['9', '777', '55555']
>>> b = ['88', '6666']
>>> list(heapq.merge(a, b, key=len))
['9', '88', '777', '6666', '55555']
>>> list(heapq.merge(reversed(a), reversed(b), key=len, reverse=True))
['55555', '6666', '777', '88', '9']

(Contributed by Raymond Hettinger in bpo-13742.)


A new HTTPStatus enum that defines a set of HTTP status codes, reason phrases and long descriptions written in English. (Contributed by Demian Brecht in bpo-21793.)


HTTPConnection.getresponse() now raises a RemoteDisconnected exception when a remote server connection is closed unexpectedly. Additionally, if a ConnectionError (of which RemoteDisconnected is a subclass) is raised, the client socket is now closed automatically, and will reconnect on the next request:

import http.client
conn = http.client.HTTPConnection('www.python.org')
for retries in range(3):
        conn.request('GET', '/')
        resp = conn.getresponse()
    except http.client.RemoteDisconnected:

(Contributed by Martin Panter in bpo-3566.)

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.


The IMAP4 class now supports the context manager 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 bpo-4972.)

The imaplib module now supports RFC 5161 (ENABLE Extension) and RFC 6855 (UTF-8 Support) via the IMAP4.enable() method. A new IMAP4.utf8_enabled attribute tracks whether or not RFC 6855 support is enabled. (Contributed by Milan Oberkirch, R. David Murray, and Maciej Szulik in bpo-21800.)

The imaplib module now automatically encodes non-ASCII string usernames and passwords using UTF-8, as recommended by the RFCs. (Contributed by Milan Oberkirch in bpo-21800.)


The what() function now recognizes the OpenEXR format (contributed by Martin Vignali and Claudiu Popa in bpo-20295), and the WebP format (contributed by Fabrice Aneche and Claudiu Popa in bpo-20197.)


The util.LazyLoader class allows for lazy loading of modules in applications where startup time is important. (Contributed by Brett Cannon in bpo-17621.)

The abc.InspectLoader.source_to_code() method is now a static method. This makes it easier to initialize a module object with code compiled from a string by running exec(code, module.__dict__). (Contributed by Brett Cannon in bpo-21156.)

The new util.module_from_spec() function is now the preferred way to create a new module. As opposed to creating a types.ModuleType instance directly, this new function will set the various import-controlled attributes based on the passed-in spec object. (Contributed by Brett Cannon in bpo-20383.)


Both the Signature and Parameter classes are now picklable and hashable. (Contributed by Yury Selivanov in bpo-20726 and bpo-20334.)

A new BoundArguments.apply_defaults() method provides a way to set default values for missing arguments:

>>> def foo(a, b='ham', *args): pass
>>> ba = inspect.signature(foo).bind('spam')
>>> ba.apply_defaults()
>>> ba.arguments
OrderedDict([('a', 'spam'), ('b', 'ham'), ('args', ())])

(Contributed by Yury Selivanov in bpo-24190.)

A new class method Signature.from_callable() makes subclassing of Signature easier. (Contributed by Yury Selivanov and Eric Snow in bpo-17373.)

The signature() function now accepts a follow_wrapped optional keyword argument, which, when set to False, disables automatic following of __wrapped__ links. (Contributed by Yury Selivanov in bpo-20691.)

A set of new functions to inspect coroutine functions and coroutine objects has been added: iscoroutine(), iscoroutinefunction(), isawaitable(), getcoroutinelocals(), and getcoroutinestate(). (Contributed by Yury Selivanov in bpo-24017 and bpo-24400.)

The stack(), trace(), getouterframes(), and getinnerframes() functions now return a list of named tuples. (Contributed by Daniel Shahaf in bpo-16808.)


A new BufferedIOBase.readinto1() method, that uses at most one call to the underlying raw stream’s RawIOBase.read() or RawIOBase.readinto() methods. (Contributed by Nikolaus Rath in bpo-20578.)


Both the IPv4Network and IPv6Network classes now accept an (address, netmask) tuple argument, so as to easily construct network objects from existing addresses:

>>> import ipaddress
>>> ipaddress.IPv4Network(('', 8))
>>> ipaddress.IPv4Network(('', ''))

(Contributed by Peter Moody and Antoine Pitrou in bpo-16531.)

A new reverse_pointer attribute for the IPv4Network and IPv6Network classes returns the name of the reverse DNS PTR record:

>>> import ipaddress
>>> addr = ipaddress.IPv4Address('')
>>> addr.reverse_pointer
>>> addr6 = ipaddress.IPv6Address('::1')
>>> addr6.reverse_pointer

(Contributed by Leon Weber in bpo-20480.)


The json.tool command line interface now preserves the order of keys in JSON objects passed in input. The new --sort-keys option can be used to sort the keys alphabetically. (Contributed by Berker Peksag in bpo-21650.)

JSON decoder now raises JSONDecodeError instead of ValueError to provide better context information about the error. (Contributed by Serhiy Storchaka in bpo-19361.)


A new lazycache() function can be used to capture information about a non-file-based module to permit getting its lines later via getline(). This avoids doing I/O until a line is actually needed, without having to carry the module globals around indefinitely. (Contributed by Robert Collins in bpo-17911.)


A new delocalize() function can be used to convert a string into a normalized number string, taking the LC_NUMERIC settings into account:

>>> import locale
>>> locale.setlocale(locale.LC_NUMERIC, 'de_DE.UTF-8')
>>> locale.delocalize('1.234,56')
>>> locale.setlocale(locale.LC_NUMERIC, 'en_US.UTF-8')
>>> locale.delocalize('1,234.56')

(Contributed by Cédric Krier in bpo-13918.)


All logging methods (Logger log(), exception(), critical(), debug(), etc.), now accept exception instances as an exc_info argument, in addition to boolean values and exception tuples:

>>> import logging
>>> try:
...     1/0
... except ZeroDivisionError as ex:
...     logging.error('exception', exc_info=ex)

(Contributed by Yury Selivanov in bpo-20537.)

The handlers.HTTPHandler class now accepts an optional ssl.SSLContext instance to configure SSL settings used in an HTTP connection. (Contributed by Alex Gaynor in bpo-22788.)

The handlers.QueueListener class now takes a respect_handler_level keyword argument which, if set to True, will pass messages to handlers taking handler levels into account. (Contributed by Vinay Sajip.)


The LZMADecompressor.decompress() method now accepts an optional max_length argument to limit the maximum size of decompressed data. (Contributed by Martin Panter in bpo-15955.)


Two new constants have been added to the math module: inf and nan. (Contributed by Mark Dickinson in bpo-23185.)

A new function isclose() provides a way to test for approximate equality. (Contributed by Chris Barker and Tal Einat in bpo-24270.)

A new gcd() function has been added. The fractions.gcd() function is now deprecated. (Contributed by Mark Dickinson and Serhiy Storchaka in bpo-22486.)


sharedctypes.synchronized() objects now support the context manager protocol. (Contributed by Charles-François Natali in bpo-21565.)


attrgetter(), itemgetter(), and methodcaller() objects now support pickling. (Contributed by Josh Rosenberg and Serhiy Storchaka in bpo-22955.)

New matmul() and imatmul() functions to perform matrix multiplication. (Contributed by Benjamin Peterson in bpo-21176.)


The new scandir() function returning an iterator of DirEntry objects has been added. If possible, scandir() extracts file attributes while scanning a directory, removing the need to perform subsequent system calls to determine file type or attributes, which may significantly improve performance. (Contributed by Ben Hoyt with the help of Victor Stinner in bpo-22524.)

On Windows, a new stat_result.st_file_attributes attribute is now available. It corresponds to the dwFileAttributes member of the BY_HANDLE_FILE_INFORMATION structure returned by GetFileInformationByHandle(). (Contributed by Ben Hoyt in bpo-21719.)

The urandom() function now uses the getrandom() syscall on Linux 3.17 or newer, and getentropy() on OpenBSD 5.6 and newer, removing the need to use /dev/urandom and avoiding failures due to potential file descriptor exhaustion. (Contributed by Victor Stinner in bpo-22181.)

New get_blocking() and set_blocking() functions allow getting and setting a file descriptor’s blocking mode (O_NONBLOCK.) (Contributed by Victor Stinner in bpo-22054.)

The truncate() and ftruncate() functions are now supported on Windows. (Contributed by Steve Dower in bpo-23668.)

There is a new os.path.commonpath() function returning the longest common sub-path of each passed pathname. Unlike the os.path.commonprefix() function, it always returns a valid path:

>>> os.path.commonprefix(['/usr/lib', '/usr/local/lib'])

>>> os.path.commonpath(['/usr/lib', '/usr/local/lib'])

(Contributed by Rafik Draoui and Serhiy Storchaka in bpo-10395.)


The new Path.samefile() method can be used to check whether the path points to the same file as another path, which can be either another Path object, or a string:

>>> import pathlib
>>> p1 = pathlib.Path('/etc/hosts')
>>> p2 = pathlib.Path('/etc/../etc/hosts')
>>> p1.samefile(p2)

(Contributed by Vajrasky Kok and Antoine Pitrou in bpo-19775.)

The Path.mkdir() method now accepts a new optional exist_ok argument to match mkdir -p and os.makedirs() functionality. (Contributed by Berker Peksag in bpo-21539.)

There is a new Path.expanduser() method to expand ~ and ~user prefixes. (Contributed by Serhiy Storchaka and Claudiu Popa in bpo-19776.)

A new Path.home() class method can be used to get a Path instance representing the user’s home directory. (Contributed by Victor Salgado and Mayank Tripathi in bpo-19777.)

New Path.write_text(), Path.read_text(), Path.write_bytes(), Path.read_bytes() methods to simplify read/write operations on files.

The following code snippet will create or rewrite existing file ~/spam42:

>>> import pathlib
>>> p = pathlib.Path('~/spam42')
>>> p.expanduser().write_text('ham')

(Contributed by Christopher Welborn in bpo-20218.)


Nested objects, such as unbound methods or nested classes, can now be pickled using pickle protocols older than protocol version 4. Protocol version 4 already supports these cases. (Contributed by Serhiy Storchaka in bpo-23611.)


A new POP3.utf8() command enables RFC 6856 (Internationalized Email) support, if a POP server supports it. (Contributed by Milan OberKirch in bpo-21804.)


References and conditional references to groups with fixed length are now allowed in lookbehind assertions:

>>> import re
>>> pat = re.compile(r'(a|b).(?<=\1)c')
>>> pat.match('aac')
<_sre.SRE_Match object; span=(0, 3), match='aac'>
>>> pat.match('bbc')
<_sre.SRE_Match object; span=(0, 3), match='bbc'>

(Contributed by Serhiy Storchaka in bpo-9179.)

The number of capturing groups in regular expressions is no longer limited to 100. (Contributed by Serhiy Storchaka in bpo-22437.)

The sub() and subn() functions now replace unmatched groups with empty strings instead of raising an exception. (Contributed by Serhiy Storchaka in bpo-1519638.)

The re.error exceptions have new attributes, msg, pattern, pos, lineno, and colno, that provide better context information about the error:

>>> re.compile("""
...     (?x)
...     .++
... """)
Traceback (most recent call last):
sre_constants.error: multiple repeat at position 16 (line 3, column 7)

(Contributed by Serhiy Storchaka in bpo-22578.)


A new append_history_file() function can be used to append the specified number of trailing elements in history to the given file. (Contributed by Bruno Cauet in bpo-22940.)


The new DevpollSelector supports efficient /dev/poll polling on Solaris. (Contributed by Giampaolo Rodola’ in bpo-18931.)


The move() function now accepts a copy_function argument, allowing, for example, the copy() function to be used instead of the default copy2() if there is a need to ignore file metadata when moving. (Contributed by Claudiu Popa in bpo-19840.)

The make_archive() function now supports the xztar format. (Contributed by Serhiy Storchaka in bpo-5411.)


On Windows, the set_wakeup_fd() function now also supports socket handles. (Contributed by Victor Stinner in bpo-22018.)

Various SIG* constants in the signal module have been converted into Enums. This allows meaningful names to be printed during debugging, instead of integer “magic numbers”. (Contributed by Giampaolo Rodola’ in bpo-21076.)


Both the SMTPServer and SMTPChannel classes now accept a decode_data keyword argument to determine if the DATA portion of the SMTP transaction is decoded using the "utf-8" codec or is instead provided to the SMTPServer.process_message() method 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 bpo-19662.)

The SMTPServer class now advertises the 8BITMIME extension (RFC 6152) if decode_data has been set True. If the client specifies BODY=8BITMIME on the MAIL command, it is passed to SMTPServer.process_message() via the mail_options keyword. (Contributed by Milan Oberkirch and R. David Murray in bpo-21795.)

The SMTPServer class now also supports the SMTPUTF8 extension (RFC 6531: Internationalized Email). If the client specified SMTPUTF8 BODY=8BITMIME on the MAIL command, they are passed to SMTPServer.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 bpo-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 bpo-14758.)


A new SMTP.auth() method provides a convenient way to implement custom authentication mechanisms. (Contributed by Milan Oberkirch in bpo-15014.)

The SMTP.set_debuglevel() method now accepts an additional debuglevel (2), which enables timestamps in debug messages. (Contributed by Gavin Chappell and Maciej Szulik in bpo-16914.)

Both the SMTP.sendmail() and SMTP.send_message() methods now support RFC 6531 (SMTPUTF8). (Contributed by Milan Oberkirch and R. David Murray in bpo-22027.)


The what() and whathdr() functions now return a namedtuple(). (Contributed by Claudiu Popa in bpo-18615.)


Functions with timeouts now use a monotonic clock, instead of a system clock. (Contributed by Victor Stinner in bpo-22043.)

A new socket.sendfile() method allows sending a file over a socket by using the high-performance os.sendfile() function on UNIX, resulting in uploads being from 2 to 3 times faster than when using plain socket.send(). (Contributed by Giampaolo Rodola’ in bpo-17552.)

The socket.sendall() method no longer resets the socket timeout every time bytes are received or sent. The socket timeout is now the maximum total duration to send all data. (Contributed by Victor Stinner in bpo-23853.)

The backlog argument of the socket.listen() method is now optional. By default it is set to SOMAXCONN or to 128, whichever is less. (Contributed by Charles-François Natali in bpo-21455.)


Memory BIO Support

(Contributed by Geert Jansen in bpo-21965.)

The new SSLObject class has been added to provide SSL protocol support for cases when the network I/O capabilities of SSLSocket are not necessary or are suboptimal. SSLObject represents an SSL protocol instance, but does not implement any network I/O methods, and instead provides a memory buffer interface. The new MemoryBIO class can be used to pass data between Python and an SSL protocol instance.

The memory BIO SSL support is primarily intended to be used in frameworks implementing asynchronous I/O for which SSLSocket‘s readiness model (“select/poll”) is inefficient.

A new SSLContext.wrap_bio() method can be used to create a new SSLObject instance.

Application-Layer Protocol Negotiation Support

(Contributed by Benjamin Peterson in bpo-20188.)

Where OpenSSL support is present, the ssl module now implements the Application-Layer Protocol Negotiation TLS extension as described in RFC 7301.

The new SSLContext.set_alpn_protocols() can be used to specify which protocols a socket should advertise during the TLS handshake.

The new SSLSocket.selected_alpn_protocol() returns the protocol that was selected during the TLS handshake. The HAS_ALPN flag indicates whether ALPN support is present.

Other Changes

There is a new SSLSocket.version() method to query the actual protocol version in use. (Contributed by Antoine Pitrou in bpo-20421.)

The SSLSocket class now implements a SSLSocket.sendfile() method. (Contributed by Giampaolo Rodola’ in bpo-17552.)

The SSLSocket.send() method now raises either the ssl.SSLWantReadError or ssl.SSLWantWriteError exception on a non-blocking socket if the operation would block. Previously, it would return 0. (Contributed by Nikolaus Rath in bpo-20951.)

The cert_time_to_seconds() function now interprets the input time as UTC and not as local time, per RFC 5280. Additionally, the return value is always an int. (Contributed by Akira Li in bpo-19940.)

New SSLObject.shared_ciphers() and SSLSocket.shared_ciphers() methods return the list of ciphers sent by the client during the handshake. (Contributed by Benjamin Peterson in bpo-23186.)

The SSLSocket.do_handshake(), SSLSocket.read(), SSLSocket.shutdown(), and SSLSocket.write() methods of the SSLSocket class no longer reset the socket timeout every time bytes are received or sent. The socket timeout is now the maximum total duration of the method. (Contributed by Victor Stinner in bpo-23853.)

The match_hostname() function now supports matching of IP addresses. (Contributed by Antoine Pitrou in bpo-23239.)


The Row class now fully supports the sequence protocol, in particular reversed() iteration and slice indexing. (Contributed by Claudiu Popa in bpo-10203; by Lucas Sinclair, Jessica McKellar, and Serhiy Storchaka in bpo-13583.)


The new run() function has been added. It runs the specified command and returns a CompletedProcess object, which describes a finished process. The new API is more consistent and is the recommended approach to invoking subprocesses in Python code that does not need to maintain compatibility with earlier Python versions. (Contributed by Thomas Kluyver in bpo-23342.)


>>> subprocess.run(["ls", "-l"])  # doesn't capture output
CompletedProcess(args=['ls', '-l'], returncode=0)

>>> subprocess.run("exit 1", shell=True, check=True)
Traceback (most recent call last):
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

>>> subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n')


A new set_coroutine_wrapper() function allows setting a global hook that will be called whenever a coroutine object is created by an async def function. A corresponding get_coroutine_wrapper() can be used to obtain a currently set wrapper. Both functions are provisional, and are intended for debugging purposes only. (Contributed by Yury Selivanov in bpo-24017.)

A new is_finalizing() function can be used to check if the Python interpreter is shutting down. (Contributed by Antoine Pitrou in bpo-22696.)


The name of the user scripts directory on Windows now includes the first two components of the Python version. (Contributed by Paul Moore in bpo-23437.)


The mode argument of the open() function now accepts "x" to request exclusive creation. (Contributed by Berker Peksag in bpo-21717.)

The TarFile.extractall() and TarFile.extract() methods now take a keyword argument numeric_owner. 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 by the named user and group in the tarfile. (Contributed by Michael Vogt and Eric Smith in bpo-23193.)

The TarFile.list() now accepts an optional members keyword argument that can be set to a subset of the list returned by TarFile.getmembers(). (Contributed by Serhiy Storchaka in bpo-21549.)


Both the Lock.acquire() and RLock.acquire() methods now use a monotonic clock for timeout management. (Contributed by Victor Stinner in bpo-22043.)


The monotonic() function is now always available. (Contributed by Victor Stinner in bpo-22043.)


A new command line option -u or --unit=U can be used to specify the time unit for the timer output. Supported options are usec, msec, or sec. (Contributed by Julian Gindi in bpo-18983.)

The timeit() function has a new globals parameter for specifying the namespace in which the code will be running. (Contributed by Ben Roberts in bpo-2527.)


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 bpo-20035.)


New walk_stack() and walk_tb() functions to conveniently traverse frame and traceback objects. (Contributed by Robert Collins in bpo-17911.)

New lightweight classes: TracebackException, StackSummary, and FrameSummary. (Contributed by Robert Collins in bpo-17911.)

Both the print_tb() and print_stack() functions now support negative values for the limit argument. (Contributed by Dmitry Kazakov in bpo-22619.)


A new coroutine() function to transform generator and generator-like objects into awaitables. (Contributed by Yury Selivanov in bpo-24017.)

A new type called CoroutineType, which is used for coroutine objects created by async def functions. (Contributed by Yury Selivanov in bpo-24400.)


The unicodedata module now uses data from Unicode 8.0.0.


The TestLoader.loadTestsFromModule() method now accepts a keyword-only argument pattern which is passed to load_tests as the third argument. Found packages are now checked for load_tests regardless of whether their path matches pattern, because it is impossible for a package name to match the default pattern. (Contributed by Robert Collins and Barry A. Warsaw in bpo-16662.)

Unittest discovery errors now are exposed in the TestLoader.errors attribute of the TestLoader instance. (Contributed by Robert Collins in bpo-19746.)

A new command line option --locals to show local variables in tracebacks. (Contributed by Robert Collins in bpo-22936.)


The Mock class has the following improvements:

  • The class constructor has a new unsafe parameter, which causes mock objects to raise AttributeError on attribute names starting with "assert". (Contributed by Kushal Das in bpo-21238.)
  • A new Mock.assert_not_called() method to check if the mock object was called. (Contributed by Kushal Das in bpo-21262.)

The MagicMock class now supports __truediv__(), __divmod__() and __matmul__() operators. (Contributed by Johannes Baiter in bpo-20968, and Håkan Lövdahl in bpo-23581 and bpo-23568.)

It is no longer necessary to explicitly pass create=True to the patch() function when patching builtin names. (Contributed by Kushal Das in bpo-17660.)


A new request.HTTPPasswordMgrWithPriorAuth class 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 bpo-19494 and Akshit Khurana in bpo-7159.)

A new quote_via argument for the parse.urlencode() function provides a way to control the encoding of query parts if needed. (Contributed by Samwyse and Arnon Yaari in bpo-13866.)

The request.urlopen() function accepts an ssl.SSLContext object as a context argument, which will be used for the HTTPS connection. (Contributed by Alex Gaynor in bpo-22366.)

The parse.urljoin() was updated to use the RFC 3986 semantics for the resolution of relative URLs, rather than RFC 1808 and RFC 2396. (Contributed by Demian Brecht and Senthil Kumaran in bpo-22118.)


The headers argument of the headers.Headers class constructor is now optional. (Contributed by Pablo Torres Navarrete and SilentGhost in bpo-5800.)


The client.ServerProxy class now supports the context manager protocol. (Contributed by Claudiu Popa in bpo-20627.)

The client.ServerProxy constructor now accepts an optional ssl.SSLContext instance. (Contributed by Alex Gaynor in bpo-22960.)


SAX parsers now support a character stream of the xmlreader.InputSource object. (Contributed by Serhiy Storchaka in bpo-2175.)

parseString() now accepts a str instance. (Contributed by Serhiy Storchaka in bpo-10590.)


ZIP output can now be written to unseekable streams. (Contributed by Serhiy Storchaka in bpo-23252.)

The mode argument of ZipFile.open() method now accepts "x" to request exclusive creation. (Contributed by Serhiy Storchaka in bpo-21717.)

Other module-level changes

Many functions in the mmap, ossaudiodev, socket, ssl, and codecs modules now accept writable bytes-like objects. (Contributed by Serhiy Storchaka in bpo-23001.)


The os.walk() function has been sped up by 3 to 5 times on POSIX systems, and by 7 to 20 times on Windows. This was done using the new os.scandir() function, which exposes file information from the underlying readdir or FindFirstFile/FindNextFile system calls. (Contributed by Ben Hoyt with help from Victor Stinner in bpo-23605.)

Construction of bytes(int) (filled by zero bytes) is faster and uses less memory for large objects. calloc() is used instead of malloc() to allocate memory for these objects. (Contributed by Victor Stinner in bpo-21233.)

Some operations on ipaddress IPv4Network and IPv6Network have been massively sped up, such as subnets(), supernet(), summarize_address_range(), collapse_addresses(). The speed up can range from 3 to 15 times. (Contributed by Antoine Pitrou, Michel Albert, and Markus in bpo-21486, bpo-21487, bpo-20826, bpo-23266.)

Pickling of ipaddress objects was optimized to produce significantly smaller output. (Contributed by Serhiy Storchaka in bpo-23133.)

Many operations on io.BytesIO are now 50% to 100% faster. (Contributed by Serhiy Storchaka in bpo-15381 and David Wilson in bpo-22003.)

The marshal.dumps() function is now faster: 65–85% with versions 3 and 4, 20–25% with versions 0 to 2 on typical data, and up to 5 times in best cases. (Contributed by Serhiy Storchaka in bpo-20416 and bpo-23344.)

The UTF-32 encoder is now 3 to 7 times faster. (Contributed by Serhiy Storchaka in bpo-15027.)

Regular expressions are now parsed up to 10% faster. (Contributed by Serhiy Storchaka in bpo-19380.)

The json.dumps() function was optimized to run with ensure_ascii=False as fast as with ensure_ascii=True. (Contributed by Naoki Inada in bpo-23206.)

The PyObject_IsInstance() and PyObject_IsSubclass() functions have been sped up in the common case that the second argument has type as its metaclass. (Contributed Georg Brandl by in bpo-22540.)

Method caching was slightly improved, yielding up to 5% performance improvement in some benchmarks. (Contributed by Antoine Pitrou in bpo-22847.)

Objects from the random module now use 50% less memory on 64-bit builds. (Contributed by Serhiy Storchaka in bpo-23488.)

The property() getter calls are up to 25% faster. (Contributed by Joe Jevnik in bpo-23910.)

Instantiation of fractions.Fraction is now up to 30% faster. (Contributed by Stefan Behnel in bpo-22464.)

String methods find(), rfind(), split(), partition() and the in string operator are now significantly faster for searching 1-character substrings. (Contributed by Serhiy Storchaka in bpo-23573.)

Build and C API Changes

New calloc functions were added:

(Contributed by Victor Stinner in bpo-21233.)

New encoding/decoding helper functions:

(Contributed by Victor Stinner in bpo-18395.)

A new PyCodec_NameReplaceErrors() function to replace the unicode encode error with \N{...} escapes. (Contributed by Serhiy Storchaka in bpo-19676.)

A new PyErr_FormatV() function similar to PyErr_Format(), but accepts a va_list argument. (Contributed by Antoine Pitrou in bpo-18711.)

A new PyExc_RecursionError exception. (Contributed by Georg Brandl in bpo-19235.)

New PyModule_FromDefAndSpec(), PyModule_FromDefAndSpec2(), and PyModule_ExecDef() functions introduced by PEP 489 – multi-phase extension module initialization. (Contributed by Petr Viktorin in bpo-24268.)

New PyNumber_MatrixMultiply() and PyNumber_InPlaceMatrixMultiply() functions to perform matrix multiplication. (Contributed by Benjamin Peterson in bpo-21176. See also PEP 465 for details.)

The PyTypeObject.tp_finalize slot is now part of the stable ABI.

Windows builds now require Microsoft Visual C++ 14.0, which is available as part of Visual Studio 2015.

Extension modules now include a platform information tag in their filename on some platforms (the tag is optional, and CPython will import extensions without it, although if the tag is present and mismatched, the extension won’t be loaded):

  • On Linux, extension module filenames end with .cpython-<major><minor>m-<architecture>-<os>.pyd:
    • <major> is the major number of the Python version; for Python 3.5 this is 3.
    • <minor> is the minor number of the Python version; for Python 3.5 this is 5.
    • <architecture> is the hardware architecture the extension module was built to run on. It’s most commonly either i386 for 32-bit Intel platforms or x86_64 for 64-bit Intel (and AMD) platforms.
    • <os> is always linux-gnu, except for extensions built to talk to the 32-bit ABI on 64-bit platforms, in which case it is linux-gnu32 (and <architecture> will be x86_64).
  • On Windows, extension module filenames end with <debug>.cp<major><minor>-<platform>.pyd:
    • <major> is the major number of the Python version; for Python 3.5 this is 3.
    • <minor> is the minor number of the Python version; for Python 3.5 this is 5.
    • <platform> is the platform the extension module was built for, either win32 for Win32, win_amd64 for Win64, win_ia64 for Windows Itanium 64, and win_arm for Windows on ARM.
    • If built in debug mode, <debug> will be _d, otherwise it will be blank.
  • On OS X platforms, extension module filenames now end with -darwin.so.
  • On all other platforms, extension module filenames are the same as they were with Python 3.4.


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.

Deprecated Python Behavior

Raising the StopIteration exception inside a generator will now generate a silent PendingDeprecationWarning, which will become a non-silent deprecation warning in Python 3.6 and will trigger a RuntimeError in Python 3.7. See PEP 479: Change StopIteration handling inside generators for details.

Unsupported Operating Systems

Windows XP is no longer supported by Microsoft, thus, per PEP 11, CPython 3.5 is no longer officially supported on this OS.

Deprecated Python modules, functions and methods

The formatter module has now graduated to full deprecation and is still slated for removal in Python 3.6.

The asyncio.async() function is deprecated in favor of ensure_future().

The smtpd module has in the past always decoded the DATA portion of email messages using the utf-8 codec. This can now be controlled by the new decode_data keyword to SMTPServer. The default value is True, but this default is deprecated. Specify the decode_data keyword with an appropriate value to avoid the deprecation warning.

Directly assigning values to the key, value and coded_value of http.cookies.Morsel objects is deprecated. Use the set() method instead. In addition, the undocumented LegalChars parameter of set() is deprecated, and is now ignored.

Passing a format string as keyword argument format_string to the format() method of the string.Formatter class has been deprecated. (Contributed by Serhiy Storchaka in bpo-23671.)

The platform.dist() and platform.linux_distribution() functions are now deprecated. Linux distributions use too many different ways of describing themselves, so the functionality is left to a package. (Contributed by Vajrasky Kok and Berker Peksag in bpo-1322.)

The previously undocumented from_function and from_builtin methods of inspect.Signature are deprecated. Use the new Signature.from_callable() method instead. (Contributed by Yury Selivanov in bpo-24248.)

The inspect.getargspec() function is deprecated and scheduled to be removed in Python 3.6. (See bpo-20438 for details.)

The inspect getfullargspec(), getcallargs(), and formatargspec() functions are deprecated in favor of the inspect.signature() API. (Contributed by Yury Selivanov in bpo-20438.)

getargvalues() and formatargvalues() functions were inadvertently marked as deprecated with the release of Python 3.5.0.

Use of re.LOCALE flag with str patterns or re.ASCII is now deprecated. (Contributed by Serhiy Storchaka in bpo-22407.)

Use of unrecognized special sequences consisting of '\' and an ASCII letter in regular expression patterns and replacement patterns now raises a deprecation warning and will be forbidden in Python 3.6. (Contributed by Serhiy Storchaka in bpo-23622.)

The undocumented and unofficial use_load_tests default argument of the unittest.TestLoader.loadTestsFromModule() method now is deprecated and ignored. (Contributed by Robert Collins and Barry A. Warsaw in bpo-16662.)


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 bpo-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. (Contributed by A. Jesse Jiryu Davis in bpo-23464.)

Porting to Python 3.5

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

Changes in Python behavior

  • Due to an oversight, earlier Python versions erroneously accepted the following syntax:

    f(1 for x in [1], *args)
    f(1 for x in [1], **kwargs)

    Python 3.5 now correctly raises a SyntaxError, as generator expressions must be put in parentheses if not a sole argument to a function.

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 bpo-13936 for full details.
  • The ssl.SSLSocket.send() method now raises either ssl.SSLWantReadError or ssl.SSLWantWriteError on a non-blocking socket if the operation would block. Previously, it would return 0. (Contributed by Nikolaus Rath in bpo-20951.)
  • The __name__ attribute of generators 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)). (Contributed by Victor Stinner in bpo-21205.)
  • The deprecated “strict” mode and argument of HTMLParser, HTMLParser.error(), and the HTMLParserError exception have been removed. (Contributed by Ezio Melotti in bpo-15114.) The convert_charrefs argument of HTMLParser is now True by default. (Contributed by Berker Peksag in bpo-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 bpo-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 (bpo-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 bpo-21793.)
  • When an import loader defines importlib.machinery.Loader.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. (Contributed by Brett Cannon in bpo-23014.)
  • The re.split() function 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. (Contributed by Serhiy Storchaka in bpo-22818.)
  • The http.cookies.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. (Contributed by Demian Brecht in bpo-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 ssl.cert_time_to_seconds() function now interprets the input time as UTC and not as local time, per RFC 5280. Additionally, the return value is always an int. (Contributed by Akira Li in bpo-19940.)
  • 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 the 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. (Contributed by Serhiy Storchaka in bpo-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. (Contributed by Serhiy Storchaka in bpo-15582.)
  • Nested functools.partial() calls are now flattened. If you were relying on the previous behavior, you can now either add an attribute to a functools.partial() object or you can create a subclass of functools.partial(). (Contributed by Alexander Belopolsky in bpo-7830.)

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. (Contributed by Serhiy Storchaka in bpo-22453.)
  • Because the lack of the __module__ attribute breaks pickling and introspection, a deprecation warning is now raised for builtin types without the __module__ attribute. This would be an AttributeError in the future. (Contributed by Serhiy Storchaka in bpo-20204.)
  • As part of the PEP 492 implementation, the tp_reserved slot of PyTypeObject was replaced with a tp_as_async slot. Refer to Coroutine Objects for new types, structures and functions.