diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst index dd8ea3c364f49a..69df09c779592a 100644 --- a/Doc/library/typing.rst +++ b/Doc/library/typing.rst @@ -3530,28 +3530,32 @@ Constant .. data:: TYPE_CHECKING A special constant that is assumed to be ``True`` by 3rd party static - type checkers. It is ``False`` at runtime. + type checkers. It's ``False`` at runtime. + + A module which is expensive to import, and which only contain types + used for typing annotations, can be safely imported inside an + ``if TYPE_CHECKING:`` block. This prevents the module from actually + being imported at runtime; annotations aren't eagerly evaluated + (see :pep:`649`) so using undefined symbols in annotations is + harmless--as long as you don't later examine them. + Your static type analysis tool will set ``TYPE_CHECKING`` to + ``True`` during static type analysis, which means the module will + be imported and the types will be checked properly during such analysis. Usage:: if TYPE_CHECKING: import expensive_mod - def fun(arg: 'expensive_mod.SomeType') -> None: + def fun(arg: expensive_mod.SomeType) -> None: local_var: expensive_mod.AnotherType = other_fun() - The first type annotation must be enclosed in quotes, making it a - "forward reference", to hide the ``expensive_mod`` reference from the - interpreter runtime. Type annotations for local variables are not - evaluated, so the second annotation does not need to be enclosed in quotes. - - .. note:: - - If ``from __future__ import annotations`` is used, - annotations are not evaluated at function definition time. - Instead, they are stored as strings in ``__annotations__``. - This makes it unnecessary to use quotes around the annotation - (see :pep:`563`). + If you occasionally need to examine type annotations at runtime + which may contain undefined symbols, use + :meth:`annotationlib.get_annotations` with a ``format`` parameter + of :attr:`annotationlib.Format.STRING` or + :attr:`annotationlib.Format.FORWARDREF` to safely retrieve the + annotations without raising :exc:`NameError`. .. versionadded:: 3.5.2