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

Release:
   3.10.1

Date:
   December 19, 2021

Editor:
   Pablo Galindo Salgado

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

For full details, see the changelog.


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

New syntax features:

* **PEP 634**, Structural Pattern Matching: Specification

* **PEP 635**, Structural Pattern Matching: Motivation and Rationale

* **PEP 636**, Structural Pattern Matching: Tutorial

* bpo-12782, Parenthesized context managers are now officially
  allowed.

New features in the standard library:

* **PEP 618**, Add Optional Length-Checking To zip.

Interpreter improvements:

* **PEP 626**, Precise line numbers for debugging and other tools.

New typing features:

* **PEP 604**, Allow writing union types as X | Y

* **PEP 613**, Explicit Type Aliases

* **PEP 612**, Parameter Specification Variables

Important deprecations, removals or restrictions:

* **PEP 644**, Require OpenSSL 1.1.1 or newer

* **PEP 632**, Deprecate distutils module.

* **PEP 623**, Deprecate and prepare for the removal of the wstr
  member in PyUnicodeObject.

* **PEP 624**, Remove Py_UNICODE encoder APIs

* **PEP 597**, Add optional EncodingWarning


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.)


Better error messages
---------------------


SyntaxErrors
~~~~~~~~~~~~

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 Python 3.10 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.)

"SyntaxError" exceptions raised by the interpreter will now highlight
the full error range of the expression that constitutes the syntax
error itself, instead of just where the problem is detected. In this
way, instead of displaying (before Python 3.10):

   >>> foo(x, z for z in range(10), t, w)
     File "<stdin>", line 1
       foo(x, z for z in range(10), t, w)
              ^
   SyntaxError: Generator expression must be parenthesized

now Python 3.10 will display the exception as:

   >>> foo(x, z for z in range(10), t, w)
     File "<stdin>", line 1
       foo(x, z for z in range(10), t, w)
              ^^^^^^^^^^^^^^^^^^^^
   SyntaxError: Generator expression must be parenthesized

This improvement was contributed by Pablo Galindo in bpo-43914.

A considerable amount of new specialized messages for "SyntaxError"
exceptions have been incorporated. Some of the most notable ones are
as follows:

* Missing ":" before blocks:

        >>> if rocket.position > event_horizon
          File "<stdin>", line 1
            if rocket.position > event_horizon
                                              ^
        SyntaxError: expected ':'

     (Contributed by Pablo Galindo in bpo-42997)

* Unparenthesised tuples in comprehensions targets:

        >>> {x,y for x,y in zip('abcd', '1234')}
          File "<stdin>", line 1
            {x,y for x,y in zip('abcd', '1234')}
             ^
        SyntaxError: did you forget parentheses around the comprehension target?

     (Contributed by Pablo Galindo in bpo-43017)

* Missing commas in collection literals and between expressions:

        >>> items = {
        ... x: 1,
        ... y: 2
        ... z: 3,
          File "<stdin>", line 3
            y: 2
               ^
        SyntaxError: invalid syntax. Perhaps you forgot a comma?

     (Contributed by Pablo Galindo in bpo-43822)

* Multiple Exception types without parentheses:

        >>> try:
        ...     build_dyson_sphere()
        ... except NotEnoughScienceError, NotEnoughResourcesError:
          File "<stdin>", line 3
            except NotEnoughScienceError, NotEnoughResourcesError:
                   ^
        SyntaxError: multiple exception types must be parenthesized

     (Contributed by Pablo Galindo in bpo-43149)

* Missing ":" and values in dictionary literals:

        >>> values = {
        ... x: 1,
        ... y: 2,
        ... z:
        ... }
          File "<stdin>", line 4
            z:
             ^
        SyntaxError: expression expected after dictionary key and ':'

        >>> values = {x:1, y:2, z w:3}
          File "<stdin>", line 1
            values = {x:1, y:2, z w:3}
                                ^
        SyntaxError: ':' expected after dictionary key

     (Contributed by Pablo Galindo in bpo-43823)

* "try" blocks without "except" or "finally" blocks:

        >>> try:
        ...     x = 2
        ... something = 3
          File "<stdin>", line 3
            something  = 3
            ^^^^^^^^^
        SyntaxError: expected 'except' or 'finally' block

     (Contributed by Pablo Galindo in bpo-44305)

* Usage of "=" instead of "==" in comparisons:

        >>> if rocket.position = event_horizon:
          File "<stdin>", line 1
            if rocket.position = event_horizon:
                               ^
        SyntaxError: cannot assign to attribute here. Maybe you meant '==' instead of '='?

     (Contributed by Pablo Galindo in bpo-43797)

* Usage of "*" in f-strings:

        >>> f"Black holes {*all_black_holes} and revelations"
          File "<stdin>", line 1
            (*all_black_holes)
             ^
        SyntaxError: f-string: cannot use starred expression here

     (Contributed by Pablo Galindo in bpo-41064)


IndentationErrors
~~~~~~~~~~~~~~~~~

Many "IndentationError" exceptions now have more context regarding
what kind of block was expecting an indentation, including the
location of the statement:

   >>> def foo():
   ...    if lel:
   ...    x = 2
     File "<stdin>", line 3
       x = 2
       ^
   IndentationError: expected an indented block after 'if' statement in line 2


AttributeErrors
~~~~~~~~~~~~~~~

When printing "AttributeError", "PyErr_Display()" will offer
suggestions of similar attribute names in the object that the
exception was raised from:

   >>> collections.namedtoplo
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?

(Contributed by Pablo Galindo in bpo-38530.)

   Warning:

     Notice this won't work if "PyErr_Display()" is not called to
     display the error which can happen if some other custom error
     display function is used. This is a common scenario in some REPLs
     like IPython.


NameErrors
~~~~~~~~~~

When printing "NameError" raised by the interpreter, "PyErr_Display()"
will offer suggestions of similar variable names in the function that
the exception was raised from:

   >>> schwarzschild_black_hole = None
   >>> schwarschild_black_hole
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   NameError: name 'schwarschild_black_hole' is not defined. Did you mean: schwarzschild_black_hole?

(Contributed by Pablo Galindo in bpo-38530.)

   Warning:

     Notice this won't work if "PyErr_Display()" is not called to
     display the error, which can happen if some other custom error
     display function is used. This is a common scenario in some REPLs
     like IPython.


PEP 626: Precise line numbers for debugging and other tools
-----------------------------------------------------------

PEP 626 brings more precise and reliable line numbers for debugging,
profiling and coverage tools. Tracing events, with the correct line
number, are generated for all lines of code executed and only for
lines of code that are executed.

The "f_lineno" attribute of frame objects will always contain the
expected line number.

The "co_lnotab" attribute of code objects is deprecated and will be
removed in 3.12. Code that needs to convert from offset to line number
should use the new "co_lines()" method instead.


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 exist, 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.


Patterns 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} occurred.")

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 a 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 is 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**.


Optional "EncodingWarning" and "encoding="locale"" option
---------------------------------------------------------

The default encoding of "TextIOWrapper" and "open()" is platform and
locale dependent. Since UTF-8 is used on most Unix platforms, omitting
"encoding" option when opening UTF-8 files (e.g. JSON, YAML, TOML,
Markdown) is a very common bug. For example:

   # BUG: "rb" mode or encoding="utf-8" should be used.
   with open("data.json") as f:
       data = json.load(f)

To find this type of bug, an optional "EncodingWarning" is added. It
is emitted when "sys.flags.warn_default_encoding" is true and locale-
specific default encoding is used.

"-X warn_default_encoding" option and "PYTHONWARNDEFAULTENCODING" are
added to enable the warning.

See Text Encoding for more information.


New Features Related to Type Hints
==================================

This section covers major changes affecting **PEP 484** type hints and
the "typing" module.


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.

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, with
additions by Yurii Karabas and Serhiy Storchaka in bpo-44490.)


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",
"typing.ParamSpecArgs", "typing.ParamSpecKwargs", and **PEP 612** for
more details.

(Contributed by Ken Jin in bpo-41559, with minor enhancements by Jelle
Zijlstra in bpo-43783.  PEP written by Mark Mendoza.)


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

**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 value "TypeAlias" which lets you
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 647: User-Defined Type Guards
---------------------------------

"TypeGuard" has been added to the "typing" module to annotate type
guard functions and improve information provided to static type
checkers during type narrowing.  For more information, please see
"TypeGuard"'s documentation, and **PEP 647**.

(Contributed by Ken Jin and Guido van Rossum in bpo-43766. PEP written
by Eric Traut.)


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

* 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.

* 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.)

* Two new builtin functions -- "aiter()" and "anext()" have been added
  to provide asynchronous counterparts to "iter()" and "next()",
  respectively. (Contributed by Joshua Bronson, Daniel Pope, and
  Justin Wang in bpo-31861.)

* Static methods ("@staticmethod") and class methods ("@classmethod")
  now inherit the method attributes ("__module__", "__name__",
  "__qualname__", "__doc__", "__annotations__") and have a new
  "__wrapped__" attribute. Moreover, static methods are now callable
  as regular functions. (Contributed by Victor Stinner in bpo-43682.)

* Annotations for complex targets (everything beside "simple name"
  targets defined by **PEP 526**) no longer cause any runtime effects
  with "from __future__ import annotations". (Contributed by Batuhan
  Taskaya in bpo-42737.)

* Class and module objects now lazy-create empty annotations dicts on
  demand. The annotations dicts are stored in the object’s "__dict__"
  for backwards compatibility.  This improves the best practices for
  working with "__annotations__"; for more information, please see
  Annotations Best Practices. (Contributed by Larry Hastings in
  bpo-43901.)

* Annotations consist of "yield", "yield from", "await" or named
  expressions are now forbidden under "from __future__ import
  annotations" due to their side effects. (Contributed by Batuhan
  Taskaya in bpo-42725.)

* Usage of unbound variables, "super()" and other expressions that
  might alter the processing of symbol table as annotations are now
  rendered effectless under "from __future__ import annotations".
  (Contributed by Batuhan Taskaya in bpo-42725.)

* Hashes of NaN values of both "float" type and "decimal.Decimal" type
  now depend on object identity. Formerly, they always hashed to "0"
  even though NaN values are not equal to one another. This caused
  potentially quadratic runtime behavior due to excessive hash
  collisions when creating dictionaries and sets containing multiple
  NaNs. (Contributed by Raymond Hettinger in bpo-43475.)

* A "SyntaxError" (instead of a "NameError") will be raised when
  deleting the "__debug__" constant.  (Contributed by Dong-hee Na in
  bpo-45000.)

* "SyntaxError" exceptions now have "end_lineno" and "end_offset"
  attributes.  They will be "None" if not determined. (Contributed by
  Pablo Galindo in bpo-43914.)


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

* None yet.


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


asyncio
-------

Add missing "connect_accepted_socket()" method. (Contributed by Alex
Grönholm in bpo-41332.)


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.)


array
-----

The "index()" method of "array.array" now has optional *start* and
*stop* parameters. (Contributed by Anders Lorentsen and Zackery Spytz
in bpo-31956.)


asynchat, asyncore, smtpd
-------------------------

These modules have been marked as deprecated in their module
documentation since Python 3.6.  An import-time "DeprecationWarning"
has now been added to all three of these modules.


base64
------

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


bdb
---

Add "clearBreakpoints()" to reset all set breakpoints. (Contributed by
Irit Katriel in bpo-24160.)


bisect
------

Added the possibility of providing a *key* function to the APIs in the
"bisect" module. (Contributed by Raymond Hettinger in bpo-4356.)


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.)

Add "AsyncContextDecorator", for supporting usage of async context
managers as decorators.


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.)


dataclasses
-----------


__slots__
~~~~~~~~~

Added "slots" parameter in "dataclasses.dataclass()" decorator.
(Contributed by Yurii Karabas in bpo-42269)


Keyword-only fields
~~~~~~~~~~~~~~~~~~~

dataclasses now supports fields that are keyword-only in the generated
__init__ method.  There are a number of ways of specifying keyword-
only fields.

You can say that every field is keyword-only:

   from dataclasses import dataclass

   @dataclass(kw_only=True)
   class Birthday:
       name: str
       birthday: datetime.date

Both "name" and "birthday" are keyword-only parameters to the
generated __init__ method.

You can specify keyword-only on a per-field basis:

   from dataclasses import dataclass

   @dataclass
   class Birthday:
       name: str
       birthday: datetime.date = field(kw_only=True)

Here only "birthday" is keyword-only.  If you set "kw_only" on
individual fields, be aware that there are rules about re-ordering
fields due to keyword-only fields needing to follow non-keyword-only
fields.  See the full dataclasses documentation for details.

You can also specify that all fields following a KW_ONLY marker are
keyword-only.  This will probably be the most common usage:

   from dataclasses import dataclass, KW_ONLY

   @dataclass
   class Point:
       x: float
       y: float
       _: KW_ONLY
       z: float = 0.0
       t: float = 0.0

Here, "z" and "t" are keyword-only parameters, while "x" and "y" are
not. (Contributed by Eric V. Smith in bpo-43532)


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.)


fileinput
---------

Add *encoding* and *errors* parameters in "fileinput.input()" and
"fileinput.FileInput". (Contributed by Inada Naoki in bpo-43712.)

"fileinput.hook_compressed()" now returns "TextIOWrapper" object when
*mode* is "r" and file is compressed, like uncompressed files.
(Contributed by Inada Naoki in bpo-5758.)


faulthandler
------------

The "faulthandler" module now detects if a fatal error occurs during a
garbage collector collection. (Contributed by Victor Stinner in
bpo-44466.)


gc
--

Add audit hooks for "gc.get_objects()", "gc.get_referrers()" and
"gc.get_referents()". (Contributed by Pablo Galindo in bpo-43439.)


glob
----

Add 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.)


hashlib
-------

The hashlib module requires OpenSSL 1.1.1 or newer. (Contributed by
Christian Heimes in **PEP 644** and bpo-43669.)

The hashlib module has preliminary support for OpenSSL 3.0.0.
(Contributed by Christian Heimes in bpo-38820 and other issues.)

The pure-Python fallback of "pbkdf2_hmac()" is deprecated. In the
future PBKDF2-HMAC will only be available when Python has been built
with OpenSSL support. (Contributed by Christian Heimes in bpo-43880.)


hmac
----

The hmac module now uses OpenSSL's HMAC implementation internally.
(Contributed by Christian Heimes in bpo-40645.)


IDLE and idlelib
----------------

Make IDLE invoke "sys.excepthook()" (when started without '-n'). User
hooks were previously ignored.  (Patch by Ken Hilton in bpo-43008.)

This change was backported to a 3.9 maintenance release.

Add a Shell sidebar.  Move the primary prompt ('>>>') to the sidebar.
Add secondary prompts ('...') to the sidebar.  Left click and optional
drag selects one or more lines of text, as with the editor line number
sidebar.  Right click after selecting text lines displays a context
menu with 'copy with prompts'.  This zips together prompts from the
sidebar with lines from the selected text.  This option also appears
on the context menu for the text.  (Contributed by Tal Einat in
bpo-37903.)

Use spaces instead of tabs to indent interactive code.  This makes
interactive code entries 'look right'.  Making this feasible was a
major motivation for adding the shell sidebar.  Contributed by Terry
Jan Reedy in bpo-37892.)

We expect to backport these shell changes to a future 3.9 maintenance
release.

Highlight the new soft keywords "match", "case", and "_" in pattern-
matching statements. However, this highlighting is not perfect and
will be incorrect in some rare cases, including some "_"-s in "case"
patterns.  (Contributed by Tal Einat in bpo-44010.)


importlib.metadata
------------------

Feature parity with "importlib_metadata" 4.6 (history).

importlib.metadata entry points now provide a nicer experience for
selecting entry points by group and name through a new
"importlib.metadata.EntryPoints" class. See the Compatibility Note in
the docs for more info on the deprecation and usage.

Added "importlib.metadata.packages_distributions()" for resolving top-
level Python modules and packages to their
"importlib.metadata.Distribution".


inspect
-------

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

Add "inspect.get_annotations()", which safely computes the annotations
defined on an object. It works around the quirks of accessing the
annotations on various types of objects, and makes very few
assumptions about the object it examines. "inspect.get_annotations()"
can also correctly un-stringize stringized annotations.
"inspect.get_annotations()" is now considered best practice for
accessing the annotations dict defined on any Python object; for more
information on best practices for working with annotations, please see
Annotations Best Practices. Relatedly, "inspect.signature()",
"inspect.Signature.from_callable()", and
"inspect.Signature.from_function()" now call
"inspect.get_annotations()" to retrieve annotations. This means
"inspect.signature()" and "inspect.Signature.from_callable()" can also
now un-stringize stringized annotations. (Contributed by Larry
Hastings in bpo-43817.)


linecache
---------

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


os
--

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

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

Add "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.)

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


os.path
-------

"os.path.realpath()" now accepts a *strict* keyword-only argument.
When set to "True", "OSError" is raised if a path doesn't exist or a
symlink loop is encountered. (Contributed by Barney Gale in
bpo-43757.)


pathlib
-------

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

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

Add "Path.hardlink_to" method that supersedes "link_to()". The new
method has the same argument order as "symlink_to()". (Contributed by
Barney Gale in bpo-39950.)

"pathlib.Path.stat()" and "chmod()" now accept a *follow_symlinks*
keyword-only argument for consistency with corresponding functions in
the "os" module. (Contributed by Barney Gale in bpo-39906.)


platform
--------

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


pprint
------

"pprint.pprint()" now accepts a new "underscore_numbers" keyword
argument. (Contributed by sblondon in bpo-42914.)

"pprint" can now pretty-print "dataclasses.dataclass" instances.
(Contributed by Lewis Gaul in bpo-43080.)


py_compile
----------

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


pyclbr
------

Add 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.)


statistics
----------

Add "covariance()", Pearson's "correlation()", and simple
"linear_regression()" functions. (Contributed by Tymoteusz Wołodźko in
bpo-38490.)


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.)

Add option to create MPTCP sockets with "IPPROTO_MPTCP" (Contributed
by Rui Cunha in bpo-43571.)

Add "IP_RECVTOS" option to receive the type of service (ToS) or
DSCP/ECN fields (Contributed by Georg Sauthoff in bpo-44077.)


ssl
---

The ssl module requires OpenSSL 1.1.1 or newer. (Contributed by
Christian Heimes in **PEP 644** and bpo-43669.)

The ssl module has preliminary support for OpenSSL 3.0.0 and new
option "OP_IGNORE_UNEXPECTED_EOF". (Contributed by Christian Heimes in
bpo-38820, bpo-43794, bpo-43788, bpo-43791, bpo-43799, bpo-43920,
bpo-43789, and bpo-43811.)

Deprecated function and use of deprecated constants now result in a
"DeprecationWarning". "ssl.SSLContext.options" has "OP_NO_SSLv2" and
"OP_NO_SSLv3" set by default and therefore cannot warn about setting
the flag again. The deprecation section has a list of deprecated
features. (Contributed by Christian Heimes in bpo-43880.)

The ssl module now has more secure default settings. Ciphers without
forward secrecy or SHA-1 MAC are disabled by default. Security level 2
prohibits weak RSA, DH, and ECC keys with less than 112 bits of
security. "SSLContext" defaults to minimum protocol version TLS 1.2.
Settings are based on Hynek Schlawack's research. (Contributed by
Christian Heimes in bpo-43998.)

The deprecated protocols SSL 3.0, TLS 1.0, and TLS 1.1 are no longer
officially supported. Python does not block them actively. However
OpenSSL build options, distro configurations, vendor patches, and
cipher suites may prevent a successful handshake.

Add a *timeout* parameter to the "ssl.get_server_certificate()"
function. (Contributed by Zackery Spytz in bpo-31870.)

The ssl module uses heap-types and multi-phase initialization.
(Contributed by Christian Heimes in bpo-42333.)

A new verify flag "VERIFY_X509_PARTIAL_CHAIN" has been added.
(Contributed by l0x in bpo-40849.)


sqlite3
-------

Add audit events for "connect/handle()", "enable_load_extension()",
and "load_extension()". (Contributed by Erlend E. Aasland in
bpo-43762.)


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.)


_thread
-------

"_thread.interrupt_main()" now takes an optional signal number to
simulate (the default is still "signal.SIGINT"). (Contributed by
Antoine Pitrou in bpo-43356.)


threading
---------

Add "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
-----

Reintroduce 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
------

For major changes, see New Features Related to Type Hints.

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 any of their parameters are not *hashable*.
   Note that declaring "Literal" with unhashable 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.)

Add new function "typing.is_typeddict()" to introspect if an
annotation is a "typing.TypedDict". (Contributed by Patrick Reader in
bpo-41792)

Subclasses of "typing.Protocol" which only have data variables
declared will now raise a "TypeError" when checked with "isinstance"
unless they are decorated with "runtime_checkable()".  Previously,
these checks passed silently.  Users should decorate their subclasses
with the "runtime_checkable()" decorator if they want runtime
protocols. (Contributed by Yurii Karabas in bpo-38908)

Importing from the "typing.io" and "typing.re" submodules will now
emit "DeprecationWarning".  These submodules have been deprecated
since Python 3.8 and will be removed in a future version of Python.
Anything belonging to those submodules should be imported directly
from "typing" instead. (Contributed by Sebastian Rittau in bpo-38291)


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.)

The presence of newline or tab characters in parts of a URL allows for
some forms of attacks. Following the WHATWG specification that updates
**RFC 3986**, ASCII newline "\n", "\r" and tab "\t" characters are
stripped from the URL by the parser in "urllib.parse" preventing such
attacks. The removal characters are controlled by a new module level
variable "urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE". (See bpo-43882)


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.)

Add "invalidate_caches()" method. (Contributed by Desmond Cheong in
bpo-14678.)


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 1.4x faster in average. On
  Linux, "python3 -I -m module-name" imports 69 modules on Python 3.9,
  whereas it only imports 51 modules (-18) on Python 3.10.
  (Contributed by Victor Stinner in bpo-41006 and bpo-41718.)

* 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.)

* Use a new output buffer management code for "bz2" / "lzma" / "zlib"
  modules, and add ".readall()" function to
  "_compression.DecompressReader" class. bz2 decompression is now
  1.09x ~ 1.17x faster, lzma decompression 1.20x ~ 1.32x faster,
  "GzipFile.read(-1)" 1.11x ~ 1.18x faster. (Contributed by Ma Lin,
  reviewed by Gregory P. Smith, in bpo-41486)

* When using stringized annotations, annotations dicts for functions
  are no longer created when the function is created.  Instead, they
  are stored as a tuple of strings, and the function object lazily
  converts this into the annotations dict on demand.  This
  optimization cuts the CPU time needed to define an annotated
  function by half. (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)

* Add micro-optimizations to "_PyType_Lookup()" to improve type
  attribute cache lookup performance in the common case of cache hits.
  This makes the interpreter 1.04 times faster on average.
  (Contributed by Dino Viehland in bpo-43452)

* The following built-in functions now support the faster **PEP 590**
  vectorcall calling convention: "map()", "filter()", "reversed()",
  "bool()" and "float()". (Contributed by Dong-hee Na and Jeroen
  Demeyer in bpo-43575, bpo-43287, bpo-41922, bpo-41873 and bpo-41870)

* "BZ2File" performance is improved by removing internal "RLock". This
  makes "BZ2File" thread unsafe in the face of multiple simultaneous
  readers or writers, just like its equivalent classes in "gzip" and
  "lzma" have always been.  (Contributed by Inada Naoki in bpo-43785).


Deprecated
==========

* Currently Python accepts numeric literals immediately followed by
  keywords, for example "0in x", "1or x", "0if 1else 2".  It allows
  confusing and ambiguous expressions like "[0x1for x in y]" (which
  can be interpreted as "[0x1 for x in y]" or "[0x1f or x in y]").
  Starting in this release, a deprecation warning is raised if the
  numeric literal is immediately followed by one of keywords "and",
  "else", "for", "if", "in", "is" and "or". In future releases it will
  be changed to syntax warning, and finally to syntax error.
  (Contributed by Serhiy Storchaka in bpo-43833).

* 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.)

* The use of "importlib.abc.MetaPathFinder.find_module()" and
  "importlib.abc.PathEntryFinder.find_module()" by the import system
  now trigger an "ImportWarning" as
  "importlib.abc.MetaPathFinder.find_spec()" and
  "importlib.abc.PathEntryFinder.find_spec()" are preferred,
  respectively. You can use "importlib.util.spec_from_loader()" to
  help in porting. (Contributed by Brett Cannon in bpo-42134.)

* The use of "importlib.abc.PathEntryFinder.find_loader()" by the
  import system now triggers an "ImportWarning" as
  "importlib.abc.PathEntryFinder.find_spec()" is preferred. You can
  use "importlib.util.spec_from_loader()" to help in porting.
  (Contributed by Brett Cannon in bpo-43672.)

* The various implementations of
  "importlib.abc.MetaPathFinder.find_module()" (
  "importlib.machinery.BuiltinImporter.find_module()",
  "importlib.machinery.FrozenImporter.find_module()",
  "importlib.machinery.WindowsRegistryFinder.find_module()",
  "importlib.machinery.PathFinder.find_module()",
  "importlib.abc.MetaPathFinder.find_module()"),
  "importlib.abc.PathEntryFinder.find_module()" (
  "importlib.machinery.FileFinder.find_module()", ), and
  "importlib.abc.PathEntryFinder.find_loader()" (
  "importlib.machinery.FileFinder.find_loader()" ) now raise
  "DeprecationWarning" and are slated for removal in Python 3.12
  (previously they were documented as deprecated in Python 3.4).
  (Contributed by Brett Cannon in bpo-42135.)

* "importlib.abc.Finder" is deprecated (including its sole method,
  "find_module()"). Both "importlib.abc.MetaPathFinder" and
  "importlib.abc.PathEntryFinder" no longer inherit from the class.
  Users should inherit from one of these two classes as appropriate
  instead. (Contributed by Brett Cannon in bpo-42135.)

* The deprecations of "imp", "importlib.find_loader()",
  "importlib.util.set_package_wrapper()",
  "importlib.util.set_loader_wrapper()",
  "importlib.util.module_for_loader()", "pkgutil.ImpImporter", and
  "pkgutil.ImpLoader" have all been updated to list Python 3.12 as the
  slated version of removal (they began raising "DeprecationWarning"
  in previous versions of Python). (Contributed by Brett Cannon in
  bpo-43720.)

* The import system now uses the "__spec__" attribute on modules
  before falling back on "module_repr()" for a module's "__repr__()"
  method. Removal of the use of "module_repr()" is scheduled for
  Python 3.12. (Contributed by Brett Cannon in bpo-42137.)

* "importlib.abc.Loader.module_repr()",
  "importlib.machinery.FrozenLoader.module_repr()", and
  "importlib.machinery.BuiltinLoader.module_repr()" are deprecated and
  slated for removal in Python 3.12. (Contributed by Brett Cannon in
  bpo-42136.)

* "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.)

* "asyncio.get_event_loop()" now emits a deprecation warning if there
  is no running event loop. In the future it will be an alias of
  "get_running_loop()". "asyncio" functions which implicitly create a
  "Future" or "Task" objects now emit a deprecation warning if there
  is no running event loop and no explicit *loop* argument is passed:
  "ensure_future()", "wrap_future()", "gather()", "shield()",
  "as_completed()" and constructors of "Future", "Task",
  "StreamReader", "StreamReaderProtocol". (Contributed by Serhiy
  Storchaka in bpo-39529.)

* 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 a 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.)

* The following "threading" methods are now deprecated:

  * "threading.currentThread" => "threading.current_thread()"

  * "threading.activeCount" => "threading.active_count()"

  * "threading.Condition.notifyAll" =>
    "threading.Condition.notify_all()"

  * "threading.Event.isSet" => "threading.Event.is_set()"

  * "threading.Thread.setName" => "threading.Thread.name"

  * "threading.thread.getName" => "threading.Thread.name"

  * "threading.Thread.isDaemon" => "threading.Thread.daemon"

  * "threading.Thread.setDaemon" => "threading.Thread.daemon"

  (Contributed by Jelle Zijlstra in bpo-21574.)

* "pathlib.Path.link_to()" is deprecated and slated for removal in
  Python 3.12. Use "pathlib.Path.hardlink_to()" instead. (Contributed
  by Barney Gale in bpo-39950.)

* "cgi.log()" is deprecated and slated for removal in Python 3.12.
  (Contributed by Inada Naoki in bpo-41139.)

* The following "ssl" features have been deprecated since Python 3.6,
  Python 3.7, or OpenSSL 1.1.0 and will be removed in 3.11:

  * "OP_NO_SSLv2", "OP_NO_SSLv3", "OP_NO_TLSv1", "OP_NO_TLSv1_1",
    "OP_NO_TLSv1_2", and "OP_NO_TLSv1_3" are replaced by
    "sslSSLContext.minimum_version" and
    "sslSSLContext.maximum_version".

  * "PROTOCOL_SSLv2", "PROTOCOL_SSLv3", "PROTOCOL_SSLv23",
    "PROTOCOL_TLSv1", "PROTOCOL_TLSv1_1", "PROTOCOL_TLSv1_2", and
    "PROTOCOL_TLS" are deprecated in favor of "PROTOCOL_TLS_CLIENT"
    and "PROTOCOL_TLS_SERVER"

  * "wrap_socket()" is replaced by "ssl.SSLContext.wrap_socket()"

  * "match_hostname()"

  * "RAND_pseudo_bytes()", "RAND_egd()"

  * NPN features like "ssl.SSLSocket.selected_npn_protocol()" and
    "ssl.SSLContext.set_npn_protocols()" are replaced by ALPN.

* The threading debug ("PYTHONTHREADDEBUG" environment variable) is
  deprecated in Python 3.10 and will be removed in Python 3.12. This
  feature requires a debug build of Python. (Contributed by Victor
  Stinner in bpo-44584.)

* Importing from the "typing.io" and "typing.re" submodules will now
  emit "DeprecationWarning".  These submodules will be removed in a
  future version of Python.  Anything belonging to these submodules
  should be imported directly from "typing" instead. (Contributed by
  Sebastian Rittau in bpo-38291)


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
  was already 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 syntax
----------------------------

* Deprecation warning is now emitted when compiling previously valid
  syntax if the numeric literal is immediately followed by a keyword
  (like in "0in x"). In future releases it will be changed to syntax
  warning, and finally to a syntax error.  To get rid of the warning
  and make the code compatible with future releases just add a space
  between the numeric literal and the following keyword. (Contributed
  by Serhiy Storchaka in bpo-43833).


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 looks 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
  overridden with this syntax: it also inherits the current builtins.
  (Contributed by Victor Stinner in bpo-42990.)


Changes in the C API
--------------------

* The C API functions "PyParser_SimpleParseStringFlags",
  "PyParser_SimpleParseStringFlagsFilename",
  "PyParser_SimpleParseFileFlags", "PyNode_Compile" and the type used
  by these functions, "struct _node", were removed due to the switch
  to the new PEG parser.

  Source should be now be compiled directly to a code object using,
  for example, "Py_CompileString()". The resulting code object can
  then be evaluated using, for example, "PyEval_EvalCode()".

  Specifically:

  * A call to "PyParser_SimpleParseStringFlags" followed by
    "PyNode_Compile" can be replaced by calling "Py_CompileString()".

  * There is no direct replacement for
    "PyParser_SimpleParseFileFlags". To compile code from a "FILE *"
    argument, you will need to read the file in C and pass the
    resulting buffer to "Py_CompileString()".

  * To compile a file given a "char *" filename, explicitly open the
    file, read it and compile the result. One way to do this is using
    the "io" module with "PyImport_ImportModule()",
    "PyObject_CallMethod()", "PyBytes_AsString()" and
    "Py_CompileString()", as sketched below. (Declarations and error
    handling are omitted.)

       io_module = Import_ImportModule("io");
       fileobject = PyObject_CallMethod(io_module, "open", "ss", filename, "rb");
       source_bytes_object = PyObject_CallMethod(fileobject, "read", "");
       result = PyObject_CallMethod(fileobject, "close", "");
       source_buf = PyBytes_AsString(source_bytes_object);
       code = Py_CompileString(source_buf, filename, Py_file_input);

  * For "FrameObject" objects, the "f_lasti" member now represents a
    wordcode offset instead of a simple offset into the bytecode
    string. This means that this number needs to be multiplied by 2 to
    be used with APIs that expect a byte offset instead (like
    "PyCode_Addr2Line()" for example). Notice as well that the
    "f_lasti" member of "FrameObject" objects is not considered
    stable: please use "PyFrame_GetLineNumber()" instead.


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

* The "MAKE_FUNCTION" instruction now accepts either a dict or a tuple
  of strings as the function's annotations. (Contributed by Yurii
  Karabas and Inada Naoki in bpo-42202)


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

* **PEP 644**: Python now requires OpenSSL 1.1.1 or newer. OpenSSL
  1.0.2 is no longer supported. (Contributed by Christian Heimes in
  bpo-43669.)

* 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.)

* Add "--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.)

* Add "--with-openssl-rpath" option to "configure" script. The option
  simplifies building Python with a custom OpenSSL installation, e.g.
  "./configure --with-openssl=/path/to/openssl --with-openssl-
  rpath=auto". (Contributed by Christian Heimes in bpo-43466.)


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


PEP 652: Maintaining the Stable ABI
-----------------------------------

The Stable ABI (Application Binary Interface) for extension modules or
embedding Python is now explicitly defined. C API Stability describes
C API and ABI stability guarantees along with best practices for using
the Stable ABI.

(Contributed by Petr Viktorin in **PEP 652** and bpo-43795.)


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.)

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

* Add "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.)

* Add "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.)

* Add "PyErr_SetInterruptEx()" which allows passing a signal number to
  simulate. (Contributed by Antoine Pitrou in bpo-43356.)

* The limited C API is now supported if Python is built in debug mode
  (if the "Py_DEBUG" macro is defined). In the limited C API, the
  "Py_INCREF()" and "Py_DECREF()" functions are now implemented as
  opaque function calls, rather than accessing directly the
  "PyObject.ob_refcnt" member, if Python is built in debug mode and
  the "Py_LIMITED_API" macro targets Python 3.10 or newer. It became
  possible to support the limited C API in debug mode because the
  "PyObject" structure is the same in release and debug mode since
  Python 3.8 (see bpo-36465).

  The limited C API is still not supported in the "--with-trace-refs"
  special build ("Py_TRACE_REFS" macro). (Contributed by Victor
  Stinner in bpo-43688.)

* Add the "Py_Is(x, y)" function to test if the *x* object is the *y*
  object, the same as "x is y" in Python. Add also the "Py_IsNone()",
  "Py_IsTrue()", "Py_IsFalse()" functions to test if an object is,
  respectively, the "None" singleton, the "True" singleton or the
  "False" singleton. (Contributed by Victor Stinner in bpo-43753.)

* Add new functions to control the garbage collector from C code:
  "PyGC_Enable()", "PyGC_Disable()", "PyGC_IsEnabled()". These
  functions allow to activate, deactivate and query the state of the
  garbage collector from C code without having to import the "gc"
  module.

* Add a new "Py_TPFLAGS_DISALLOW_INSTANTIATION" type flag to disallow
  creating type instances. (Contributed by Victor Stinner in
  bpo-43916.)

* Add a new "Py_TPFLAGS_IMMUTABLETYPE" type flag for creating
  immutable type objects: type attributes cannot be set nor deleted.
  (Contributed by Victor Stinner and Erlend E. Aasland in bpo-43908.)


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)

* Use the "Py_TPFLAGS_IMMUTABLETYPE" type flag to create immutable
  type objects. Do not rely on "Py_TPFLAGS_HEAPTYPE" to decide if a
  type object is mutable or not; check if "Py_TPFLAGS_IMMUTABLETYPE"
  is set instead. (Contributed by Victor Stinner and Erlend E. Aasland
  in bpo-43908.)

* The undocumented function "Py_FrozenMain" has been removed from the
  limited API. The function is mainly useful for custom builds of
  Python. (Contributed by Petr Viktorin in bpo-26241)


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
-------

* 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.)

* Remove the "PyAST_Validate()" function. It is no longer possible to
  build a AST object ("mod_ty" type) with the public C API. The
  function was already excluded from the limited C API (**PEP 384**).
  (Contributed by Victor Stinner in bpo-43244.)

* Remove the "symtable.h" header file and the undocumented functions:

  * "PyST_GetScope()"

  * "PySymtable_Build()"

  * "PySymtable_BuildObject()"

  * "PySymtable_Free()"

  * "Py_SymtableString()"

  * "Py_SymtableStringObject()"

  The "Py_SymtableString()" function was part the stable ABI by
  mistake but it could not be used, because the "symtable.h" header
  file was excluded from the limited C API.

  Use Python "symtable" module instead. (Contributed by Victor Stinner
  in bpo-43244.)

* Remove "PyOS_ReadlineFunctionPointer()" from the limited C API
  headers and from "python3.dll", the library that provides the stable
  ABI on Windows. Since the function takes a "FILE*" argument, its ABI
  stability cannot be guaranteed. (Contributed by Petr Viktorin in
  bpo-43868.)

* Remove "ast.h", "asdl.h", and "Python-ast.h" header files. These
  functions were undocumented and excluded from the limited C API.
  Most names defined by these header files were not prefixed by "Py"
  and so could create names conflicts. For example, "Python-ast.h"
  defined a "Yield" macro which was conflict with the "Yield" name
  used by the Windows "<winbase.h>" header. Use the Python "ast"
  module instead. (Contributed by Victor Stinner in bpo-43244.)

* Remove the compiler and parser functions using "struct _mod" type,
  because the public AST C API was removed:

  * "PyAST_Compile()"

  * "PyAST_CompileEx()"

  * "PyAST_CompileObject()"

  * "PyFuture_FromAST()"

  * "PyFuture_FromASTObject()"

  * "PyParser_ASTFromFile()"

  * "PyParser_ASTFromFileObject()"

  * "PyParser_ASTFromFilename()"

  * "PyParser_ASTFromString()"

  * "PyParser_ASTFromStringObject()"

  These functions were undocumented and excluded from the limited C
  API. (Contributed by Victor Stinner in bpo-43244.)

* Remove the "pyarena.h" header file with functions:

  * "PyArena_New()"

  * "PyArena_Free()"

  * "PyArena_Malloc()"

  * "PyArena_AddPyObject()"

  These functions were undocumented, excluded from the limited C API,
  and were only used internally by the compiler. (Contributed by
  Victor Stinner in bpo-43244.)

* The "PyThreadState.use_tracing" member has been removed to optimize
  Python. (Contributed by Mark Shannon in bpo-43760.)