What's New In Python 3.10
*************************

Release:
   3.10.0a5

Date:
   March 01, 2021

This article explains the new features in Python 3.10, compared to
3.9.

For full details, see the changelog.

Note:

  Prerelease users should be aware that this document is currently in
  draft form. It will be updated substantially as Python 3.10 moves
  towards release, so it's worth checking back even after reading
  earlier versions.


Summary -- Release highlights
=============================


New Features
============


Parenthesized context managers
------------------------------

Using enclosing parentheses for continuation across multiple lines in
context managers is now supported. This allows formatting a long
collection of context managers in multiple lines in a similar way as
it was previously possible with import statements. For instance, all
these examples are now valid:

   with (CtxManager() as example):
       ...

   with (
       CtxManager1(),
       CtxManager2()
   ):
       ...

   with (CtxManager1() as example,
         CtxManager2()):
       ...

   with (CtxManager1(),
         CtxManager2() as example):
       ...

   with (
       CtxManager1() as example1,
       CtxManager2() as example2
   ):
       ...

it is also possible to use a trailing comma at the end of the enclosed
group:

   with (
       CtxManager1() as example1,
       CtxManager2() as example2,
       CtxManager3() as example3,
   ):
       ...

This new syntax uses the non LL(1) capacities of the new parser. Check
**PEP 617** for more details.

(Contributed by Guido van Rossum, Pablo Galindo and Lysandros Nikolaou
in bpo-12782 and bpo-40334.)


PEP 563: Postponed Evaluation of Annotations Becomes Default
------------------------------------------------------------

In Python 3.7, postponed evaluation of annotations was added, to be
enabled with a "from __future__ import annotations" directive.  In
3.10 this became the default behavior, even without that future
directive.  With this being default, all annotations stored in
"__annotations__" will be strings. If needed, annotations can be
resolved at runtime using "typing.get_type_hints()".  See **PEP 563**
for a full description.  Also, the "inspect.signature()" will try to
resolve types from now on, and when it fails it will fall back to
showing the string annotations.  (Contributed by Batuhan Taskaya in
bpo-38605.)

* The "int" type has a new method "int.bit_count()", returning the
  number of ones in the binary expansion of a given integer, also
  known as the population count. (Contributed by Niklas Fiekas in
  bpo-29882.)

* The views returned by "dict.keys()", "dict.values()" and
  "dict.items()" now all have a "mapping" attribute that gives a
  "types.MappingProxyType" object wrapping the original dictionary.
  (Contributed by Dennis Sweeney in bpo-40890.)

* **PEP 618**: The "zip()" function now has an optional "strict" flag,
  used to require that all the iterables have an equal length.


PEP 613: TypeAlias Annotation
-----------------------------

**PEP 484** introduced the concept of type aliases, only requiring
them to be top-level unannotated assignments. This simplicity
sometimes made it difficult for type checkers to distinguish between
type aliases and ordinary assignments, especially when forward
references or invalid types were involved. Compare:

   StrCache = 'Cache[str]'  # a type alias
   LOG_PREFIX = 'LOG[DEBUG]'  # a module constant

Now the "typing" module has a special annotation "TypeAlias" to
declare type aliases more explicitly:

   StrCache: TypeAlias = 'Cache[str]'  # a type alias
   LOG_PREFIX = 'LOG[DEBUG]'  # a module constant

See **PEP 613** for more details.

(Contributed by Mikhail Golubev in bpo-41923.)


PEP 604: New Type Union Operator
--------------------------------

A new type union operator was introduced which enables the syntax "X |
Y". This provides a cleaner way of expressing 'either type X or type
Y' instead of using "typing.Union", especially in type hints
(annotations).

In previous versions of Python, to apply a type hint for functions
accepting arguments of multiple types, "typing.Union" was used:

   def square(number: Union[int, float]) -> Union[int, float]:
       return number ** 2

Type hints can now be written in a more succinct manner:

   def square(number: int | float) -> int | float:
       return number ** 2

This new syntax is also accepted as the second argument to
"isinstance()" and "issubclass()":

   >>> isinstance(1, int | str)
   True

See Union Type and **PEP 604** for more details.

(Contributed by Maggie Moss and Philippe Prados in bpo-41428.)


PEP 612: Parameter Specification Variables
------------------------------------------

Two new options to improve the information provided to static type
checkers for **PEP 484**'s "Callable" have been added to the "typing"
module.

The first is the parameter specification variable.  They are used to
forward the parameter types of one callable to another callable -- a
pattern commonly found in higher order functions and decorators.
Examples of usage can be found in "typing.ParamSpec". Previously,
there was no easy way to type annotate dependency of parameter types
in such a precise manner.

The second option is the new "Concatenate" operator. It's used in
conjunction with parameter specification variables to type annotate a
higher order callable which adds or removes parameters of another
callable.  Examples of usage can be found in "typing.Concatenate".

See "typing.Callable", "typing.ParamSpec", "typing.Concatenate" and
**PEP 612** for more details.

(Contributed by Ken Jin in bpo-41559.)


PEP 634: Structural Pattern Matching
------------------------------------

Structural pattern matching has been added in the form of a *match
statement* and *case statements* of patterns with associated actions.
Patterns consist of sequences, mappings, primitive data types as well
as class instances. Pattern matching enables programs to extract
information from complex data types, branch on the structure of data,
and apply specific actions based on different forms of data.


Syntax and operations
~~~~~~~~~~~~~~~~~~~~~

The generic syntax of pattern matching is:

   match subject:
       case <pattern_1>:
           <action_1>
       case <pattern_2>:
           <action_2>
       case <pattern_3>:
           <action_3>
       case _:
           <action_wildcard>

A match statement takes an expression and compares its value to
successive patterns given as one or more case blocks.  Specifically,
pattern matching operates by:

   1. using data with type and shape (the "subject")

   2. evaluating the "subject" in the "match" statement

   3. comparing the subject with each pattern in a "case" statement
      from top to bottom until a match is confirmed.

   4. executing the action associated with the pattern of the
      confirmed match

   5. If an exact match is not confirmed, the last case, a wildcard
      "_", if provided, will be used as the matching case. If an exact
      match is not confirmed and a wildcard case does not exists, the
      entire match block is a no-op.


Declarative approach
~~~~~~~~~~~~~~~~~~~~

Readers may be aware of pattern matching through the simple example of
matching a subject (data object) to a literal (pattern) with the
switch statement found in C, Java or JavaScript (and many other
languages). Often the switch statement is used for comparison of an
object/expression with case statements containing literals.

More powerful examples of pattern matching can be found in languages,
such as Scala and Elixir. With structural pattern matching, the
approach is "declarative" and explicitly states the conditions (the
patterns) for data to match.

While an "imperative" series of instructions using nested "if"
statements could be used to accomplish something similar to structural
pattern matching, it is less clear than the "declarative" approach.
Instead the "declarative" approach states the conditions to meet for a
match and is more readable through its explicit patterns. While
structural pattern matching can be used in its simplest form comparing
a variable to a literal in a case statement, its true value for Python
lies in its handling of the subject's type and shape.


Simple pattern: match to a literal
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's look at this example as pattern matching in its simplest form: a
value, the subject, being matched to several literals, the patterns.
In the example below, "status" is the subject of the match statement.
The patterns are each of the case statements, where literals represent
request status codes. The associated action to the case is executed
after a match:

   def http_error(status):
       match status:
           case 400:
               return "Bad request"
           case 404:
               return "Not found"
           case 418:
               return "I'm a teapot"
           case _:
               return "Something's wrong with the Internet"

If the above function is passed a "status" of 418, "I'm a teapot" is
returned. If the above function is passed a "status" of 500, the case
statement with "_" will match as a wildcard, and "Something's wrong
with the Internet" is returned. Note the last block: the variable
name, "_", acts as a *wildcard* and insures the subject will always
match. The use of "_" is optional.

You can combine several literals in a single pattern using "|" ("or"):

   case 401 | 403 | 404:
       return "Not allowed"


Behavior without the wildcard
"""""""""""""""""""""""""""""

If we modify the above example by removing the last case block, the
example becomes:

   def http_error(status):
       match status:
           case 400:
               return "Bad request"
           case 404:
               return "Not found"
           case 418:
               return "I'm a teapot"

Without the use of "_" in a case statement, a match may not exist. If
no match exists, the behavior is a no-op. For example, if "status" of
500 is passed, a no-op occurs.


Pattterns with a literal and variable
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Patterns can look like unpacking assignments, and a pattern may be
used to bind variables. In this example, a data point can be unpacked
to its x-coordinate and y-coordinate:

   # point is an (x, y) tuple
   match point:
       case (0, 0):
           print("Origin")
       case (0, y):
           print(f"Y={y}")
       case (x, 0):
           print(f"X={x}")
       case (x, y):
           print(f"X={x}, Y={y}")
       case _:
           raise ValueError("Not a point")

The first pattern has two literals, "(0, 0)", and may be thought of as
an extension of the literal pattern shown above. The next two patterns
combine a literal and a variable, and the variable *binds* a value
from the subject ("point").  The fourth pattern captures two values,
which makes it conceptually similar to the unpacking assignment "(x,
y) = point".


Patterns and classes
~~~~~~~~~~~~~~~~~~~~

If you are using classes to structure your data, you can use as a
pattern the class name followed by an argument list resembling a
constructor. This pattern has the ability to capture class attributes
into variables:

   class Point:
       x: int
       y: int

   def location(point):
       match point:
           case Point(x=0, y=0):
               print("Origin is the point's location.")
           case Point(x=0, y=y):
               print(f"Y={y} and the point is on the y-axis.")
           case Point(x=x, y=0):
               print(f"X={x} and the point is on the x-axis.")
           case Point():
               print("The point is located somewhere else on the plane.")
           case _:
               print("Not a point")


Patterns with positional parameters
"""""""""""""""""""""""""""""""""""

You can use positional parameters with some builtin classes that
provide an ordering for their attributes (e.g. dataclasses). You can
also define a specific position for attributes in patterns by setting
the "__match_args__" special attribute in your classes. If it's set to
("x", "y"), the following patterns are all equivalent (and all bind
the "y" attribute to the "var" variable):

   Point(1, var)
   Point(1, y=var)
   Point(x=1, y=var)
   Point(y=var, x=1)


Nested patterns
~~~~~~~~~~~~~~~

Patterns can be arbitrarily nested.  For example, if our data is a
short list of points, it could be matched like this:

   match points:
       case []:
           print("No points in the list.")
       case [Point(0, 0)]:
           print("The origin is the only point in the list.")
       case [Point(x, y)]:
           print(f"A single point {x}, {y} is in the list.")
       case [Point(0, y1), Point(0, y2)]:
           print(f"Two points on the Y axis at {y1}, {y2} are in the list.")
       case _:
           print("Something else is found in the list.")


Complex patterns and the wildcard
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To this point, the examples have used "_" alone in the last case
statement. A wildcard can be used in more complex patterns, such as
"('error', code, _)". For example:

   match test_variable:
       case ('warning', code, 40):
           print("A warning has been received.")
       case ('error', code, _):
           print(f"An error {code} occured.")

In the above case, "test_variable" will match for ('error', code, 100)
and ('error', code, 800).


Guard
~~~~~

We can add an "if" clause to a pattern, known as a "guard".  If the
guard is false, "match" goes on to try the next case block.  Note that
value capture happens before the guard is evaluated:

   match point:
       case Point(x, y) if x == y:
           print(f"The point is located on the diagonal Y=X at {x}.")
       case Point(x, y):
           print(f"Point is not on the diagonal.")


Other Key Features
~~~~~~~~~~~~~~~~~~

Several other key features:

* Like unpacking assignments, tuple and list patterns have exactly the
  same meaning and actually match arbitrary sequences. Technically,
  the subject must be an instance of "collections.abc.Sequence".
  Therefore, an important exception is that patterns don't match
  iterators. Also, to prevent a common mistake, sequence patterns
  don't match strings.

* Sequence patterns support wildcards: "[x, y, *rest]" and "(x, y,
  *rest)" work similar to wildcards in unpacking assignments.  The
  name after "*" may also be "_", so "(x, y, *_)" matches a sequence
  of at least two items without binding the remaining items.

* Mapping patterns: "{"bandwidth": b, "latency": l}" captures the
  ""bandwidth"" and ""latency"" values from a dict.  Unlike sequence
  patterns, extra keys are ignored.  A wildcard "**rest" is also
  supported.  (But "**_" would be redundant, so it not allowed.)

* Subpatterns may be captured using the "as" keyword:

     case (Point(x1, y1), Point(x2, y2) as p2): ...

  This binds x1, y1, x2, y2 like you would expect without the "as"
  clause, and p2 to the entire second item of the subject.

* Most literals are compared by equality. However, the singletons
  "True", "False" and "None" are compared by identity.

* Named constants may be used in patterns.  These named constants must
  be dotted names to prevent the constant from being interpreted as a
  capture variable:

     from enum import Enum
     class Color(Enum):
         RED = 0
         GREEN = 1
         BLUE = 2

     match color:
         case Color.RED:
             print("I see red!")
         case Color.GREEN:
             print("Grass is green")
         case Color.BLUE:
             print("I'm feeling the blues :(")

For the full specification see **PEP 634**.  Motivation and rationale
are in **PEP 635**, and a longer tutorial is in **PEP 636**.


Better error messages in the parser
-----------------------------------

When parsing code that contains unclosed parentheses or brackets the
interpreter now includes the location of the unclosed bracket of
parentheses instead of displaying *SyntaxError: unexpected EOF while
parsing* or pointing to some incorrect location. For instance,
consider the following code (notice the unclosed '{'):

   expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
               38: 4, 39: 4, 45: 5, 46: 5, 47: 5, 48: 5, 49: 5, 54: 6,
   some_other_code = foo()

previous versions of the interpreter reported confusing places as the
location of the syntax error:

   File "example.py", line 3
       some_other_code = foo()
                       ^
   SyntaxError: invalid syntax

but in Python3.10 a more informative error is emitted:

   File "example.py", line 1
       expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
                  ^
   SyntaxError: '{' was never closed

In a similar way, errors involving unclosed string literals (single
and triple quoted) now point to the start of the string instead of
reporting EOF/EOL.

These improvements are inspired by previous work in the PyPy
interpreter.

(Contributed by Pablo Galindo in bpo-42864 and Batuhan Taskaya in
bpo-40176.)


Other Language Changes
======================

* Builtin and extension functions that take integer arguments no
  longer accept "Decimal"s, "Fraction"s and other objects that can be
  converted to integers only with a loss (e.g. that have the
  "__int__()" method but do not have the "__index__()" method).
  (Contributed by Serhiy Storchaka in bpo-37999.)

* If "object.__ipow__()" returns "NotImplemented", the operator will
  correctly fall back to "object.__pow__()" and "object.__rpow__()" as
  expected. (Contributed by Alex Shkop in bpo-38302.)

* Assignment expressions can now be used unparenthesized within set
  literals and set comprehensions, as well as in sequence indexes (but
  not slices).

* Functions have a new "__builtins__" attribute which is used to look
  for builtin symbols when a function is executed, instead of looking
  into "__globals__['__builtins__']". The attribute is initialized
  from "__globals__["__builtins__"]" if it exists, else from the
  current builtins. (Contributed by Mark Shannon in bpo-42990.)


New Modules
===========

* None yet.


Improved Modules
================


argparse
--------

Misleading phrase "optional arguments" was replaced with "options" in
argparse help. Some tests might require adaptation if they rely on
exact output match. (Contributed by Raymond Hettinger in bpo-9694.)


base64
------

Add "base64.b32hexencode()" and "base64.b32hexdecode()" to support the
Base32 Encoding with Extended Hex Alphabet.


codecs
------

Add a "codecs.unregister()" function to unregister a codec search
function. (Contributed by Hai Shi in bpo-41842.)


collections.abc
---------------

The "__args__" of the parameterized generic for
"collections.abc.Callable" are now consistent with "typing.Callable".
"collections.abc.Callable" generic now flattens type parameters,
similar to what "typing.Callable" currently does.  This means that
"collections.abc.Callable[[int, str], str]" will have "__args__" of
"(int, str, str)"; previously this was "([int, str], str)".  To allow
this change, "types.GenericAlias" can now be subclassed, and a
subclass will be returned when subscripting the
"collections.abc.Callable" type.  Note that a "TypeError" may be
raised for invalid forms of parameterizing "collections.abc.Callable"
which may have passed silently in Python 3.9. (Contributed by Ken Jin
in bpo-42195.)


contextlib
----------

Add a "contextlib.aclosing()" context manager to safely close async
generators and objects representing asynchronously released resources.
(Contributed by Joongi Kim and John Belmonte in bpo-41229.)

Add asynchronous context manager support to
"contextlib.nullcontext()". (Contributed by Tom Gringauz in
bpo-41543.)


curses
------

The extended color functions added in ncurses 6.1 will be used
transparently by "curses.color_content()", "curses.init_color()",
"curses.init_pair()", and "curses.pair_content()". A new function,
"curses.has_extended_color_support()", indicates whether extended
color support is provided by the underlying ncurses library.
(Contributed by Jeffrey Kintscher and Hans Petter Jansson in
bpo-36982.)

The "BUTTON5_*" constants are now exposed in the "curses" module if
they are provided by the underlying curses library. (Contributed by
Zackery Spytz in bpo-39273.)


distutils
---------

The entire "distutils" package is deprecated, to be removed in Python
3.12. Its functionality for specifying package builds has already been
completely replaced by third-party packages "setuptools" and
"packaging", and most other commonly used APIs are available elsewhere
in the standard library (such as "platform", "shutil", "subprocess" or
"sysconfig"). There are no plans to migrate any other functionality
from "distutils", and applications that are using other functions
should plan to make private copies of the code. Refer to **PEP 632**
for discussion.

The "bdist_wininst" command deprecated in Python 3.8 has been removed.
The "bdist_wheel" command is now recommended to distribute binary
packages on Windows. (Contributed by Victor Stinner in bpo-42802.)


doctest
-------

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


encodings
---------

"encodings.normalize_encoding()" now ignores non-ASCII characters.
(Contributed by Hai Shi in bpo-39337.)


glob
----

Added the *root_dir* and *dir_fd* parameters in "glob()" and "iglob()"
which allow to specify the root directory for searching. (Contributed
by Serhiy Storchaka in bpo-38144.)


inspect
-------

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)

Added *globalns* and *localns* parameters in "signature()" and
"inspect.Signature.from_callable()" to retrieve the annotations in
given local and global namespaces. (Contributed by Batuhan Taskaya in
bpo-41960.)


linecache
---------

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


os
--

Added "os.cpu_count()" support for VxWorks RTOS. (Contributed by
Peixing Xin in bpo-41440.)

Added a new function "os.eventfd()" and related helpers to wrap the
"eventfd2" syscall on Linux. (Contributed by Christian Heimes in
bpo-41001.)

Added "os.splice()" that allows to move data between two file
descriptors without copying between kernel address space and user
address space, where one of the file descriptors must refer to a pipe.
(Contributed by Pablo Galindo in bpo-41625.)

Added "O_EVTONLY", "O_FSYNC", "O_SYMLINK" and "O_NOFOLLOW_ANY" for
macOS. (Contributed by Dong-hee Na in bpo-43106.)


pathlib
-------

Added slice support to "PurePath.parents". (Contributed by Joshua
Cannon in bpo-35498)

Added negative indexing support to "PurePath.parents". (Contributed by
Yaroslav Pankovych in bpo-21041)


platform
--------

Added "platform.freedesktop_os_release()" to retrieve operation system
identification from freedesktop.org os-release standard file.
(Contributed by Christian Heimes in bpo-28468)


py_compile
----------

Added "--quiet" option to command-line interface of "py_compile".
(Contributed by Gregory Schevchenko in bpo-38731.)


pyclbr
------

Added an "end_lineno" attribute to the "Function" and "Class" objects
in the tree returned by "pyclbr.readline()" and
"pyclbr.readline_ex()".  It matches the existing (start) "lineno".
(Contributed by Aviral Srivastava in bpo-38307.)


shelve
------

The "shelve" module now uses "pickle.DEFAULT_PROTOCOL" by default
instead of "pickle" protocol "3" when creating shelves. (Contributed
by Zackery Spytz in bpo-34204.)


site
----

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


socket
------

The exception "socket.timeout" is now an alias of "TimeoutError".
(Contributed by Christian Heimes in bpo-42413.)


sys
---

Add "sys.orig_argv" attribute: the list of the original command line
arguments passed to the Python executable. (Contributed by Victor
Stinner in bpo-23427.)

Add "sys.stdlib_module_names", containing the list of the standard
library module names. (Contributed by Victor Stinner in bpo-42955.)


threading
---------

Added "threading.gettrace()" and "threading.getprofile()" to retrieve
the functions set by "threading.settrace()" and
"threading.setprofile()" respectively. (Contributed by Mario Corchero
in bpo-42251.)

Add "threading.__excepthook__" to allow retrieving the original value
of "threading.excepthook()" in case it is set to a broken or a
different value. (Contributed by Mario Corchero in bpo-42308.)


traceback
---------

The "format_exception()", "format_exception_only()", and
"print_exception()" functions can now take an exception object as a
positional-only argument. (Contributed by Zackery Spytz and Matthias
Bussonnier in bpo-26389.)


types
-----

Reintroduced the "types.EllipsisType", "types.NoneType" and
"types.NotImplementedType" classes, providing a new set of types
readily interpretable by type checkers. (Contributed by Bas van Beek
in bpo-41810.)


typing
------

The behavior of "typing.Literal" was changed to conform with **PEP
586** and to match the behavior of static type checkers specified in
the PEP.

1. "Literal" now de-duplicates parameters.

2. Equality comparisons between "Literal" objects are now order
   independent.

3. "Literal" comparisons now respects types.  For example, "Literal[0]
   == Literal[False]" previously evaluated to "True".  It is now
   "False".  To support this change, the internally used type cache
   now supports differentiating types.

4. "Literal" objects will now raise a "TypeError" exception during
   equality comparisons if one of their parameters are not
   *immutable*. Note that declaring "Literal" with mutable parameters
   will not throw an error:

      >>> from typing import Literal
      >>> Literal[{0}]
      >>> Literal[{0}] == Literal[{False}]
      Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
      TypeError: unhashable type: 'set'

(Contributed by Yurii Karabas in bpo-42345.)


unittest
--------

Add new method "assertNoLogs()" to complement the existing
"assertLogs()". (Contributed by Kit Yan Choi in bpo-39385.)


urllib.parse
------------

Python versions earlier than Python 3.10 allowed using both ";" and
"&" as query parameter separators in "urllib.parse.parse_qs()" and
"urllib.parse.parse_qsl()".  Due to security concerns, and to conform
with newer W3C recommendations, this has been changed to allow only a
single separator key, with "&" as the default.  This change also
affects "cgi.parse()" and "cgi.parse_multipart()" as they use the
affected functions internally.  For more details, please see their
respective documentation. (Contributed by Adam Goldschmidt, Senthil
Kumaran and Ken Jin in bpo-42967.)


xml
---

Add a "LexicalHandler" class to the "xml.sax.handler" module.
(Contributed by Jonathan Gossage and Zackery Spytz in bpo-35018.)


zipimport
---------

Add methods related to **PEP 451**: "find_spec()",
"zipimport.zipimporter.create_module()", and
"zipimport.zipimporter.exec_module()". (Contributed by Brett Cannon in
bpo-42131.


Optimizations
=============

* Constructors "str()", "bytes()" and "bytearray()" are now faster
  (around 30--40% for small objects). (Contributed by Serhiy Storchaka
  in bpo-41334.)

* The "runpy" module now imports fewer modules. The "python3 -m
  module-name" command startup time is 1.3x faster in average.
  (Contributed by Victor Stinner in bpo-41006.)

* The "LOAD_ATTR" instruction now uses new "per opcode cache"
  mechanism.  It is about 36% faster now for regular attributes and
  44% faster for slots. (Contributed by Pablo Galindo and Yury
  Selivanov in bpo-42093 and Guido van Rossum in bpo-42927, based on
  ideas implemented originally in PyPy and MicroPython.)

* When building Python with "--enable-optimizations" now "-fno-
  semantic-interposition" is added to both the compile and link line.
  This speeds builds of the Python interpreter created with "--enable-
  shared" with "gcc" by up to 30%. See this article for more details.
  (Contributed by Victor Stinner and Pablo Galindo in bpo-38980.)

* Function parameters and their annotations are no longer computed at
  runtime, but rather at compilation time.  They are stored as a tuple
  of strings at the bytecode level.  It is now around 100% faster to
  create a function with parameter annotations.  (Contributed by Yurii
  Karabas and Inada Naoki in bpo-42202)

* Substring search functions such as "str1 in str2" and
  "str2.find(str1)" now sometimes use Crochemore & Perrin's "Two-Way"
  string searching algorithm to avoid quadratic behavior on long
  strings.  (Contributed by Dennis Sweeney in bpo-41972)


Deprecated
==========

* Starting in this release, there will be a concerted effort to begin
  cleaning up old import semantics that were kept for Python 2.7
  compatibility. Specifically, "find_loader()"/"find_module()"
  (superseded by "find_spec()"), "load_module()" (superseded by
  "exec_module()"), "module_repr()" (which the import system takes
  care of for you), the "__package__" attribute (superseded by
  "__spec__.parent"), the "__loader__" attribute (superseded by
  "__spec__.loader"), and the "__cached__" attribute (superseded by
  "__spec__.cached") will slowly be removed (as well as other classes
  and methods in "importlib"). "ImportWarning" and/or
  "DeprecationWarning" will be raised as appropriate to help identify
  code which needs updating during this transition.

* The entire "distutils" namespace is deprecated, to be removed in
  Python 3.12. Refer to the module changes section for more
  information.

* Non-integer arguments to "random.randrange()" are deprecated. The
  "ValueError" is deprecated in favor of a "TypeError". (Contributed
  by Serhiy Storchaka and Raymond Hettinger in bpo-37319.)

* The various "load_module()" methods of "importlib" have been
  documented as deprecated since Python 3.6, but will now also trigger
  a "DeprecationWarning". Use "exec_module()" instead. (Contributed by
  Brett Cannon in bpo-26131.)

* "zimport.zipimporter.load_module()" has been deprecated in
  preference for "exec_module()". (Contributed by Brett Cannon in
  bpo-26131.)

* The use of "load_module()" by the import system now triggers an
  "ImportWarning" as "exec_module()" is preferred. (Contributed by
  Brett Cannon in bpo-26131.)

* "sqlite3.OptimizedUnicode" has been undocumented and obsolete since
  Python 3.3, when it was made an alias to "str".  It is now
  deprecated, scheduled for removal in Python 3.12. (Contributed by
  Erlend E. Aasland in bpo-42264.)

* The undocumented built-in function "sqlite3.enable_shared_cache" is
  now deprecated, scheduled for removal in Python 3.12.  Its use is
  strongly discouraged by the SQLite3 documentation.  See the SQLite3
  docs for more details. If shared cache must be used, open the
  database in URI mode using the "cache=shared" query parameter.
  (Contributed by Erlend E. Aasland in bpo-24464.)


Removed
=======

* Removed special methods "__int__", "__float__", "__floordiv__",
  "__mod__", "__divmod__", "__rfloordiv__", "__rmod__" and
  "__rdivmod__" of the "complex" class.  They always raised a
  "TypeError". (Contributed by Serhiy Storchaka in bpo-41974.)

* The "ParserBase.error()" method from the private and undocumented
  "_markupbase" module has been removed.  "html.parser.HTMLParser" is
  the only subclass of "ParserBase" and its "error()" implementation
  has already been removed in Python 3.5. (Contributed by Berker
  Peksag in bpo-31844.)

* Removed the "unicodedata.ucnhash_CAPI" attribute which was an
  internal PyCapsule object. The related private
  "_PyUnicode_Name_CAPI" structure was moved to the internal C API.
  (Contributed by Victor Stinner in bpo-42157.)

* Removed the "parser" module, which was deprecated in 3.9 due to the
  switch to the new PEG parser, as well as all the C source and header
  files that were only being used by the old parser, including
  "node.h", "parser.h", "graminit.h" and "grammar.h".

* Removed the Public C API functions
  "PyParser_SimpleParseStringFlags()",
  "PyParser_SimpleParseStringFlagsFilename()",
  "PyParser_SimpleParseFileFlags()" and "PyNode_Compile()" that were
  deprecated in 3.9 due to the switch to the new PEG parser.

* Removed the "formatter" module, which was deprecated in Python 3.4.
  It is somewhat obsolete, little used, and not tested. It was
  originally scheduled to be removed in Python 3.6, but such removals
  were delayed until after Python 2.7 EOL. Existing users should copy
  whatever classes they use into their code. (Contributed by Dong-hee
  Na and Terry J. Reedy in bpo-42299.)

* Removed the "PyModule_GetWarningsModule()" function that was useless
  now due to the _warnings module was converted to a builtin module in
  2.6. (Contributed by Hai Shi in bpo-42599.)

* Remove deprecated aliases to Collections Abstract Base Classes from
  the "collections" module. (Contributed by Victor Stinner in
  bpo-37324.)

* The "loop" parameter has been removed from most of "asyncio"'s high-
  level API following deprecation in Python 3.8.  The motivation
  behind this change is multifold:

  1. This simplifies the high-level API.

  2. The functions in the high-level API have been implicitly getting
     the current thread's running event loop since Python 3.7.  There
     isn't a need to pass the event loop to the API in most normal use
     cases.

  3. Event loop passing is error-prone especially when dealing with
     loops running in different threads.

  Note that the low-level API will still accept "loop". See Changes in
  the Python API for examples of how to replace existing code.

  (Contributed by Yurii Karabas, Andrew Svetlov, Yury Selivanov and
  Kyle Stanley in bpo-42392.)


Porting to Python 3.10
======================

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


Changes in the Python API
-------------------------

* The *etype* parameters of the "format_exception()",
  "format_exception_only()", and "print_exception()" functions in the
  "traceback" module have been renamed to *exc*. (Contributed by
  Zackery Spytz and Matthias Bussonnier in bpo-26389.)

* "atexit": At Python exit, if a callback registered with
  "atexit.register()" fails, its exception is now logged. Previously,
  only some exceptions were logged, and the last exception was always
  silently ignored. (Contributed by Victor Stinner in bpo-42639.)

* "collections.abc.Callable" generic now flattens type parameters,
  similar to what "typing.Callable" currently does.  This means that
  "collections.abc.Callable[[int, str], str]" will have "__args__" of
  "(int, str, str)"; previously this was "([int, str], str)".  Code
  which accesses the arguments via "typing.get_args()" or "__args__"
  need to account for this change.  Furthermore, "TypeError" may be
  raised for invalid forms of parameterizing
  "collections.abc.Callable" which may have passed silently in Python
  3.9. (Contributed by Ken Jin in bpo-42195.)

* "socket.htons()" and "socket.ntohs()" now raise "OverflowError"
  instead of "DeprecationWarning" if the given parameter will not fit
  in a 16-bit unsigned integer. (Contributed by Erlend E. Aasland in
  bpo-42393.)

* The "loop" parameter has been removed from most of "asyncio"'s high-
  level API following deprecation in Python 3.8.

  A coroutine that currently look like this:

     async def foo(loop):
         await asyncio.sleep(1, loop=loop)

  Should be replaced with this:

     async def foo():
         await asyncio.sleep(1)

  If "foo()" was specifically designed *not* to run in the current
  thread's running event loop (e.g. running in another thread's event
  loop), consider using "asyncio.run_coroutine_threadsafe()" instead.

  (Contributed by Yurii Karabas, Andrew Svetlov, Yury Selivanov and
  Kyle Stanley in bpo-42392.)

* The "types.FunctionType" constructor now inherits the current
  builtins if the *globals* dictionary has no ""__builtins__"" key,
  rather than using "{"None": None}" as builtins: same behavior as
  "eval()" and "exec()" functions.  Defining a function with "def
  function(...): ..." in Python is not affected, globals cannot be
  overriden with this syntax: it also inherits the current builtins.
  (Contributed by Victor Stinner in bpo-42990.)


CPython bytecode changes
========================

* The "MAKE_FUNCTION" instruction accepts tuple of strings as
  annotations instead of dictionary. (Contributed by Yurii Karabas and
  Inada Naoki in bpo-42202)


Build Changes
=============

* The C99 functions "snprintf()" and "vsnprintf()" are now required to
  build Python. (Contributed by Victor Stinner in bpo-36020.)

* "sqlite3" requires SQLite 3.7.15 or higher. (Contributed by Sergey
  Fedoseev and Erlend E. Aasland bpo-40744 and bpo-40810.)

* The "atexit" module must now always be built as a built-in module.
  (Contributed by Victor Stinner in bpo-42639.)

* Added "--disable-test-modules" option to the "configure" script:
  don't build nor install test modules. (Contributed by Xavier de
  Gaye, Thomas Petazzoni and Peixing Xin in bpo-27640.)

* Add "--with-wheel-pkg-dir=PATH" option to the "./configure" script.
  If specified, the "ensurepip" module looks for "setuptools" and
  "pip" wheel packages in this directory: if both are present, these
  wheel packages are used instead of ensurepip bundled wheel packages.

  Some Linux distribution packaging policies recommend against
  bundling dependencies. For example, Fedora installs wheel packages
  in the "/usr/share/python-wheels/" directory and don't install the
  "ensurepip._bundled" package.

  (Contributed by Victor Stinner in bpo-42856.)

* Add a new configure "--without-static-libpython" option to not build
  the "libpythonMAJOR.MINOR.a" static library and not install the
  "python.o" object file.

  (Contributed by Victor Stinner in bpo-43103.)

* The "configure" script now uses the "pkg-config" utility, if
  available, to detect the location of Tcl/Tk headers and libraries.
  As before, those locations can be explicitly specified with the "--
  with-tcltk-includes" and "--with-tcltk-libs" configuration options.
  (Contributed by Manolis Stamatogiannakis in bpo-42603.)


C API Changes
=============


New Features
------------

* The result of "PyNumber_Index()" now always has exact type "int".
  Previously, the result could have been an instance of a subclass of
  "int". (Contributed by Serhiy Storchaka in bpo-40792.)

* Add a new "orig_argv" member to the "PyConfig" structure: the list
  of the original command line arguments passed to the Python
  executable. (Contributed by Victor Stinner in bpo-23427.)

* The "PyDateTime_DATE_GET_TZINFO()" and
  "PyDateTime_TIME_GET_TZINFO()" macros have been added for accessing
  the "tzinfo" attributes of "datetime.datetime" and "datetime.time"
  objects. (Contributed by Zackery Spytz in bpo-30155.)

* Add a "PyCodec_Unregister()" function to unregister a codec search
  function. (Contributed by Hai Shi in bpo-41842.)

* The "PyIter_Send()" function was added to allow sending value into
  iterator without raising "StopIteration" exception. (Contributed by
  Vladimir Matveev in bpo-41756.)

* Added "PyUnicode_AsUTF8AndSize()" to the limited C API. (Contributed
  by Alex Gaynor in bpo-41784.)

* Added "PyModule_AddObjectRef()" function: similar to
  "PyModule_AddObject()" but don't steal a reference to the value on
  success. (Contributed by Victor Stinner in bpo-1635741.)

* Added "Py_NewRef()" and "Py_XNewRef()" functions to increment the
  reference count of an object and return the object. (Contributed by
  Victor Stinner in bpo-42262.)

* The "PyType_FromSpecWithBases()" and "PyType_FromModuleAndSpec()"
  functions now accept a single class as the *bases* argument.
  (Contributed by Serhiy Storchaka in bpo-42423.)

* The "PyType_FromModuleAndSpec()" function now accepts NULL "tp_doc"
  slot. (Contributed by Hai Shi in bpo-41832.)

* The "PyType_GetSlot()" function can accept static types.
  (Contributed by Hai Shi and Petr Viktorin in bpo-41073.)

* Add a new "PySet_CheckExact()" function to the C-API to check if an
  object is an instance of "set" but not an instance of a subtype.
  (Contributed by Pablo Galindo in bpo-43277.)


Porting to Python 3.10
----------------------

* The "PY_SSIZE_T_CLEAN" macro must now be defined to use
  "PyArg_ParseTuple()" and "Py_BuildValue()" formats which use "#":
  "es#", "et#", "s#", "u#", "y#", "z#", "U#" and "Z#". See Parsing
  arguments and building values and the **PEP 353**. (Contributed by
  Victor Stinner in bpo-40943.)

* Since "Py_REFCNT()" is changed to the inline static function,
  "Py_REFCNT(obj) = new_refcnt" must be replaced with
  "Py_SET_REFCNT(obj, new_refcnt)": see "Py_SET_REFCNT()" (available
  since Python 3.9). For backward compatibility, this macro can be
  used:

     #if PY_VERSION_HEX < 0x030900A4
     #  define Py_SET_REFCNT(obj, refcnt) ((Py_REFCNT(obj) = (refcnt)), (void)0)
     #endif

  (Contributed by Victor Stinner in bpo-39573.)

* Calling "PyDict_GetItem()" without *GIL* held had been allowed for
  historical reason. It is no longer allowed. (Contributed by Victor
  Stinner in bpo-40839.)

* "PyUnicode_FromUnicode(NULL, size)" and
  "PyUnicode_FromStringAndSize(NULL, size)" raise "DeprecationWarning"
  now.  Use "PyUnicode_New()" to allocate Unicode object without
  initial data. (Contributed by Inada Naoki in bpo-36346.)

* The private "_PyUnicode_Name_CAPI" structure of the PyCapsule API
  "unicodedata.ucnhash_CAPI" has been moved to the internal C API.
  (Contributed by Victor Stinner in bpo-42157.)

* "Py_GetPath()", "Py_GetPrefix()", "Py_GetExecPrefix()",
  "Py_GetProgramFullPath()", "Py_GetPythonHome()" and
  "Py_GetProgramName()" functions now return "NULL" if called before
  "Py_Initialize()" (before Python is initialized). Use the new Python
  Initialization Configuration API to get the Python Path
  Configuration.. (Contributed by Victor Stinner in bpo-42260.)

* "PyList_SET_ITEM()", "PyTuple_SET_ITEM()" and "PyCell_SET()" macros
  can no longer be used as l-value or r-value. For example, "x =
  PyList_SET_ITEM(a, b, c)" and "PyList_SET_ITEM(a, b, c) = x" now
  fail with a compiler error. It prevents bugs like "if
  (PyList_SET_ITEM (a, b, c) < 0) ..." test. (Contributed by Zackery
  Spytz and Victor Stinner in bpo-30459.)

* The non-limited API files "odictobject.h", "parser_interface.h",
  "picklebufobject.h", "pyarena.h", "pyctype.h", "pydebug.h",
  "pyfpe.h", and "pytime.h" have been moved to the "Include/cpython"
  directory. These files must not be included directly, as they are
  already included in "Python.h": Include Files. If they have been
  included directly, consider including "Python.h" instead.
  (Contributed by Nicholas Sim in bpo-35134)


Deprecated
----------

* The "PyUnicode_InternImmortal()" function is now deprecated and will
  be removed in Python 3.12: use "PyUnicode_InternInPlace()" instead.
  (Contributed by Victor Stinner in bpo-41692.)


Removed
-------

* "PyObject_AsCharBuffer()", "PyObject_AsReadBuffer()",
  "PyObject_CheckReadBuffer()", and "PyObject_AsWriteBuffer()" are
  removed. Please migrate to new buffer protocol;
  "PyObject_GetBuffer()" and "PyBuffer_Release()". (Contributed by
  Inada Naoki in bpo-41103.)

* Removed "Py_UNICODE_str*" functions manipulating "Py_UNICODE*"
  strings. (Contributed by Inada Naoki in bpo-41123.)

     * "Py_UNICODE_strlen": use "PyUnicode_GetLength()" or
       "PyUnicode_GET_LENGTH"

     * "Py_UNICODE_strcat": use "PyUnicode_CopyCharacters()" or
       "PyUnicode_FromFormat()"

     * "Py_UNICODE_strcpy", "Py_UNICODE_strncpy": use
       "PyUnicode_CopyCharacters()" or "PyUnicode_Substring()"

     * "Py_UNICODE_strcmp": use "PyUnicode_Compare()"

     * "Py_UNICODE_strncmp": use "PyUnicode_Tailmatch()"

     * "Py_UNICODE_strchr", "Py_UNICODE_strrchr": use
       "PyUnicode_FindChar()"

* Removed "PyUnicode_GetMax()". Please migrate to new (**PEP 393**)
  APIs. (Contributed by Inada Naoki in bpo-41103.)

* Removed "PyLong_FromUnicode()". Please migrate to
  "PyLong_FromUnicodeObject()". (Contributed by Inada Naoki in
  bpo-41103.)

* Removed "PyUnicode_AsUnicodeCopy()". Please use
  "PyUnicode_AsUCS4Copy()" or "PyUnicode_AsWideCharString()"
  (Contributed by Inada Naoki in bpo-41103.)

* Removed "_Py_CheckRecursionLimit" variable: it has been replaced by
  "ceval.recursion_limit" of the "PyInterpreterState" structure.
  (Contributed by Victor Stinner in bpo-41834.)

* Removed undocumented macros "Py_ALLOW_RECURSION" and
  "Py_END_ALLOW_RECURSION" and the "recursion_critical" field of the
  "PyInterpreterState" structure. (Contributed by Serhiy Storchaka in
  bpo-41936.)

* Removed the undocumented "PyOS_InitInterrupts()" function.
  Initializing Python already implicitly installs signal handlers: see
  "PyConfig.install_signal_handlers". (Contributed by Victor Stinner
  in bpo-41713.)
