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Backported and Experimental Type Hints for Python 3.7+

Project description

Chat at https://gitter.im/python/typing

Overview

The typing_extensions module serves two related purposes:

  • Enable use of new type system features on older Python versions. For example, typing.TypeGuard is new in Python 3.10, but typing_extensions allows users on Python 3.6 through 3.9 to use it too.
  • Enable experimentation with new type system PEPs before they are accepted and added to the typing module.

New features may be added to typing_extensions as soon as they are specified in a PEP that has been added to the python/peps repository. If the PEP is accepted, the feature will then be added to typing for the next CPython release. No typing PEP has been rejected so far, so we haven’t yet figured out how to deal with that possibility.

Starting with version 4.0.0, typing_extensions uses Semantic Versioning. The major version is incremented for all backwards-incompatible changes. Therefore, it’s safe to depend on typing_extensions like this: typing_extensions >=x.y, <(x+1), where x.y is the first version that includes all features you need.

typing_extensions supports Python versions 3.7 and higher. In the future, support for older Python versions will be dropped some time after that version reaches end of life.

Included items

This module currently contains the following:

  • Experimental features
    • @dataclass_transform() (see PEP 681)
  • In typing since Python 3.11
    • assert_never
    • assert_type
    • clear_overloads
    • get_overloads
    • LiteralString (see PEP 675)
    • Never
    • NotRequired (see PEP 655)
    • reveal_type
    • Required (see PEP 655)
    • Self (see PEP 673)
    • TypeVarTuple (see PEP 646)
    • Unpack (see PEP 646)
  • In typing since Python 3.10
    • Concatenate (see PEP 612)
    • ParamSpec (see PEP 612)
    • ParamSpecArgs (see PEP 612)
    • ParamSpecKwargs (see PEP 612)
    • TypeAlias (see PEP 613)
    • TypeGuard (see PEP 647)
    • is_typeddict
  • In typing since Python 3.9
    • Annotated (see PEP 593)
  • In typing since Python 3.8
    • final (see PEP 591)
    • Final (see PEP 591)
    • Literal (see PEP 586)
    • Protocol (see PEP 544)
    • runtime_checkable (see PEP 544)
    • TypedDict (see PEP 589)
    • get_origin (typing_extensions provides this function only in Python 3.7+)
    • get_args (typing_extensions provides this function only in Python 3.7+)
  • In typing since Python 3.7
    • OrderedDict
  • In typing since Python 3.5 or 3.6 (see the typing documentation for details)
    • AsyncContextManager
    • AsyncGenerator
    • AsyncIterable
    • AsyncIterator
    • Awaitable
    • ChainMap
    • ClassVar (see PEP 526)
    • ContextManager
    • Coroutine
    • Counter
    • DefaultDict
    • Deque
    • NewType
    • NoReturn
    • overload
    • Text
    • Type
    • TYPE_CHECKING
    • get_type_hints

Other Notes and Limitations

Certain objects were changed after they were added to typing, and typing_extensions provides a backport even on newer Python versions:

  • TypedDict does not store runtime information about which (if any) keys are non-required in Python 3.8, and does not honor the “total” keyword with old-style TypedDict() in Python 3.9.0 and 3.9.1.
  • get_origin and get_args lack support for Annotated in Python 3.8 and lack support for ParamSpecArgs and ParamSpecKwargs in 3.9.
  • @final was changed in Python 3.11 to set the .__final__ attribute.
  • @overload was changed in Python 3.11 to make function overloads introspectable at runtime. In order to access overloads with typing_extensions.get_overloads(), you must use @typing_extensions.overload.

There are a few types whose interface was modified between different versions of typing. For example, typing.Sequence was modified to subclass typing.Reversible as of Python 3.5.3.

These changes are _not_ backported to prevent subtle compatibility issues when mixing the differing implementations of modified classes.

Certain types have incorrect runtime behavior due to limitations of older versions of the typing module:

  • ParamSpec and Concatenate will not work with get_args and get_origin. Certain PEP 612 special cases in user-defined Generics are also not available.

These types are only guaranteed to work for static type checking.

Running tests

To run tests, navigate into the appropriate source directory and run test_typing_extensions.py.

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