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DOCS: NewType can subclass NewType #3695

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DOCS: NewType can subclass NewType #3695

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12 changes: 9 additions & 3 deletions Doc/library/typing.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -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

Expand All @@ -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::
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