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| -rw-r--r-- | Doc/library/typing.rst | 31 |
1 files changed, 21 insertions, 10 deletions
diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst index 60bf06e..d060066 100644 --- a/Doc/library/typing.rst +++ b/Doc/library/typing.rst @@ -953,13 +953,17 @@ using ``[]``. For example:: - from typing import Self + from typing import Self, reveal_type class Foo: def return_self(self) -> Self: ... return self + class SubclassOfFoo(Foo): pass + + reveal_type(Foo().return_self()) # Revealed type is "Foo" + reveal_type(SubclassOfFoo().return_self()) # Revealed type is "SubclassOfFoo" This annotation is semantically equivalent to the following, albeit in a more succinct fashion:: @@ -973,15 +977,11 @@ using ``[]``. ... return self - In general if something currently follows the pattern of:: - - class Foo: - def return_self(self) -> "Foo": - ... - return self - - You should use :data:`Self` as calls to ``SubclassOfFoo.return_self`` would have - ``Foo`` as the return type and not ``SubclassOfFoo``. + In general, if something returns ``self``, as in the above examples, you + should use ``Self`` as the return annotation. If ``Foo.return_self`` was + annotated as returning ``"Foo"``, then the type checker would infer the + object returned from ``SubclassOfFoo.return_self`` as being of type ``Foo`` + rather than ``SubclassOfFoo``. Other common use cases include: @@ -989,6 +989,17 @@ using ``[]``. of the ``cls`` parameter. - Annotating an :meth:`~object.__enter__` method which returns self. + You should not use ``Self`` as the return annotation if the method is not + guaranteed to return an instance of a subclass when the class is + subclassed:: + + class Eggs: + # Self would be an incorrect return annotation here, + # as the object returned is always an instance of Eggs, + # even in subclasses + def returns_eggs(self) -> "Eggs": + return Eggs() + See :pep:`673` for more details. .. versionadded:: 3.11 |