What’s New In Python 3.10

Release

3.9.9

Date

January 02, 2022

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.

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 Decimals, Fractions 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.)

enum

Enum __repr__() now returns enum_name.member_name and __str__() now returns member_name. Stdlib enums available as module constants have a repr() of module_name.member_name. (Contributed by Ethan Furman in bpo-40066.)

Add enum.StrEnum for enums where all members are strings. (Contributed by Ethan Furman in bpo-41816.)

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 respect 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.)

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

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 in 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

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.)

  • 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.)