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bpo-31564: Update typing documentation (GH-3696) 

Mention that ``NewType`` can derive from another ``NewType``.
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topper-123 2017-09-24 04:37:48 +02:00 committed by Mariatta
parent d6238a76c6
commit 039b25d8fd
1 changed files with 9 additions and 3 deletions
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12
Doc/library/typing.rst
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@ -111,8 +111,7 @@ More precisely, the expression ``some_value is Derived(some_value)`` is always
true at runtime.
This also means that it is not possible to create a subtype of ``Derived``
since it is an identity function at runtime, not an actual type. Similarly, it
is not possible to create another :func:`NewType` based on a ``Derived`` type::
since it is an identity function at runtime, not an actual type::
from typing import NewType
@ -121,9 +120,16 @@ is not possible to create another :func:`NewType` based on a ``Derived`` type::
# Fails at runtime and does not typecheck
class AdminUserId(UserId): pass
# Also does not typecheck
However, it is possible to create a :func:`NewType` based on a 'derived' ``NewType``::
from typing import NewType
UserId = NewType('UserId', int)
ProUserId = NewType('ProUserId', UserId)
and typechecking for ``ProUserId`` will work as expected.
See :pep:`484` for more details.
.. note::