What’s New in Python

Release 3.6.4

A. M. Kuchling

February 24, 2018

Python Software Foundation
Email: docs@python.org

Contents

1 Summary – Release highlights 4

2 New Features 5
2.1 PEP 498: Formatted string literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 PEP 526: Syntax for variable annotations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 PEP 515: Underscores in Numeric Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 PEP 525: Asynchronous Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5 PEP 530: Asynchronous Comprehensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.6 PEP 487: Simpler customization of class creation . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.7 PEP 487: Descriptor Protocol Enhancements . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.8 PEP 519: Adding a file system path protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.9 PEP 495: Local Time Disambiguation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.10 PEP 529: Change Windows filesystem encoding to UTF-8 . . . . . . . . . . . . . . . . . . . . 9
2.11 PEP 528: Change Windows console encoding to UTF-8 . . . . . . . . . . . . . . . . . . . . . 9
2.12 PEP 520: Preserving Class Attribute Definition Order . . . . . . . . . . . . . . . . . . . . . . 10
2.13 PEP 468: Preserving Keyword Argument Order . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.14 New dict implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.15 PEP 523: Adding a frame evaluation API to CPython . . . . . . . . . . . . . . . . . . . . . . 10
2.16 PYTHONMALLOC environment variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.17 DTrace and SystemTap probing support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Other Language Changes 12

4 New Modules 12
4.1 secrets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Improved Modules 13
5.1 array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2 ast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.3 asyncio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.4 binascii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.5 cmath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.6 collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.7 concurrent.futures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.8 contextlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.9 datetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.10 decimal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.11 distutils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.12 email . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.13 encodings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.14 enum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.15 faulthandler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.16 fileinput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.17 hashlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.18 http.client . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.19 idlelib and IDLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.20 importlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.21 inspect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.22 json . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.23 logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.24 math . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.25 multiprocessing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.26 os . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.27 pathlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.28 pdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.29 pickle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.30 pickletools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.31 pydoc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.32 random . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.33 re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.34 readline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.35 rlcompleter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.36 shlex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.37 site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.38 sqlite3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.39 socket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.40 socketserver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.41 ssl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.42 statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.43 struct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.44 subprocess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.45 sys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.46 telnetlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.47 time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.48 timeit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.49 tkinter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.50 traceback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.51 tracemalloc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.52 typing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.53 unicodedata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.54 unittest.mock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.55 urllib.request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.56 urllib.robotparser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.57 venv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.58 warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.59 winreg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.60 winsound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.61 xmlrpc.client . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.62 zipfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
 5.63 zlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6 Optimizations 24

7 Build and C API Changes 25

8 Other Improvements 26

9 Deprecated 26
9.1 New Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.2 Deprecated Python behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.3 Deprecated Python modules, functions and methods . . . . . . . . . . . . . . . . . . . . . . . 27
asynchat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
asyncore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
dbm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
distutils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
grp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
importlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
os . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
ssl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
tkinter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
venv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.4 Deprecated functions and types of the C API . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.5 Deprecated Build Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

10 Removed 28
10.1 API and Feature Removals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

11 Porting to Python 3.6 29

11.1 Changes in ‘python’ Command Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.2 Changes in the Python API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.3 Changes in the C API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
11.4 CPython bytecode changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

12 Notable changes in Python 3.6.2 32

12.1 New make regen-all build target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.2 Removal of make touch build target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

13 Notable changes in Python 3.6.5 32

Index 33

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

This article explains the new features in Python 3.6, compared to 3.5. Python 3.6 was released on December
23, 2016. For full details, see the changelog.
See also:
PEP 494 - Python 3.6 Release Schedule
1 Summary – Release highlights
New syntax features:
• PEP 498, formatted string literals.
• PEP 515, underscores in numeric literals.
• PEP 526, syntax for variable annotations.
• PEP 525, asynchronous generators.
• PEP 530: asynchronous comprehensions.
New library modules:
• secrets: PEP 506 – Adding A Secrets Module To The Standard Library.
CPython implementation improvements:
• The dict type has been reimplemented to use a more compact representation based on a proposal by
Raymond Hettinger and similar to the PyPy dict implementation. This resulted in dictionaries using
20% to 25% less memory when compared to Python 3.5.
• Customization of class creation has been simplified with the new protocol.
• The class attribute definition order is now preserved.
• The order of elements in **kwargs now corresponds to the order in which keyword arguments were
passed to the function.
• DTrace and SystemTap probing support has been added.
• The new PYTHONMALLOC environment variable can now be used to debug the interpreter memory
allocation and access errors.
Significant improvements in the standard library:
• The asyncio module has received new features, significant usability and performance improvements,
and a fair amount of bug fixes. Starting with Python 3.6 the asyncio module is no longer provisional
and its API is considered stable.
• A new file system path protocol has been implemented to support path-like objects. All standard library
functions operating on paths have been updated to work with the new protocol.
• The datetime module has gained support for Local Time Disambiguation.
• The typing module received a number of improvements.
• The tracemalloc module has been significantly reworked and is now used to provide better output
for ResourceWarning as well as provide better diagnostics for memory allocation errors. See the
PYTHONMALLOC section for more information.
Security improvements:
• The new secrets module has been added to simplify the generation of cryptographically strong pseudo-
random numbers suitable for managing secrets such as account authentication, tokens, and similar.
• On Linux, os.urandom() now blocks until the system urandom entropy pool is initialized to increase
the security. See the PEP 524 for the rationale.
• The hashlib and ssl modules now support OpenSSL 1.1.0.
• The default settings and feature set of the ssl module have been improved.
• The hashlib module received support for the BLAKE2, SHA-3 and SHAKE hash algorithms and the
scrypt() key derivation function.
Windows improvements:
 • PEP 528 and PEP 529, Windows filesystem and console encoding changed to UTF-8.
• The py.exe launcher, when used interactively, no longer prefers Python 2 over Python 3 when the
user doesn’t specify a version (via command line arguments or a config file). Handling of shebang lines
remains unchanged - “python” refers to Python 2 in that case.
• python.exe and pythonw.exe have been marked as long-path aware, which means that the 260 char-
acter path limit may no longer apply. See removing the MAX_PATH limitation for details.
• A ._pth file can be added to force isolated mode and fully specify all search paths to avoid registry
and environment lookup. See the documentation for more information.
• A python36.zip file now works as a landmark to infer PYTHONHOME. See the documentation for more
information.

2 New Features

2.1 PEP 498: Formatted string literals

PEP 498 introduces a new kind of string literals: f-strings, or formatted string literals.
Formatted string literals are prefixed with 'f' and are similar to the format strings accepted by str.
format(). They contain replacement fields surrounded by curly braces. The replacement fields are expres-
sions, which are evaluated at run time, and then formatted using the format() protocol:

>>> name = "Fred"

>>> f"He said his name is {name}."
'He said his name is Fred.'
>>> width = 10
>>> precision = 4
>>> value = decimal.Decimal("12.34567")
>>> f"result: {value:{width}.{precision}}" # nested fields
'result: 12.35'

See also:
PEP 498 – Literal String Interpolation. PEP written and implemented by Eric V. Smith.
Feature documentation.

2.2 PEP 526: Syntax for variable annotations

PEP 484 introduced the standard for type annotations of function parameters, a.k.a. type hints. This PEP
adds syntax to Python for annotating the types of variables including class variables and instance variables:

primes: List[int] = []

captain: str # Note: no initial value!

class Starship:
stats: Dict[str, int] = {}

Just as for function annotations, the Python interpreter does not attach any particular meaning to variable
annotations and only stores them in the __annotations__ attribute of a class or module.
In contrast to variable declarations in statically typed languages, the goal of annotation syntax is to provide
an easy way to specify structured type metadata for third party tools and libraries via the abstract syntax
tree and the __annotations__ attribute.
See also:
PEP 526 – Syntax for variable annotations. PEP written by Ryan Gonzalez, Philip House, Ivan Lev-
kivskyi, Lisa Roach, and Guido van Rossum. Implemented by Ivan Levkivskyi.
Tools that use or will use the new syntax: mypy, pytype, PyCharm, etc.

2.3 PEP 515: Underscores in Numeric Literals

PEP 515 adds the ability to use underscores in numeric literals for improved readability. For example:

>>> 1_000_000_000_000_000
1000000000000000
>>> 0x_FF_FF_FF_FF
4294967295

Single underscores are allowed between digits and after any base specifier. Leading, trailing, or multiple
underscores in a row are not allowed.
The string formatting language also now has support for the '_' option to signal the use of an underscore
for a thousands separator for floating point presentation types and for integer presentation type 'd'. For
integer presentation types 'b', 'o', 'x', and 'X', underscores will be inserted every 4 digits:

>>> '{:_}'.format(1000000)
'1_000_000'
>>> '{:_x}'.format(0xFFFFFFFF)
'ffff_ffff'

See also:
PEP 515 – Underscores in Numeric Literals PEP written by Georg Brandl and Serhiy Storchaka.

2.4 PEP 525: Asynchronous Generators

PEP 492 introduced support for native coroutines and async / await syntax to Python 3.5. A notable
limitation of the Python 3.5 implementation is that it was not possible to use await and yield in the same
function body. In Python 3.6 this restriction has been lifted, making it possible to define asynchronous
generators:

async def ticker(delay, to):

"""Yield numbers from 0 to *to* every *delay* seconds."""
for i in range(to):
yield i
await asyncio.sleep(delay)

The new syntax allows for faster and more concise code.
See also:
PEP 525 – Asynchronous Generators PEP written and implemented by Yury Selivanov.

2.5 PEP 530: Asynchronous Comprehensions

PEP 530 adds support for using async for in list, set, dict comprehensions and generator expressions:

result = [i async for i in aiter() if i % 2]

Additionally, await expressions are supported in all kinds of comprehensions:
result = [await fun() for fun in funcs if await condition()]

See also:
PEP 530 – Asynchronous Comprehensions PEP written and implemented by Yury Selivanov.

2.6 PEP 487: Simpler customization of class creation

It is now possible to customize subclass creation without using a metaclass. The new __init_subclass__
classmethod will be called on the base class whenever a new subclass is created:
class PluginBase:
subclasses = []

def __init_subclass__(cls, **kwargs):

super().__init_subclass__(**kwargs)
cls.subclasses.append(cls)

class Plugin1(PluginBase):
pass

class Plugin2(PluginBase):
pass

In order to allow zero-argument super() calls to work correctly from __init_subclass__() implemen-
tations, custom metaclasses must ensure that the new __classcell__ namespace entry is propagated to
type.__new__ (as described in class-object-creation).
See also:
PEP 487 – Simpler customization of class creation PEP written and implemented by Martin Teich-
mann.
Feature documentation

2.7 PEP 487: Descriptor Protocol Enhancements

PEP 487 extends the descriptor protocol to include the new optional __set_name__() method. Whenever
a new class is defined, the new method will be called on all descriptors included in the definition, providing
them with a reference to the class being defined and the name given to the descriptor within the class
namespace. In other words, instances of descriptors can now know the attribute name of the descriptor in
the owner class:
class IntField:
def __get__(self, instance, owner):
return instance.__dict__[self.name]

def __set__(self, instance, value):

if not isinstance(value, int):
raise ValueError(f'expecting integer in {self.name}')
instance.__dict__[self.name] = value

# this is the new initializer:

def __set_name__(self, owner, name):
self.name = name
class Model:
int_field = IntField()

See also:
PEP 487 – Simpler customization of class creation PEP written and implemented by Martin Teich-
mann.
Feature documentation

2.8 PEP 519: Adding a file system path protocol

File system paths have historically been represented as str or bytes objects. This has led to people who
write code which operate on file system paths to assume that such objects are only one of those two types (an
int representing a file descriptor does not count as that is not a file path). Unfortunately that assumption
prevents alternative object representations of file system paths like pathlib from working with pre-existing
code, including Python’s standard library.
To fix this situation, a new interface represented by os.PathLike has been defined. By implementing the
__fspath__() method, an object signals that it represents a path. An object can then provide a low-level
representation of a file system path as a str or bytes object. This means an object is considered path-like
if it implements os.PathLike or is a str or bytes object which represents a file system path. Code can use
os.fspath(), os.fsdecode(), or os.fsencode() to explicitly get a str and/or bytes representation of a
path-like object.
The built-in open() function has been updated to accept os.PathLike objects, as have all relevant functions
in the os and os.path modules, and most other functions and classes in the standard library. The os.
DirEntry class and relevant classes in pathlib have also been updated to implement os.PathLike.
The hope is that updating the fundamental functions for operating on file system paths will lead to third-
party code to implicitly support all path-like objects without any code changes, or at least very minimal
ones (e.g. calling os.fspath() at the beginning of code before operating on a path-like object).
Here are some examples of how the new interface allows for pathlib.Path to be used more easily and
transparently with pre-existing code:

>>> import pathlib

>>> with open(pathlib.Path("README")) as f:
... contents = f.read()
...
>>> import os.path
>>> os.path.splitext(pathlib.Path("some_file.txt"))
('some_file', '.txt')
>>> os.path.join("/a/b", pathlib.Path("c"))
'/a/b/c'
>>> import os
>>> os.fspath(pathlib.Path("some_file.txt"))
'some_file.txt'

(Implemented by Brett Cannon, Ethan Furman, Dusty Phillips, and Jelle Zijlstra.)
See also:
PEP 519 – Adding a file system path protocol PEP written by Brett Cannon and Koos Zevenhoven.
2.9 PEP 495: Local Time Disambiguation
In most world locations, there have been and will be times when local clocks are moved back. In those times,
intervals are introduced in which local clocks show the same time twice in the same day. In these situations,
the information displayed on a local clock (or stored in a Python datetime instance) is insufficient to identify
a particular moment in time.
PEP 495 adds the new fold attribute to instances of datetime.datetime and datetime.time classes to
differentiate between two moments in time for which local times are the same:

>>> u0 = datetime(2016, 11, 6, 4, tzinfo=timezone.utc)

>>> for i in range(4):
... u = u0 + i*HOUR
... t = u.astimezone(Eastern)
... print(u.time(), 'UTC =', t.time(), t.tzname(), t.fold)
...
04:00:00 UTC = 00:00:00 EDT 0
05:00:00 UTC = 01:00:00 EDT 0
06:00:00 UTC = 01:00:00 EST 1
07:00:00 UTC = 02:00:00 EST 0

The values of the fold attribute have the value 0 for all instances except those that represent the second
(chronologically) moment in time in an ambiguous case.
See also:
PEP 495 – Local Time Disambiguation PEP written by Alexander Belopolsky and Tim Peters, imple-
mentation by Alexander Belopolsky.

2.10 PEP 529: Change Windows filesystem encoding to UTF-8

Representing filesystem paths is best performed with str (Unicode) rather than bytes. However, there are
some situations where using bytes is sufficient and correct.
Prior to Python 3.6, data loss could result when using bytes paths on Windows. With this change, using
bytes to represent paths is now supported on Windows, provided those bytes are encoded with the encoding
returned by sys.getfilesystemencoding(), which now defaults to 'utf-8'.
Applications that do not use str to represent paths should use os.fsencode() and os.fsdecode() to ensure
their bytes are correctly encoded. To revert to the previous behaviour, set PYTHONLEGACYWINDOWSFSENCODING
or call sys._enablelegacywindowsfsencoding().
See PEP 529 for more information and discussion of code modifications that may be required.

2.11 PEP 528: Change Windows console encoding to UTF-8

The default console on Windows will now accept all Unicode characters and provide correctly read str objects
to Python code. sys.stdin, sys.stdout and sys.stderr now default to utf-8 encoding.
This change only applies when using an interactive console, and not when redirecting files or pipes. To revert
to the previous behaviour for interactive console use, set PYTHONLEGACYWINDOWSSTDIO.
See also:
PEP 528 – Change Windows console encoding to UTF-8 PEP written and implemented by Steve
Dower.
2.12 PEP 520: Preserving Class Attribute Definition Order
Attributes in a class definition body have a natural ordering: the same order in which the names appear in
the source. This order is now preserved in the new class’s __dict__ attribute.
Also, the effective default class execution namespace (returned from type.__prepare__()) is now an
insertion-order-preserving mapping.
See also:
PEP 520 – Preserving Class Attribute Definition Order PEP written and implemented by Eric
Snow.

2.13 PEP 468: Preserving Keyword Argument Order

**kwargs in a function signature is now guaranteed to be an insertion-order-preserving mapping.
See also:
PEP 468 – Preserving Keyword Argument Order PEP written and implemented by Eric Snow.

2.14 New dict implementation

The dict type now uses a “compact” representation based on a proposal by Raymond Hettinger which was
first implemented by PyPy. The memory usage of the new dict() is between 20% and 25% smaller compared
to Python 3.5.
The order-preserving aspect of this new implementation is considered an implementation detail and should
not be relied upon (this may change in the future, but it is desired to have this new dict implementation in
the language for a few releases before changing the language spec to mandate order-preserving semantics for
all current and future Python implementations; this also helps preserve backwards-compatibility with older
versions of the language where random iteration order is still in effect, e.g. Python 3.5).
(Contributed by INADA Naoki in bpo-27350. Idea originally suggested by Raymond Hettinger.)

2.15 PEP 523: Adding a frame evaluation API to CPython

While Python provides extensive support to customize how code executes, one place it has not done so is
in the evaluation of frame objects. If you wanted some way to intercept frame evaluation in Python there
really wasn’t any way without directly manipulating function pointers for defined functions.
PEP 523 changes this by providing an API to make frame evaluation pluggable at the C level. This will
allow for tools such as debuggers and JITs to intercept frame evaluation before the execution of Python
code begins. This enables the use of alternative evaluation implementations for Python code, tracking frame
evaluation, etc.
This API is not part of the limited C API and is marked as private to signal that usage of this API is
expected to be limited and only applicable to very select, low-level use-cases. Semantics of the API will
change with Python as necessary.
See also:
PEP 523 – Adding a frame evaluation API to CPython PEP written by Brett Cannon and Dino
Viehland.
2.16 PYTHONMALLOC environment variable
The new PYTHONMALLOC environment variable allows setting the Python memory allocators and installing
debug hooks.
It is now possible to install debug hooks on Python memory allocators on Python compiled in release mode
using PYTHONMALLOC=debug. Effects of debug hooks:
• Newly allocated memory is filled with the byte 0xCB
• Freed memory is filled with the byte 0xDB
• Detect violations of the Python memory allocator API. For example, PyObject_Free() called on a
memory block allocated by PyMem_Malloc().
• Detect writes before the start of a buffer (buffer underflows)
• Detect writes after the end of a buffer (buffer overflows)
• Check that the GIL is held when allocator functions of PYMEM_DOMAIN_OBJ (ex: PyObject_Malloc())
and PYMEM_DOMAIN_MEM (ex: PyMem_Malloc()) domains are called.
Checking if the GIL is held is also a new feature of Python 3.6.
See the PyMem_SetupDebugHooks() function for debug hooks on Python memory allocators.
It is now also possible to force the usage of the malloc() allocator of the C library for all Python memory
allocations using PYTHONMALLOC=malloc. This is helpful when using external memory debuggers like Valgrind
on a Python compiled in release mode.
On error, the debug hooks on Python memory allocators now use the tracemalloc module to get the
traceback where a memory block was allocated.
Example of fatal error on buffer overflow using python3.6 -X tracemalloc=5 (store 5 frames in traces):

Debug memory block at address p=0x7fbcd41666f8: API 'o'

4 bytes originally requested
The 7 pad bytes at p-7 are FORBIDDENBYTE, as expected.
The 8 pad bytes at tail=0x7fbcd41666fc are not all FORBIDDENBYTE (0xfb):
at tail+0: 0x02 *** OUCH
at tail+1: 0xfb
at tail+2: 0xfb
at tail+3: 0xfb
at tail+4: 0xfb
at tail+5: 0xfb
at tail+6: 0xfb
at tail+7: 0xfb
The block was made by call #1233329 to debug malloc/realloc.
Data at p: 1a 2b 30 00

Memory block allocated at (most recent call first):

File "test/test_bytes.py", line 323
File "unittest/case.py", line 600
File "unittest/case.py", line 648
File "unittest/suite.py", line 122
File "unittest/suite.py", line 84

Fatal Python error: bad trailing pad byte

Current thread 0x00007fbcdbd32700 (most recent call first):

File "test/test_bytes.py", line 323 in test_hex
File "unittest/case.py", line 600 in run
File "unittest/case.py", line 648 in __call__
 File "unittest/suite.py", line 122 in run
File "unittest/suite.py", line 84 in __call__
File "unittest/suite.py", line 122 in run
File "unittest/suite.py", line 84 in __call__
...

(Contributed by Victor Stinner in bpo-26516 and bpo-26564.)

2.17 DTrace and SystemTap probing support

Python can now be built --with-dtrace which enables static markers for the following events in the inter-
preter:
• function call/return
• garbage collection started/finished
• line of code executed.
This can be used to instrument running interpreters in production, without the need to recompile specific
debug builds or providing application-specific profiling/debugging code.
More details in instrumentation.
The current implementation is tested on Linux and macOS. Additional markers may be added in the future.
(Contributed by Łukasz Langa in bpo-21590, based on patches by Jesús Cea Avión, David Malcolm, and
Nikhil Benesch.)

3 Other Language Changes

Some smaller changes made to the core Python language are:
• A global or nonlocal statement must now textually appear before the first use of the affected name
in the same scope. Previously this was a SyntaxWarning.
• It is now possible to set a special method to None to indicate that the corresponding operation is not
available. For example, if a class sets __iter__() to None, the class is not iterable. (Contributed by
Andrew Barnert and Ivan Levkivskyi in bpo-25958.)
• Long sequences of repeated traceback lines are now abbreviated as "[Previous line repeated
{count} more times]" (see traceback for an example). (Contributed by Emanuel Barry in bpo-
26823.)
• Import now raises the new exception ModuleNotFoundError (subclass of ImportError) when it cannot
find a module. Code that currently checks for ImportError (in try-except) will still work. (Contributed
by Eric Snow in bpo-15767.)
• Class methods relying on zero-argument super() will now work correctly when called from metaclass
methods during class creation. (Contributed by Martin Teichmann in bpo-23722.)

4 New Modules

4.1 secrets
The main purpose of the new secrets module is to provide an obvious way to reliably generate cryptograph-
ically strong pseudo-random values suitable for managing secrets, such as account authentication, tokens,
and similar.

Warning: Note that the pseudo-random generators in the random module should NOT be used for
security purposes. Use secrets on Python 3.6+ and os.urandom() on Python 3.5 and earlier.

See also:
PEP 506 – Adding A Secrets Module To The Standard Library PEP written and implemented by
Steven D’Aprano.

5 Improved Modules

5.1 array
Exhausted iterators of array.array will now stay exhausted even if the iterated array is extended. This is
consistent with the behavior of other mutable sequences.
Contributed by Serhiy Storchaka in bpo-26492.

5.2 ast
The new ast.Constant AST node has been added. It can be used by external AST optimizers for the
purposes of constant folding.
Contributed by Victor Stinner in bpo-26146.

5.3 asyncio
Starting with Python 3.6 the asyncio module is no longer provisional and its API is considered stable.
Notable changes in the asyncio module since Python 3.5.0 (all backported to 3.5.x due to the provisional
status):
• The get_event_loop() function has been changed to always return the currently running loop when
called from couroutines and callbacks. (Contributed by Yury Selivanov in bpo-28613.)
• 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.)
• 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 in bpo-27041.)
• New loop.get_exception_handler() method to get the current exception handler. (Contributed by
Yury Selivanov in bpo-27040.)
• New StreamReader.readuntil() method to read data from the stream until a separator bytes se-
quence appears. (Contributed by Mark Korenberg.)
 • The performance of StreamReader.readexactly() has been improved. (Contributed by Mark Koren-
berg in bpo-28370.)
• The loop.getaddrinfo() method is optimized to avoid calling the system getaddrinfo function if
the address is already resolved. (Contributed by A. Jesse Jiryu Davis.)
• The loop.stop() method has been changed to stop the loop immediately after the current iteration.
Any new callbacks scheduled as a result of the last iteration will be discarded. (Contributed by Guido
van Rossum in bpo-25593.)
• Future.set_exception will now raise TypeError when passed an instance of the StopIteration
exception. (Contributed by Chris Angelico in bpo-26221.)
• New loop.connect_accepted_socket() method to be used by servers that accept connections outside
of asyncio, but that use asyncio to handle them. (Contributed by Jim Fulton in bpo-27392.)
• TCP_NODELAY flag is now set for all TCP transports by default. (Contributed by Yury Selivanov in
bpo-27456.)
• New loop.shutdown_asyncgens() to properly close pending asynchronous generators before closing
the loop. (Contributed by Yury Selivanov in bpo-28003.)
• Future and Task classes now have an optimized C implementation which makes asyncio code up to
30% faster. (Contributed by Yury Selivanov and INADA Naoki in bpo-26081 and bpo-28544.)

5.4 binascii
The b2a_base64() function now accepts an optional newline keyword argument to control whether the
newline character is appended to the return value. (Contributed by Victor Stinner in bpo-25357.)

5.5 cmath
The new cmath.tau (τ ) constant has been added. (Contributed by Lisa Roach in bpo-12345, see PEP 628
for details.)
New constants: cmath.inf and cmath.nan to match math.inf and math.nan, and also cmath.infj and
cmath.nanj to match the format used by complex repr. (Contributed by Mark Dickinson in bpo-23229.)

5.6 collections
The new Collection abstract base class has been added to represent sized iterable container classes. (Con-
tributed by Ivan Levkivskyi, docs by Neil Girdhar in bpo-27598.)
The new Reversible abstract base class represents iterable classes that also provide the __reversed__()
method. (Contributed by Ivan Levkivskyi in bpo-25987.)
The new AsyncGenerator abstract base class represents asynchronous generators. (Contributed by Yury
Selivanov in bpo-28720.)
The namedtuple() function now accepts an optional keyword argument module, which, when specified, is
used for the __module__ attribute of the returned named tuple class. (Contributed by Raymond Hettinger
in bpo-17941.)
The verbose and rename arguments for namedtuple() are now keyword-only. (Contributed by Raymond
Hettinger in bpo-25628.)
Recursive collections.deque instances can now be pickled. (Contributed by Serhiy Storchaka in bpo-
26482.)
5.7 concurrent.futures
The ThreadPoolExecutor class constructor now accepts an optional thread_name_prefix argument to make
it possible to customize the names of the threads created by the pool. (Contributed by Gregory P. Smith in
bpo-27664.)

5.8 contextlib
The contextlib.AbstractContextManager class has been added to provide an abstract base class for
context managers. It provides a sensible default implementation for __enter__() which returns self and
leaves __exit__() an abstract method. A matching class has been added to the typing module as typing.
ContextManager. (Contributed by Brett Cannon in bpo-25609.)

5.9 datetime
The datetime and time classes have the new fold attribute used to disambiguate local time when necessary.
Many functions in the datetime have been updated to support local time disambiguation. See Local Time
Disambiguation section for more information. (Contributed by Alexander Belopolsky in bpo-24773.)
The datetime.strftime() and date.strftime() methods now support ISO 8601 date directives %G, %u
and %V. (Contributed by Ashley Anderson in bpo-12006.)
The datetime.isoformat() function now accepts an optional timespec argument that specifies the number
of additional components of the time value to include. (Contributed by Alessandro Cucci and Alexander
Belopolsky in bpo-19475.)
The datetime.combine() now accepts an optional tzinfo argument. (Contributed by Alexander Belopolsky
in bpo-27661.)

5.10 decimal
New Decimal.as_integer_ratio() method that returns a pair (n, d) of integers that represent the given
Decimal instance as a fraction, in lowest terms and with a positive denominator:

>>> Decimal('-3.14').as_integer_ratio()
(-157, 50)

(Contributed by Stefan Krah amd Mark Dickinson in bpo-25928.)

5.11 distutils
The default_format attribute has been removed from distutils.command.sdist.sdist and the formats
attribute defaults to ['gztar']. Although not anticipated, any code relying on the presence of
default_format may need to be adapted. See bpo-27819 for more details.
The upload command now longer tries to change CR end-of-line characters to CRLF. This fixes a corruption
issue with sdists that ended with a byte equivalent to CR. (Contributed by Bo Bayles in bpo-32304.)

5.12 email
The new email API, enabled via the policy keyword to various constructors, is no longer provisional. The
email documentation has been reorganized and rewritten to focus on the new API, while retaining the old
documentation for the legacy API. (Contributed by R. David Murray in bpo-24277.)
The email.mime classes now all accept an optional policy keyword. (Contributed by Berker Peksag in
bpo-27331.)
The DecodedGenerator now supports the policy keyword.
There is a new policy attribute, message_factory, that controls what class is used by default when the
parser creates new message objects. For the email.policy.compat32 policy this is Message, for the new
policies it is EmailMessage. (Contributed by R. David Murray in bpo-20476.)

5.13 encodings
On Windows, added the 'oem' encoding to use CP_OEMCP, and the 'ansi' alias for the existing 'mbcs'
encoding, which uses the CP_ACP code page. (Contributed by Steve Dower in bpo-27959.)

5.14 enum
Two new enumeration base classes have been added to the enum module: Flag and IntFlags. Both are used
to define constants that can be combined using the bitwise operators. (Contributed by Ethan Furman in
bpo-23591.)
Many standard library modules have been updated to use the IntFlags class for their constants.
The new enum.auto value can be used to assign values to enum members automatically:

>>> from enum import Enum, auto

>>> class Color(Enum):
... red = auto()
... blue = auto()
... green = auto()
...
>>> list(Color)
[<Color.red: 1>, <Color.blue: 2>, <Color.green: 3>]

5.15 faulthandler
On Windows, the faulthandler module now installs a handler for Windows exceptions: see faulthandler.
enable(). (Contributed by Victor Stinner in bpo-23848.)

5.16 fileinput
hook_encoded() now supports the errors argument. (Contributed by Joseph Hackman in bpo-25788.)

5.17 hashlib
hashlib supports OpenSSL 1.1.0. The minimum recommend version is 1.0.2. (Contributed by Christian
Heimes in bpo-26470.)
BLAKE2 hash functions were added to the module. blake2b() and blake2s() are always available and
support the full feature set of BLAKE2. (Contributed by Christian Heimes in bpo-26798 based on code by
Dmitry Chestnykh and Samuel Neves. Documentation written by Dmitry Chestnykh.)
The SHA-3 hash functions sha3_224(), sha3_256(), sha3_384(), sha3_512(), and SHAKE hash functions
shake_128() and shake_256() were added. (Contributed by Christian Heimes in bpo-16113. Keccak Code
Package by Guido Bertoni, Joan Daemen, Michaël Peeters, Gilles Van Assche, and Ronny Van Keer.)
The password-based key derivation function scrypt() is now available with OpenSSL 1.1.0 and newer.
(Contributed by Christian Heimes in bpo-27928.)

5.18 http.client
HTTPConnection.request() and endheaders() both now support chunked encoding request bodies. (Con-
tributed by Demian Brecht and Rolf Krahl in bpo-12319.)

5.19 idlelib and IDLE

The idlelib package is being modernized and refactored to make IDLE look and work better and to make
the code easier to understand, test, and improve. Part of making IDLE look better, especially on Linux and
Mac, is using ttk widgets, mostly in the dialogs. As a result, IDLE no longer runs with tcl/tk 8.4. It now
requires tcl/tk 8.5 or 8.6. We recommend running the latest release of either.
‘Modernizing’ includes renaming and consolidation of idlelib modules. The renaming of files with partial
uppercase names is similar to the renaming of, for instance, Tkinter and TkFont to tkinter and tkinter.font
in 3.0. As a result, imports of idlelib files that worked in 3.5 will usually not work in 3.6. At least a module
name change will be needed (see idlelib/README.txt), sometimes more. (Name changes contributed by
Al Swiegart and Terry Reedy in bpo-24225. Most idlelib patches since have been and will be part of the
process.)
In compensation, the eventual result with be that some idlelib classes will be easier to use, with better APIs
and docstrings explaining them. Additional useful information will be added to idlelib when available.

5.20 importlib
Import now raises the new exception ModuleNotFoundError (subclass of ImportError) when it cannot find
a module. Code that current checks for ImportError (in try-except) will still work. (Contributed by Eric
Snow in bpo-15767.)
importlib.util.LazyLoader now calls create_module() on the wrapped loader, removing the restriction
that importlib.machinery.BuiltinImporter and importlib.machinery.ExtensionFileLoader couldn’t
be used with importlib.util.LazyLoader.
importlib.util.cache_from_source(), importlib.util.source_from_cache(), and importlib.util.
spec_from_file_location() now accept a path-like object.

5.21 inspect
The inspect.signature() function now reports the implicit .0 parameters generated by the compiler for
comprehension and generator expression scopes as if they were positional-only parameters called implicit0.
(Contributed by Jelle Zijlstra in bpo-19611.)
To reduce code churn when upgrading from Python 2.7 and the legacy inspect.getargspec() API, the
previously documented deprecation of inspect.getfullargspec() has been reversed. While this function
is convenient for single/source Python 2/3 code bases, the richer inspect.signature() interface remains
the recommended approach for new code. (Contributed by Nick Coghlan in bpo-27172)

5.22 json
json.load() and json.loads() now support binary input. Encoded JSON should be represented using
either UTF-8, UTF-16, or UTF-32. (Contributed by Serhiy Storchaka in bpo-17909.)
5.23 logging
The new WatchedFileHandler.reopenIfNeeded() method has been added to add the ability to check if
the log file needs to be reopened. (Contributed by Marian Horban in bpo-24884.)

5.24 math
The tau (τ ) constant has been added to the math and cmath modules. (Contributed by Lisa Roach in
bpo-12345, see PEP 628 for details.)

5.25 multiprocessing
Proxy Objects returned by multiprocessing.Manager() can now be nested. (Contributed by Davin Potts
in bpo-6766.)

5.26 os
See the summary of PEP 519 for details on how the os and os.path modules now support path-like objects.
scandir() now supports bytes paths on Windows.
A new close() method allows explicitly closing a scandir() iterator. The scandir() iterator now sup-
ports the context manager protocol. If a scandir() iterator is neither exhausted nor explicitly closed a
ResourceWarning will be emitted in its destructor. (Contributed by Serhiy Storchaka in bpo-25994.)
On Linux, os.urandom() now blocks until the system urandom entropy pool is initialized to increase the
security. See the PEP 524 for the rationale.
The Linux getrandom() syscall (get random bytes) is now exposed as the new os.getrandom() function.
(Contributed by Victor Stinner, part of the PEP 524)

5.27 pathlib
pathlib now supports path-like objects. (Contributed by Brett Cannon in bpo-27186.)
See the summary of PEP 519 for details.

5.28 pdb
The Pdb class constructor has a new optional readrc argument to control whether .pdbrc files should be
read.

5.29 pickle
Objects that need __new__ called with keyword arguments can now be pickled using pickle protocols older
than protocol version 4. Protocol version 4 already supports this case. (Contributed by Serhiy Storchaka in
bpo-24164.)

5.30 pickletools
pickletools.dis() now outputs the implicit memo index for the MEMOIZE opcode. (Contributed by Serhiy
Storchaka in bpo-25382.)
5.31 pydoc
The pydoc module has learned to respect the MANPAGER environment variable. (Contributed by Matthias
Klose in bpo-8637.)
help() and pydoc can now list named tuple fields in the order they were defined rather than alphabetically.
(Contributed by Raymond Hettinger in bpo-24879.)

5.32 random
The new choices() function returns a list of elements of specified size from the given population with
optional weights. (Contributed by Raymond Hettinger in bpo-18844.)

5.33 re
Added support of modifier spans in regular expressions. Examples: '(?i:p)ython' matches 'python' and
'Python', but not 'PYTHON'; '(?i)g(?-i:v)r' matches 'GvR' and 'gvr', but not 'GVR'. (Contributed by
Serhiy Storchaka in bpo-433028.)
Match object groups can be accessed by __getitem__, which is equivalent to group(). So mo['name'] is
now equivalent to mo.group('name'). (Contributed by Eric Smith in bpo-24454.)
Match objects now support index-like objects as group indices. (Contributed by Jeroen Demeyer and
Xiang Zhang in bpo-27177.)

5.34 readline
Added set_auto_history() to enable or disable automatic addition of input to the history list. (Contributed
by Tyler Crompton in bpo-26870.)

5.35 rlcompleter
Private and special attribute names now are omitted unless the prefix starts with underscores. A space
or a colon is added after some completed keywords. (Contributed by Serhiy Storchaka in bpo-25011 and
bpo-25209.)

5.36 shlex
The shlex has much improved shell compatibility through the new punctuation_chars argument to control
which characters are treated as punctuation. (Contributed by Vinay Sajip in bpo-1521950.)

5.37 site
When specifying paths to add to sys.path in a .pth file, you may now specify file paths on top of directories
(e.g. zip files). (Contributed by Wolfgang Langner in bpo-26587).

5.38 sqlite3
sqlite3.Cursor.lastrowid now supports the REPLACE statement. (Contributed by Alex LordThorsen in
bpo-16864.)
5.39 socket
The ioctl() function now supports the SIO_LOOPBACK_FAST_PATH control code. (Contributed by Daniel
Stokes in bpo-26536.)
The getsockopt() constants SO_DOMAIN, SO_PROTOCOL, SO_PEERSEC, and SO_PASSSEC are now supported.
(Contributed by Christian Heimes in bpo-26907.)
The setsockopt() now supports the setsockopt(level, optname, None, optlen: int) form. (Con-
tributed by Christian Heimes in bpo-27744.)
The socket module now supports the address family AF_ALG to interface with Linux Kernel crypto API.
ALG_*, SOL_ALG and sendmsg_afalg() were added. (Contributed by Christian Heimes in bpo-27744 with
support from Victor Stinner.)
New Linux constants TCP_USER_TIMEOUT and TCP_CONGESTION were added. (Contributed by Omar Sandoval,
issue:26273).

5.40 socketserver
Servers based on the socketserver module, including those defined in http.server, xmlrpc.server and
wsgiref.simple_server, now support the context manager protocol. (Contributed by Aviv Palivoda in
bpo-26404.)
The wfile attribute of StreamRequestHandler classes now implements the io.BufferedIOBase writable
interface. In particular, calling write() is now guaranteed to send the data in full. (Contributed by Martin
Panter in bpo-26721.)

5.41 ssl
ssl supports OpenSSL 1.1.0. The minimum recommend version is 1.0.2. (Contributed by Christian Heimes
in bpo-26470.)
3DES has been removed from the default cipher suites and ChaCha20 Poly1305 cipher suites have been
added. (Contributed by Christian Heimes in bpo-27850 and bpo-27766.)
SSLContext has better default configuration for options and ciphers. (Contributed by Christian Heimes in
bpo-28043.)
SSL session can be copied from one client-side connection to another with the new SSLSession class. TLS
session resumption can speed up the initial handshake, reduce latency and improve performance (Contributed
by Christian Heimes in bpo-19500 based on a draft by Alex Warhawk.)
The new get_ciphers() method can be used to get a list of enabled ciphers in order of cipher priority.
All constants and flags have been converted to IntEnum and IntFlags. (Contributed by Christian Heimes
in bpo-28025.)
Server and client-side specific TLS protocols for SSLContext were added. (Contributed by Christian Heimes
in bpo-28085.)

5.42 statistics
A new harmonic_mean() function has been added. (Contributed by Steven D’Aprano in bpo-27181.)
5.43 struct
struct now supports IEEE 754 half-precision floats via the 'e' format specifier. (Contributed by Eli Stevens,
Mark Dickinson in bpo-11734.)

5.44 subprocess
subprocess.Popen destructor now emits a ResourceWarning warning if the child process is still running.
Use the context manager protocol (with proc: ...) or explicitly call the wait() method to read the exit
status of the child process. (Contributed by Victor Stinner in bpo-26741.)
The subprocess.Popen constructor and all functions that pass arguments through to it now accept encoding
and errors arguments. Specifying either of these will enable text mode for the stdin, stdout and stderr
streams. (Contributed by Steve Dower in bpo-6135.)

5.45 sys
The new getfilesystemencodeerrors() function returns the name of the error mode used to convert
between Unicode filenames and bytes filenames. (Contributed by Steve Dower in bpo-27781.)
On Windows the return value of the getwindowsversion() function now includes the platform_version field
which contains the accurate major version, minor version and build number of the current operating system,
rather than the version that is being emulated for the process (Contributed by Steve Dower in bpo-27932.)

5.46 telnetlib
Telnet is now a context manager (contributed by Stéphane Wirtel in bpo-25485).

5.47 time
The struct_time attributes tm_gmtoff and tm_zone are now available on all platforms.

5.48 timeit
The new Timer.autorange() convenience method has been added to call Timer.timeit() repeatedly so that
the total run time is greater or equal to 200 milliseconds. (Contributed by Steven D’Aprano in bpo-6422.)
timeit now warns when there is substantial (4x) variance between best and worst times. (Contributed by
Serhiy Storchaka in bpo-23552.)

5.49 tkinter
Added methods trace_add(), trace_remove() and trace_info() in the tkinter.Variable class. They
replace old methods trace_variable(), trace(), trace_vdelete() and trace_vinfo() that use obsolete
Tcl commands and might not work in future versions of Tcl. (Contributed by Serhiy Storchaka in bpo-22115).
5.50 traceback
Both the traceback module and the interpreter’s builtin exception display now abbreviate long sequences of
repeated lines in tracebacks as shown in the following example:

>>> def f(): f()

...
>>> f()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in f
File "<stdin>", line 1, in f
File "<stdin>", line 1, in f
[Previous line repeated 995 more times]
RecursionError: maximum recursion depth exceeded

(Contributed by Emanuel Barry in bpo-26823.)

5.51 tracemalloc
The tracemalloc module now supports tracing memory allocations in multiple different address spaces.
The new DomainFilter filter class has been added to filter block traces by their address space (domain).
(Contributed by Victor Stinner in bpo-26588.)

5.52 typing
Since the typing module is provisional, all changes introduced in Python 3.6 have also been backported to
Python 3.5.x.
The typing module has a much improved support for generic type aliases. For example Dict[str, Tuple[S,
T]] is now a valid type annotation. (Contributed by Guido van Rossum in Github #195.)
The typing.ContextManager class has been added for representing contextlib.AbstractContextManager.
(Contributed by Brett Cannon in bpo-25609.)
The typing.Collection class has been added for representing collections.abc.Collection. (Con-
tributed by Ivan Levkivskyi in bpo-27598.)
The typing.ClassVar type construct has been added to mark class variables. As introduced in PEP 526,
a variable annotation wrapped in ClassVar indicates that a given attribute is intended to be used as a class
variable and should not be set on instances of that class. (Contributed by Ivan Levkivskyi in Github #280.)
A new TYPE_CHECKING constant that is assumed to be True by the static type chekers, but is False at
runtime. (Contributed by Guido van Rossum in Github #230.)
A new NewType() helper function has been added to create lightweight distinct types for annotations:

from typing import NewType

UserId = NewType('UserId', int)

some_id = UserId(524313)

The static type checker will treat the new type as if it were a subclass of the original type. (Contributed by
Ivan Levkivskyi in Github #189.)
5.53 unicodedata
The unicodedata module now uses data from Unicode 9.0.0. (Contributed by Benjamin Peterson.)

5.54 unittest.mock
The Mock class has the following improvements:
• Two new methods, Mock.assert_called() and Mock.assert_called_once() to check if the mock
object was called. (Contributed by Amit Saha in bpo-26323.)
• The Mock.reset_mock() method now has two optional keyword only arguments: return_value and
side_effect. (Contributed by Kushal Das in bpo-21271.)

5.55 urllib.request
If a HTTP request has a file or iterable body (other than a bytes object) but no Content-Length header,
rather than throwing an error, AbstractHTTPHandler now falls back to use chunked transfer encoding.
(Contributed by Demian Brecht and Rolf Krahl in bpo-12319.)

5.56 urllib.robotparser
RobotFileParser now supports the Crawl-delay and Request-rate extensions. (Contributed by Nikolay
Bogoychev in bpo-16099.)

5.57 venv
venv accepts a new parameter --prompt. This parameter provides an alternative prefix for the virtual
environment. (Proposed by Łukasz Balcerzak and ported to 3.6 by Stéphane Wirtel in bpo-22829.)

5.58 warnings
A new optional source parameter has been added to the warnings.warn_explicit() function: the de-
stroyed object which emitted a ResourceWarning. A source attribute has also been added to warnings.
WarningMessage (contributed by Victor Stinner in bpo-26568 and bpo-26567).
When a ResourceWarning warning is logged, the tracemalloc module is now used to try to retrieve the
traceback where the destroyed object was allocated.
Example with the script example.py:

import warnings

def func():
return open(__file__)

f = func()
f = None

Output of the command python3.6 -Wd -X tracemalloc=5 example.py:

example.py:7: ResourceWarning: unclosed file <_io.TextIOWrapper name='example.py' mode='r'␣
,→encoding='UTF-8'>

f = None
Object allocated at (most recent call first):
File "example.py", lineno 4
return open(__file__)
File "example.py", lineno 6
f = func()

The “Object allocated at” traceback is new and is only displayed if tracemalloc is tracing Python memory
allocations and if the warnings module was already imported.

5.59 winreg
Added the 64-bit integer type REG_QWORD. (Contributed by Clement Rouault in bpo-23026.)

5.60 winsound
Allowed keyword arguments to be passed to Beep, MessageBeep, and PlaySound (bpo-27982).

5.61 xmlrpc.client
The xmlrpc.client module now supports unmarshalling additional data types used by the Apache XML-
RPC implementation for numerics and None. (Contributed by Serhiy Storchaka in bpo-26885.)

5.62 zipfile
A new ZipInfo.from_file() class method allows making a ZipInfo instance from a filesystem file. A new
ZipInfo.is_dir() method can be used to check if the ZipInfo instance represents a directory. (Contributed
by Thomas Kluyver in bpo-26039.)
The ZipFile.open() method can now be used to write data into a ZIP file, as well as for extracting data.
(Contributed by Thomas Kluyver in bpo-26039.)

5.63 zlib
The compress() and decompress() functions now accept keyword arguments. (Contributed by Aviv
Palivoda in bpo-26243 and Xiang Zhang in bpo-16764 respectively.)

6 Optimizations
• The Python interpreter now uses a 16-bit wordcode instead of bytecode which made a number of
opcode optimizations possible. (Contributed by Demur Rumed with input and reviews from Serhiy
Storchaka and Victor Stinner in bpo-26647 and bpo-28050.)
• The asyncio.Future class now has an optimized C implementation. (Contributed by Yury Selivanov
and INADA Naoki in bpo-26081.)
• The asyncio.Task class now has an optimized C implementation. (Contributed by Yury Selivanov in
bpo-28544.)
 • Various implementation improvements in the typing module (such as caching of generic types) allow
up to 30 times performance improvements and reduced memory footprint.
• The ASCII decoder is now up to 60 times as fast for error handlers surrogateescape, ignore and
replace (Contributed by Victor Stinner in bpo-24870).
• The ASCII and the Latin1 encoders are now up to 3 times as fast for the error handler surrogateescape
(Contributed by Victor Stinner in bpo-25227).
• The UTF-8 encoder is now up to 75 times as fast for error handlers ignore, replace, surrogateescape,
surrogatepass (Contributed by Victor Stinner in bpo-25267).
• The UTF-8 decoder is now up to 15 times as fast for error handlers ignore, replace and
surrogateescape (Contributed by Victor Stinner in bpo-25301).
• bytes % args is now up to 2 times faster. (Contributed by Victor Stinner in bpo-25349).
• bytearray % args is now between 2.5 and 5 times faster. (Contributed by Victor Stinner in bpo-
25399).
• Optimize bytes.fromhex() and bytearray.fromhex(): they are now between 2x and 3.5x faster.
(Contributed by Victor Stinner in bpo-25401).
• Optimize bytes.replace(b'', b'.') and bytearray.replace(b'', b'.'): up to 80% faster. (Con-
tributed by Josh Snider in bpo-26574).
• Allocator functions of the PyMem_Malloc() domain (PYMEM_DOMAIN_MEM) now use the pymalloc memory
allocator instead of malloc() function of the C library. The pymalloc allocator is optimized for
objects smaller or equal to 512 bytes with a short lifetime, and use malloc() for larger memory blocks.
(Contributed by Victor Stinner in bpo-26249).
• pickle.load() and pickle.loads() are now up to 10% faster when deserializing many small objects
(Contributed by Victor Stinner in bpo-27056).
• Passing keyword arguments to a function has an overhead in comparison with passing positional argu-
ments. Now in extension functions implemented with using Argument Clinic this overhead is signifi-
cantly decreased. (Contributed by Serhiy Storchaka in bpo-27574).
• Optimized glob() and iglob() functions in the glob module; they are now about 3–6 times faster.
(Contributed by Serhiy Storchaka in bpo-25596).
• Optimized globbing in pathlib by using os.scandir(); it is now about 1.5–4 times faster. (Con-
tributed by Serhiy Storchaka in bpo-26032).
• xml.etree.ElementTree parsing, iteration and deepcopy performance has been significantly improved.
(Contributed by Serhiy Storchaka in bpo-25638, bpo-25873, and bpo-25869.)
• Creation of fractions.Fraction instances from floats and decimals is now 2 to 3 times faster. (Con-
tributed by Serhiy Storchaka in bpo-25971.)

7 Build and C API Changes

• Python now requires some C99 support in the toolchain to build. Most notably, Python now uses
standard integer types and macros in place of custom macros like PY_LONG_LONG. For more information,
see PEP 7 and bpo-17884.
• Cross-compiling CPython with the Android NDK and the Android API level set to 21 (Android 5.0
Lollilop) or greater runs successfully. While Android is not yet a supported platform, the Python test
suite runs on the Android emulator with only about 16 tests failures. See the Android meta-issue
bpo-26865.
 • The --enable-optimizations configure flag has been added. Turning it on will activate expensive
optimizations like PGO. (Original patch by Alecsandru Patrascu of Intel in bpo-26359.)
• The GIL must now be held when allocator functions of PYMEM_DOMAIN_OBJ (ex: PyObject_Malloc())
and PYMEM_DOMAIN_MEM (ex: PyMem_Malloc()) domains are called.
• New Py_FinalizeEx() API which indicates if flushing buffered data failed. (Contributed by Martin
Panter in bpo-5319.)
• PyArg_ParseTupleAndKeywords() now supports positional-only parameters. Positional-only parame-
ters are defined by empty names. (Contributed by Serhiy Storchaka in bpo-26282).
• PyTraceback_Print method now abbreviates long sequences of repeated lines as "[Previous line
repeated {count} more times]". (Contributed by Emanuel Barry in bpo-26823.)
• The new PyErr_SetImportErrorSubclass() function allows for specifying a subclass of ImportError
to raise. (Contributed by Eric Snow in bpo-15767.)
• The new PyErr_ResourceWarning() function can be used to generate a ResourceWarning providing
the source of the resource allocation. (Contributed by Victor Stinner in bpo-26567.)
• The new PyOS_FSPath() function returns the file system representation of a path-like object. (Con-
tributed by Brett Cannon in bpo-27186.)
• The PyUnicode_FSConverter() and PyUnicode_FSDecoder() functions will now accept path-like ob-
jects.
• The PyExc_RecursionErrorInst singleton that was part of the public API has been removed as its
members being never cleared may cause a segfault during finalization of the interpreter. Contributed
by Xavier de Gaye in bpo-22898 and bpo-30697.

8 Other Improvements

• When --version (short form: -V) is supplied twice, Python prints sys.version for detailed informa-
tion.

$ ./python -VV
Python 3.6.0b4+ (3.6:223967b49e49+, Nov 21 2016, 20:55:04)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)]

9 Deprecated

9.1 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. Starting in Python 3.6, the
use of async or await as names will generate a DeprecationWarning.

9.2 Deprecated Python behavior

Raising the StopIteration exception inside a generator will now generate a DeprecationWarning, and will
trigger a RuntimeError in Python 3.7. See whatsnew-pep-479 for details.
The __aiter__() method is now expected to return an asynchronous iterator directly instead of returning
an awaitable as previously. Doing the former will trigger a DeprecationWarning. Backward compatibility
will be removed in Python 3.7. (Contributed by Yury Selivanov in bpo-27243.)
A backslash-character pair that is not a valid escape sequence now generates a DeprecationWarning. Al-
though this will eventually become a SyntaxError, that will not be for several Python releases. (Contributed
by Emanuel Barry in bpo-27364.)
When performing a relative import, falling back on __name__ and __path__ from the calling module when
__spec__ or __package__ are not defined now raises an ImportWarning. (Contributed by Rose Ames in
bpo-25791.)

9.3 Deprecated Python modules, functions and methods

asynchat

The asynchat has been deprecated in favor of asyncio. (Contributed by Mariatta in bpo-25002.)

asyncore

The asyncore has been deprecated in favor of asyncio. (Contributed by Mariatta in bpo-25002.)

dbm

Unlike other dbm implementations, the dbm.dumb module creates databases with the 'rw' mode and allows
modifying the database opened with the 'r' mode. This behavior is now deprecated and will be removed
in 3.8. (Contributed by Serhiy Storchaka in bpo-21708.)

distutils

The undocumented extra_path argument to the Distribution constructor is now considered deprecated
and will raise a warning if set. Support for this parameter will be removed in a future Python release. See
bpo-27919 for details.

grp

The support of non-integer arguments in getgrgid() has been deprecated. (Contributed by Serhiy Storchaka
in bpo-26129.)

importlib

The importlib.machinery.SourceFileLoader.load_module() and importlib.machinery.

SourcelessFileLoader.load_module() methods are now deprecated. They were the only remaining
implementations of importlib.abc.Loader.load_module() in importlib that had not been deprecated
in previous versions of Python in favour of importlib.abc.Loader.exec_module().
The importlib.machinery.WindowsRegistryFinder class is now deprecated. As of 3.6.0, it is still added
to sys.meta_path by default (on Windows), but this may change in future releases.

os

Undocumented support of general bytes-like objects as paths in os functions, compile() and similar functions
is now deprecated. (Contributed by Serhiy Storchaka in bpo-25791 and bpo-26754.)
re

Support for inline flags (?letters) in the middle of the regular expression has been deprecated and will be
removed in a future Python version. Flags at the start of a regular expression are still allowed. (Contributed
by Serhiy Storchaka in bpo-22493.)

ssl

OpenSSL 0.9.8, 1.0.0 and 1.0.1 are deprecated and no longer supported. In the future the ssl module will
require at least OpenSSL 1.0.2 or 1.1.0.
SSL-related arguments like certfile, keyfile and check_hostname in ftplib, http.client, imaplib,
poplib, and smtplib have been deprecated in favor of context. (Contributed by Christian Heimes in
bpo-28022.)
A couple of protocols and functions of the ssl module are now deprecated. Some features will no longer
be available in future versions of OpenSSL. Other features are deprecated in favor of a different API. (Con-
tributed by Christian Heimes in bpo-28022 and bpo-26470.)

tkinter

The tkinter.tix module is now deprecated. tkinter users should use tkinter.ttk instead.

venv

The pyvenv script has been deprecated in favour of python3 -m venv. This prevents confusion as to what
Python interpreter pyvenv is connected to and thus what Python interpreter will be used by the virtual
environment. (Contributed by Brett Cannon in bpo-25154.)

9.4 Deprecated functions and types of the C API

Undocumented functions PyUnicode_AsEncodedObject(), PyUnicode_AsDecodedObject(),
PyUnicode_AsEncodedUnicode() and PyUnicode_AsDecodedUnicode() are deprecated now. Use the
generic codec based API instead.

9.5 Deprecated Build Options

The --with-system-ffi configure flag is now on by default on non-macOS UNIX platforms. It may
be disabled by using --without-system-ffi, but using the flag is deprecated and will not be accepted
in Python 3.7. macOS is unaffected by this change. Note that many OS distributors already use the
--with-system-ffi flag when building their system Python.

10 Removed

10.1 API and Feature Removals

• Unknown escapes consisting of '\' and an ASCII letter in regular expressions will now cause an error.
In replacement templates for re.sub() they are still allowed, but deprecated. The re.LOCALE flag can
now only be used with binary patterns.
 • inspect.getmoduleinfo() was removed (was deprecated since CPython 3.3). inspect.
getmodulename() should be used for obtaining the module name for a given path. (Contributed
by Yury Selivanov in bpo-13248.)
• traceback.Ignore class and traceback.usage, traceback.modname, traceback.fullmodname,
traceback.find_lines_from_code, traceback.find_lines, traceback.find_strings,
traceback.find_executable_lines methods were removed from the traceback module. They were
undocumented methods deprecated since Python 3.2 and equivalent functionality is available from
private methods.
• The tk_menuBar() and tk_bindForTraversal() dummy methods in tkinter widget classes were
removed (corresponding Tk commands were obsolete since Tk 4.0).
• The open() method of the zipfile.ZipFile class no longer supports the 'U' mode (was deprecated
since Python 3.4). Use io.TextIOWrapper for reading compressed text files in universal newlines mode.
• The undocumented IN, CDROM, DLFCN, TYPES, CDIO, and STROPTS modules have been removed. They
had been available in the platform specific Lib/plat-*/ directories, but were chronically out of date,
inconsistently available across platforms, and unmaintained. The script that created these modules is
still available in the source distribution at Tools/scripts/h2py.py.
• The deprecated asynchat.fifo class has been removed.

11 Porting to Python 3.6

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

11.1 Changes in ‘python’ Command Behavior

• The output of a special Python build with defined COUNT_ALLOCS, SHOW_ALLOC_COUNT or
SHOW_TRACK_COUNT macros is now off by default. It can be re-enabled using the -X showalloccount
option. It now outputs to stderr instead of stdout. (Contributed by Serhiy Storchaka in bpo-23034.)

11.2 Changes in the Python API

• open() will no longer allow combining the 'U' mode flag with '+'. (Contributed by Jeff Balogh and
John O’Connor in bpo-2091.)
• sqlite3 no longer implicitly commits an open transaction before DDL statements.
• On Linux, os.urandom() now blocks until the system urandom entropy pool is initialized to increase
the security.
• When importlib.abc.Loader.exec_module() is defined, importlib.abc.Loader.create_module()
must also be defined.
• PyErr_SetImportError() now sets TypeError when its msg argument is not set. Previously only
NULL was returned.
• The format of the co_lnotab attribute of code objects changed to support a negative line number delta.
By default, Python does not emit bytecode with a negative line number delta. Functions using frame.
f_lineno, PyFrame_GetLineNumber() or PyCode_Addr2Line() are not affected. Functions directly
decoding co_lnotab should be updated to use a signed 8-bit integer type for the line number delta,
but this is only required to support applications using a negative line number delta. See Objects/
lnotab_notes.txt for the co_lnotab format and how to decode it, and see the PEP 511 for the
rationale.
• The functions in the compileall module now return booleans instead of 1 or 0 to represent success
or failure, respectively. Thanks to booleans being a subclass of integers, this should only be an issue if
you were doing identity checks for 1 or 0. See bpo-25768.
• Reading the port attribute of urllib.parse.urlsplit() and urlparse() results now raises
ValueError for out-of-range values, rather than returning None. See bpo-20059.
• The imp module now raises a DeprecationWarning instead of PendingDeprecationWarning.
• The following modules have had missing APIs added to their __all__ attributes to match the doc-
umented APIs: calendar, cgi, csv, ElementTree, enum, fileinput, ftplib, logging, mailbox,
mimetypes, optparse, plistlib, smtpd, subprocess, tarfile, threading and wave. This means
they will export new symbols when import * is used. (Contributed by Joel Taddei and Jacek Kołodziej
in bpo-23883.)
• When performing a relative import, if __package__ does not compare equal to __spec__.parent then
ImportWarning is raised. (Contributed by Brett Cannon in bpo-25791.)
• When a relative import is performed and no parent package is known, then ImportError will be raised.
Previously, SystemError could be raised. (Contributed by Brett Cannon in bpo-18018.)
• Servers based on the socketserver module, including those defined in http.server, xmlrpc.server
and wsgiref.simple_server, now only catch exceptions derived from Exception. Therefore if a
request handler raises an exception like SystemExit or KeyboardInterrupt, handle_error() is no
longer called, and the exception will stop a single-threaded server. (Contributed by Martin Panter in
bpo-23430.)
• spwd.getspnam() now raises a PermissionError instead of KeyError if the user doesn’t have privi-
leges.
• The socket.socket.close() method now raises an exception if an error (e.g. EBADF) was reported
by the underlying system call. (Contributed by Martin Panter in bpo-26685.)
• The decode_data argument for the smtpd.SMTPChannel and smtpd.SMTPServer constructors is now
False by default. This means that the argument passed to process_message() is now a bytes object
by default, and process_message() will be passed keyword arguments. Code that has already been
updated in accordance with the deprecation warning generated by 3.5 will not be affected.
• All optional arguments of the dump(), dumps(), load() and loads() functions and JSONEncoder and
JSONDecoder class constructors in the json module are now keyword-only. (Contributed by Serhiy
Storchaka in bpo-18726.)
• Subclasses of type which don’t override type.__new__ may no longer use the one-argument form to
get the type of an object.
• As part of PEP 487, the handling of keyword arguments passed to type (other than the metaclass
hint, metaclass) is now consistently delegated to object.__init_subclass__(). This means that
type.__new__() and type.__init__() both now accept arbitrary keyword arguments, but object.
__init_subclass__() (which is called from type.__new__()) will reject them by default. Custom
metaclasses accepting additional keyword arguments will need to adjust their calls to type.__new__()
(whether direct or via super) accordingly.
• In distutils.command.sdist.sdist, the default_format attribute has been removed and is no
longer honored. Instead, the gzipped tarfile format is the default on all platforms and no platform-
specific selection is made. In environments where distributions are built on Windows and zip distribu-
tions are required, configure the project with a setup.cfg file containing the following:

[sdist]
formats=zip

This behavior has also been backported to earlier Python versions by Setuptools 26.0.0.
 • In the urllib.request module and the http.client.HTTPConnection.request() method, if no
Content-Length header field has been specified and the request body is a file object, it is now sent with
HTTP 1.1 chunked encoding. If a file object has to be sent to a HTTP 1.0 server, the Content-Length
value now has to be specified by the caller. (Contributed by Demian Brecht and Rolf Krahl with tweaks
from Martin Panter in bpo-12319.)
• The DictReader now returns rows of type OrderedDict. (Contributed by Steve Holden in bpo-27842.)
• The crypt.METHOD_CRYPT will no longer be added to crypt.methods if unsupported by the platform.
(Contributed by Victor Stinner in bpo-25287.)
• The verbose and rename arguments for namedtuple() are now keyword-only. (Contributed by Ray-
mond Hettinger in bpo-25628.)
• On Linux, ctypes.util.find_library() now looks in LD_LIBRARY_PATH for shared libraries. (Con-
tributed by Vinay Sajip in bpo-9998.)
• The imaplib.IMAP4 class now handles flags containing the ']' character in messages sent from the
server to improve real-world compatibility. (Contributed by Lita Cho in bpo-21815.)
• The mmap.write() function now returns the number of bytes written like other write methods. (Con-
tributed by Jakub Stasiak in bpo-26335.)
• The pkgutil.iter_modules() and pkgutil.walk_packages() functions now return ModuleInfo
named tuples. (Contributed by Ramchandra Apte in bpo-17211.)
• re.sub() now raises an error for invalid numerical group references in replacement templates even if
the pattern is not found in the string. The error message for invalid group references now includes
the group index and the position of the reference. (Contributed by SilentGhost, Serhiy Storchaka in
bpo-25953.)
• zipfile.ZipFile will now raise NotImplementedError for unrecognized compression values. Previ-
ously a plain RuntimeError was raised. Additionally, calling ZipFile methods on a closed ZipFile or
calling the write() method on a ZipFile created with mode 'r' will raise a ValueError. Previously,
a RuntimeError was raised in those scenarios.
• when custom metaclasses are combined with zero-argument super() or direct references from methods
to the implicit __class__ closure variable, the implicit __classcell__ namespace entry must now be
passed up to type.__new__ for initialisation. Failing to do so will result in a DeprecationWarning in
3.6 and a RuntimeWarning in the future.

11.3 Changes in the C API

• The PyMem_Malloc() allocator family now uses the pymalloc allocator rather than the system
malloc(). Applications calling PyMem_Malloc() without holding the GIL can now crash. Set the
PYTHONMALLOC environment variable to debug to validate the usage of memory allocators in your ap-
plication. See bpo-26249.
• Py_Exit() (and the main interpreter) now override the exit status with 120 if flushing buffered data
failed. See bpo-5319.

11.4 CPython bytecode changes

There have been several major changes to the bytecode in Python 3.6.
• The Python interpreter now uses a 16-bit wordcode instead of bytecode. (Contributed by Demur Rumed
with input and reviews from Serhiy Storchaka and Victor Stinner in bpo-26647 and bpo-28050.)
• The new FORMAT_VALUE and BUILD_STRING opcodes as part of the formatted string literal implemen-
tation. (Contributed by Eric Smith in bpo-25483 and Serhiy Storchaka in bpo-27078.)
 • The new BUILD_CONST_KEY_MAP opcode to optimize the creation of dictionaries with constant keys.
(Contributed by Serhiy Storchaka in bpo-27140.)
• The function call opcodes have been heavily reworked for better performance and simpler implemen-
tation. The MAKE_FUNCTION, CALL_FUNCTION, CALL_FUNCTION_KW and BUILD_MAP_UNPACK_WITH_CALL
opcodes have been modified, the new CALL_FUNCTION_EX and BUILD_TUPLE_UNPACK_WITH_CALL have
been added, and CALL_FUNCTION_VAR, CALL_FUNCTION_VAR_KW and MAKE_CLOSURE opcodes have been
removed. (Contributed by Demur Rumed in bpo-27095, and Serhiy Storchaka in bpo-27213, bpo-
28257.)
• The new SETUP_ANNOTATIONS and STORE_ANNOTATION opcodes have been added to support the new
variable annotation syntax. (Contributed by Ivan Levkivskyi in bpo-27985.)

12 Notable changes in Python 3.6.2

12.1 New make regen-all build target

To simplify cross-compilation, and to ensure that CPython can reliably be compiled without requiring an
existing version of Python to already be available, the autotools-based build system no longer attempts to
implicitly recompile generated files based on file modification times.
Instead, a new make regen-all command has been added to force regeneration of these files when desired
(e.g. after an initial version of Python has already been built based on the pregenerated versions).
More selective regeneration targets are also defined - see Makefile.pre.in for details.
(Contributed by Victor Stinner in bpo-23404.)
New in version 3.6.2.

12.2 Removal of make touch build target

The make touch build target previously used to request implicit regeneration of generated files by updating
their modification times has been removed.
It has been replaced by the new make regen-all target.
(Contributed by Victor Stinner in bpo-23404.)
Changed in version 3.6.2.

13 Notable changes in Python 3.6.5

The locale.localeconv() function now sets temporarily the LC_CTYPE locale to the LC_NUMERIC locale in
some cases. (Contributed by Victor Stinner in bpo-31900.)
Index
E
environment variable
PYTHONHOME, 5
PYTHONLEGACYWINDOWSFSENCOD-
ING, 9
PYTHONLEGACYWINDOWSSTDIO, 9
PYTHONMALLOC, 11, 31

P
Python Enhancement Proposals
PEP 468, 10
PEP 484, 5
PEP 487, 7, 8, 30
PEP 492, 6, 26
PEP 494, 3
PEP 495, 9
PEP 498, 5
PEP 506, 13
PEP 511, 29
PEP 515, 6
PEP 519, 8
PEP 520, 10
PEP 523, 10
PEP 524, 4, 18
PEP 525, 6
PEP 526, 6, 22
PEP 528, 9
PEP 529, 9
PEP 530, 6, 7
PEP 628, 14, 18
PEP 7, 25
PYTHONHOME, 5
PYTHONLEGACYWINDOWSFSENCODING, 9
PYTHONLEGACYWINDOWSSTDIO, 9
PYTHONMALLOC, 11, 31

33