What’s New In Python 3.12¶

PEP 695: Type Parameter Syntax¶

Generic classes and functions under PEP 484 were declared using a verbose syntax that left the scope of type parameters unclear and required explicit declarations of variance.

PEP 695 introduces a new, more compact and explicit way to create generic classes and functions:

def max[T](args: Iterable[T]) -> T:
    ...

class list[T]:
    def __getitem__(self, index: int, /) -> T:
        ...

    def append(self, element: T) -> None:
        ...

In addition, the PEP introduces a new way to declare type aliases using the type statement, which creates an instance of TypeAliasType:

type Point = tuple[float, float]

Type aliases can also be generic:

type Point[T] = tuple[T, T]

The new syntax allows declaring TypeVarTuple and ParamSpec parameters, as well as TypeVar parameters with bounds or constraints:

type IntFunc[**P] = Callable[P, int]  # ParamSpec
type LabeledTuple[*Ts] = tuple[str, *Ts]  # TypeVarTuple
type HashableSequence[T: Hashable] = Sequence[T]  # TypeVar with bound
type IntOrStrSequence[T: (int, str)] = Sequence[T]  # TypeVar with constraints

The value of type aliases and the bound and constraints of type variables created through this syntax are evaluated only on demand (see lazy evaluation). This means type aliases are able to refer to other types defined later in the file.

Type parameters declared through a type parameter list are visible within the scope of the declaration and any nested scopes, but not in the outer scope. For example, they can be used in the type annotations for the methods of a generic class or in the class body. However, they cannot be used in the module scope after the class is defined. See Type parameter lists for a detailed description of the runtime semantics of type parameters.

In order to support these scoping semantics, a new kind of scope is introduced, the annotation scope. Annotation scopes behave for the most part like function scopes, but interact differently with enclosing class scopes. In Python 3.13, annotations will also be evaluated in annotation scopes.

See PEP 695 for more details.

(PEP written by Eric Traut. Implementation by Jelle Zijlstra, Eric Traut, and others in gh-103764.)

PEP 701: Syntactic formalization of f-strings¶

PEP 701 lifts some restrictions on the usage of f-strings. Expression components inside f-strings can now be any valid Python expression, including strings reusing the same quote as the containing f-string, multi-line expressions, comments, backslashes, and unicode escape sequences. Let’s cover these in detail:

• Quote reuse: in Python 3.11, reusing the same quotes as the enclosing f-string raises a SyntaxError, forcing the user to either use other available quotes (like using double quotes or triple quotes if the f-string uses single quotes). In Python 3.12, you can now do things like this:

  >>> songs = ['Take me back to Eden', 'Alkaline', 'Ascensionism']
  >>> f"This is the playlist: {", ".join(songs)}"
  'This is the playlist: Take me back to Eden, Alkaline, Ascensionism'
  

  Note that before this change there was no explicit limit in how f-strings can be nested, but the fact that string quotes cannot be reused inside the expression component of f-strings made it impossible to nest f-strings arbitrarily. In fact, this is the most nested f-string that could be written:

  >>> f"""{f'''{f'{f"{1+1}"}'}'''}"""
  '2'
  

  As now f-strings can contain any valid Python expression inside expression components, it is now possible to nest f-strings arbitrarily:

  >>> f"{f"{f"{f"{f"{f"{1+1}"}"}"}"}"}"
  '2'
  

• Multi-line expressions and comments: In Python 3.11, f-string expressions must be defined in a single line, even if the expression within the f-string could normally span multiple lines (like literal lists being defined over multiple lines), making them harder to read. In Python 3.12 you can now define f-strings spanning multiple lines, and add inline comments:

  >>> f"This is the playlist: {", ".join([
  ...     'Take me back to Eden',  # My, my, those eyes like fire
  ...     'Alkaline',              # Not acid nor alkaline
  ...     'Ascensionism'           # Take to the broken skies at last
  ... ])}"
  'This is the playlist: Take me back to Eden, Alkaline, Ascensionism'
  

• Backslashes and unicode characters: before Python 3.12 f-string expressions couldn’t contain any \ character. This also affected unicode escape sequences (such as \N{snowman}) as these contain the \N part that previously could not be part of expression components of f-strings. Now, you can define expressions like this:

  >>> print(f"This is the playlist: {"\n".join(songs)}")
  This is the playlist: Take me back to Eden
  Alkaline
  Ascensionism
  >>> print(f"This is the playlist: {"\N{BLACK HEART SUIT}".join(songs)}")
  This is the playlist: Take me back to Eden♥Alkaline♥Ascensionism
  

See PEP 701 for more details.

As a positive side-effect of how this feature has been implemented (by parsing f-strings with the PEG parser), now error messages for f-strings are more precise and include the exact location of the error. For example, in Python 3.11, the following f-string raises a SyntaxError:

>>> my_string = f"{x z y}" + f"{1 + 1}"
  File "<stdin>", line 1
    (x z y)
     ^^^
SyntaxError: f-string: invalid syntax. Perhaps you forgot a comma?

but the error message doesn’t include the exact location of the error within the line and also has the expression artificially surrounded by parentheses. In Python 3.12, as f-strings are parsed with the PEG parser, error messages can be more precise and show the entire line:

>>> my_string = f"{x z y}" + f"{1 + 1}"
  File "<stdin>", line 1
    my_string = f"{x z y}" + f"{1 + 1}"
                   ^^^
SyntaxError: invalid syntax. Perhaps you forgot a comma?

(Contributed by Pablo Galindo, Batuhan Taskaya, Lysandros Nikolaou, Cristián Maureira-Fredes and Marta Gómez in gh-102856. PEP written by Pablo Galindo, Batuhan Taskaya, Lysandros Nikolaou and Marta Gómez).

PEP 684: A Per-Interpreter GIL¶

PEP 684 introduces a per-interpreter GIL, so that sub-interpreters may now be created with a unique GIL per interpreter. This allows Python programs to take full advantage of multiple CPU cores. This is currently only available through the C-API, though a Python API is anticipated for 3.13.

Use the new Py_NewInterpreterFromConfig() function to create an interpreter with its own GIL:

PyInterpreterConfig config = {
    .check_multi_interp_extensions = 1,
    .gil = PyInterpreterConfig_OWN_GIL,
};
PyThreadState *tstate = NULL;
PyStatus status = Py_NewInterpreterFromConfig(&tstate, &config);
if (PyStatus_Exception(status)) {
    return -1;
}
/* The new interpreter is now active in the current thread. */

For further examples how to use the C-API for sub-interpreters with a per-interpreter GIL, see Modules/_xxsubinterpretersmodule.c.

(Contributed by Eric Snow in gh-104210, etc.)

PEP 669: Low impact monitoring for CPython¶

PEP 669 defines a new API for profilers, debuggers, and other tools to monitor events in CPython. It covers a wide range of events, including calls, returns, lines, exceptions, jumps, and more. This means that you only pay for what you use, providing support for near-zero overhead debuggers and coverage tools. See sys.monitoring for details.

(Contributed by Mark Shannon in gh-103082.)

PEP 688: Making the buffer protocol accessible in Python¶

PEP 688 introduces a way to use the buffer protocol from Python code. Classes that implement the __buffer__() method are now usable as buffer types.

The new collections.abc.Buffer ABC provides a standard way to represent buffer objects, for example in type annotations. The new inspect.BufferFlags enum represents the flags that can be used to customize buffer creation. (Contributed by Jelle Zijlstra in gh-102500.)

PEP 709: Comprehension inlining¶

Dictionary, list, and set comprehensions are now inlined, rather than creating a new single-use function object for each execution of the comprehension. This speeds up execution of a comprehension by up to two times. See PEP 709 for further details.

Comprehension iteration variables remain isolated and don’t overwrite a variable of the same name in the outer scope, nor are they visible after the comprehension. Inlining does result in a few visible behavior changes:

• There is no longer a separate frame for the comprehension in tracebacks, and tracing/profiling no longer shows the comprehension as a function call.

• The symtable module will no longer produce child symbol tables for each comprehension; instead, the comprehension’s locals will be included in the parent function’s symbol table.

• Calling locals() inside a comprehension now includes variables from outside the comprehension, and no longer includes the synthetic .0 variable for the comprehension “argument”.

• A comprehension iterating directly over locals() (e.g. [k for k in locals()]) may see “RuntimeError: dictionary changed size during iteration” when run under tracing (e.g. code coverage measurement). This is the same behavior already seen in e.g. for k in locals():. To avoid the error, first create a list of keys to iterate over: keys = list(locals()); [k for k in keys].

(Contributed by Carl Meyer and Vladimir Matveev in PEP 709.)

Improved Error Messages¶

• Modules from the standard library are now potentially suggested as part of the error messages displayed by the interpreter when a NameError is raised to the top level. (Contributed by Pablo Galindo in gh-98254.)

  >>> sys.version_info
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'sys' is not defined. Did you forget to import 'sys'?
  

• Improve the error suggestion for NameError exceptions for instances. Now if a NameError is raised in a method and the instance has an attribute that’s exactly equal to the name in the exception, the suggestion will include self.<NAME> instead of the closest match in the method scope. (Contributed by Pablo Galindo in gh-99139.)

  >>> class A:
  ...    def __init__(self):
  ...        self.blech = 1
  ...
  ...    def foo(self):
  ...        somethin = blech
  ...
  >>> A().foo()
  Traceback (most recent call last):
    File "<stdin>", line 1
      somethin = blech
                 ^^^^^
  NameError: name 'blech' is not defined. Did you mean: 'self.blech'?
  

• Improve the SyntaxError error message when the user types import x from y instead of from y import x. (Contributed by Pablo Galindo in gh-98931.)

  >>> import a.y.z from b.y.z
  Traceback (most recent call last):
    File "<stdin>", line 1
      import a.y.z from b.y.z
      ^^^^^^^^^^^^^^^^^^^^^^^
  SyntaxError: Did you mean to use 'from ... import ...' instead?
  

• ImportError exceptions raised from failed from <module> import <name> statements now include suggestions for the value of <name> based on the available names in <module>. (Contributed by Pablo Galindo in gh-91058.)

  >>> from collections import chainmap
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ImportError: cannot import name 'chainmap' from 'collections'. Did you mean: 'ChainMap'?
  

PEP 692: Using TypedDict for more precise **kwargs typing¶

Typing **kwargs in a function signature as introduced by PEP 484 allowed for valid annotations only in cases where all of the **kwargs were of the same type.

PEP 692 specifies a more precise way of typing **kwargs by relying on typed dictionaries:

from typing import TypedDict, Unpack

class Movie(TypedDict):
  name: str
  year: int

def foo(**kwargs: Unpack[Movie]): ...

See PEP 692 for more details.

(Contributed by Franek Magiera in gh-103629.)

PEP 698: Override Decorator for Static Typing¶

A new decorator typing.override() has been added to the typing module. It indicates to type checkers that the method is intended to override a method in a superclass. This allows type checkers to catch mistakes where a method that is intended to override something in a base class does not in fact do so.

Example:

from typing import override

class Base:
  def get_color(self) -> str:
    return "blue"

class GoodChild(Base):
  @override  # ok: overrides Base.get_color
  def get_color(self) -> str:
    return "yellow"

class BadChild(Base):
  @override  # type checker error: does not override Base.get_color
  def get_colour(self) -> str:
    return "red"

See PEP 698 for more details.

(Contributed by Steven Troxler in gh-101561.)

array¶

• The array.array class now supports subscripting, making it a generic type. (Contributed by Jelle Zijlstra in gh-98658.)

asyncio¶

• The performance of writing to sockets in asyncio has been significantly improved. asyncio now avoids unnecessary copying when writing to sockets and uses sendmsg() if the platform supports it. (Contributed by Kumar Aditya in gh-91166.)

• Add asyncio.eager_task_factory() and asyncio.create_eager_task_factory() functions to allow opting an event loop in to eager task execution, making some use-cases 2x to 5x faster. (Contributed by Jacob Bower & Itamar Oren in gh-102853, gh-104140, and gh-104138)

• On Linux, asyncio uses asyncio.PidfdChildWatcher by default if os.pidfd_open() is available and functional instead of asyncio.ThreadedChildWatcher. (Contributed by Kumar Aditya in gh-98024.)

• The event loop now uses the best available child watcher for each platform (asyncio.PidfdChildWatcher if supported and asyncio.ThreadedChildWatcher otherwise), so manually configuring a child watcher is not recommended. (Contributed by Kumar Aditya in gh-94597.)

• Add loop_factory parameter to asyncio.run() to allow specifying a custom event loop factory. (Contributed by Kumar Aditya in gh-99388.)

• Add C implementation of asyncio.current_task() for 4x-6x speedup. (Contributed by Itamar Oren and Pranav Thulasiram Bhat in gh-100344.)

• asyncio.iscoroutine() now returns False for generators as asyncio does not support legacy generator-based coroutines. (Contributed by Kumar Aditya in gh-102748.)

• asyncio.wait() and asyncio.as_completed() now accepts generators yielding tasks. (Contributed by Kumar Aditya in gh-78530.)

calendar¶

• Add enums calendar.Month and calendar.Day defining months of the year and days of the week. (Contributed by Prince Roshan in gh-103636.)

csv¶

• Add csv.QUOTE_NOTNULL and csv.QUOTE_STRINGS flags to provide finer grained control of None and empty strings by reader and writer objects.

dis¶

• Pseudo instruction opcodes (which are used by the compiler but do not appear in executable bytecode) are now exposed in the dis module. HAVE_ARGUMENT is still relevant to real opcodes, but it is not useful for pseudo instructions. Use the new dis.hasarg collection instead. (Contributed by Irit Katriel in gh-94216.)

• Add the dis.hasexc collection to signify instructions that set an exception handler. (Contributed by Irit Katriel in gh-94216.)

fractions¶

• Objects of type fractions.Fraction now support float-style formatting. (Contributed by Mark Dickinson in gh-100161.)

importlib.resources¶

• importlib.resources.as_file() now supports resource directories. (Contributed by Jason R. Coombs in gh-97930.)

• Rename first parameter of importlib.resources.files() to anchor. (Contributed by Jason R. Coombs in gh-100598.)

inspect¶

• Add inspect.markcoroutinefunction() to mark sync functions that return a coroutine for use with inspect.iscoroutinefunction(). (Contributed by Carlton Gibson in gh-99247.)

• Add inspect.getasyncgenstate() and inspect.getasyncgenlocals() for determining the current state of asynchronous generators. (Contributed by Thomas Krennwallner in gh-79940.)

• The performance of inspect.getattr_static() has been considerably improved. Most calls to the function should be at least 2x faster than they were in Python 3.11. (Contributed by Alex Waygood in gh-103193.)

itertools¶

• Add itertools.batched() for collecting into even-sized tuples where the last batch may be shorter than the rest. (Contributed by Raymond Hettinger in gh-98363.)

math¶

• Add math.sumprod() for computing a sum of products. (Contributed by Raymond Hettinger in gh-100485.)

• Extend math.nextafter() to include a steps argument for moving up or down multiple steps at a time. (Contributed by Matthias Goergens, Mark Dickinson, and Raymond Hettinger in gh-94906.)

os¶

• Add os.PIDFD_NONBLOCK to open a file descriptor for a process with os.pidfd_open() in non-blocking mode. (Contributed by Kumar Aditya in gh-93312.)

• os.DirEntry now includes an os.DirEntry.is_junction() method to check if the entry is a junction. (Contributed by Charles Machalow in gh-99547.)

• Add os.listdrives(), os.listvolumes() and os.listmounts() functions on Windows for enumerating drives, volumes and mount points. (Contributed by Steve Dower in gh-102519.)

• os.stat() and os.lstat() are now more accurate on Windows. The st_birthtime field will now be filled with the creation time of the file, and st_ctime is deprecated but still contains the creation time (but in the future will return the last metadata change, for consistency with other platforms). st_dev may be up to 64 bits and st_ino up to 128 bits depending on your file system, and st_rdev is always set to zero rather than incorrect values. Both functions may be significantly faster on newer releases of Windows. (Contributed by Steve Dower in gh-99726.)

os.path¶

• Add os.path.isjunction() to check if a given path is a junction. (Contributed by Charles Machalow in gh-99547.)

• Add os.path.splitroot() to split a path into a triad (drive, root, tail). (Contributed by Barney Gale in gh-101000.)

pathlib¶

• Add support for subclassing pathlib.PurePath and pathlib.Path, plus their Posix- and Windows-specific variants. Subclasses may override the pathlib.PurePath.with_segments() method to pass information between path instances.

• Add pathlib.Path.walk() for walking the directory trees and generating all file or directory names within them, similar to os.walk(). (Contributed by Stanislav Zmiev in gh-90385.)

• Add walk_up optional parameter to pathlib.PurePath.relative_to() to allow the insertion of .. entries in the result; this behavior is more consistent with os.path.relpath(). (Contributed by Domenico Ragusa in gh-84538.)

• Add pathlib.Path.is_junction() as a proxy to os.path.isjunction(). (Contributed by Charles Machalow in gh-99547.)

• Add case_sensitive optional parameter to pathlib.Path.glob(), pathlib.Path.rglob() and pathlib.PurePath.match() for matching the path’s case sensitivity, allowing for more precise control over the matching process.

platform¶

• Add support for detecting Windows 11 and Windows Server releases past 2012. Previously, lookups on Windows Server platforms newer than Windows Server 2012 and on Windows 11 would return Windows-10. (Contributed by Steve Dower in gh-89545.)

pdb¶

• Add convenience variables to hold values temporarily for debug session and provide quick access to values like the current frame or the return value. (Contributed by Tian Gao in gh-103693.)

random¶

• Add random.binomialvariate(). (Contributed by Raymond Hettinger in gh-81620.)

• Add a default of lambd=1.0 to random.expovariate(). (Contributed by Raymond Hettinger in gh-100234.)

shutil¶

• shutil.make_archive() now passes the root_dir argument to custom archivers which support it. In this case it no longer temporarily changes the current working directory of the process to root_dir to perform archiving. (Contributed by Serhiy Storchaka in gh-74696.)

• shutil.rmtree() now accepts a new argument onexc which is an error handler like onerror but which expects an exception instance rather than a (typ, val, tb) triplet. onerror is deprecated. (Contributed by Irit Katriel in gh-102828.)

• shutil.which() now consults the PATHEXT environment variable to find matches within PATH on Windows even when the given cmd includes a directory component. (Contributed by Charles Machalow in gh-103179.)

  shutil.which() will call NeedCurrentDirectoryForExePathW when querying for executables on Windows to determine if the current working directory should be prepended to the search path. (Contributed by Charles Machalow in gh-103179.)

  shutil.which() will return a path matching the cmd with a component from PATHEXT prior to a direct match elsewhere in the search path on Windows. (Contributed by Charles Machalow in gh-103179.)

sqlite3¶

• Add a command-line interface. (Contributed by Erlend E. Aasland in gh-77617.)

• Add the sqlite3.Connection.autocommit attribute to sqlite3.Connection and the autocommit parameter to sqlite3.connect() to control PEP 249-compliant transaction handling. (Contributed by Erlend E. Aasland in gh-83638.)

• Add entrypoint keyword-only parameter to sqlite3.Connection.load_extension(), for overriding the SQLite extension entry point. (Contributed by Erlend E. Aasland in gh-103015.)

• Add sqlite3.Connection.getconfig() and sqlite3.Connection.setconfig() to sqlite3.Connection to make configuration changes to a database connection. (Contributed by Erlend E. Aasland in gh-103489.)

statistics¶

• Extend statistics.correlation() to include as a ranked method for computing the Spearman correlation of ranked data. (Contributed by Raymond Hettinger in gh-95861.)

sys¶

• Add the sys.monitoring namespace to expose the new PEP 669 monitoring API. (Contributed by Mark Shannon in gh-103082.)

• Add sys.activate_stack_trampoline() and sys.deactivate_stack_trampoline() for activating and deactivating stack profiler trampolines, and sys.is_stack_trampoline_active() for querying if stack profiler trampolines are active. (Contributed by Pablo Galindo and Christian Heimes with contributions from Gregory P. Smith [Google] and Mark Shannon in gh-96123.)

• Add sys.last_exc which holds the last unhandled exception that was raised (for post-mortem debugging use cases). Deprecate the three fields that have the same information in its legacy form: