From 039b25d8fd21f8d5d9e3cb536402d952cf068dc1 Mon Sep 17 00:00:00 2001 From: topper-123 Date: Sun, 24 Sep 2017 04:37:48 +0200 Subject: bpo-31564: Update typing documentation (GH-3696) Mention that ``NewType`` can derive from another ``NewType``. --- Doc/library/typing.rst | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst index 1e48fec..bd04f73 100644 --- a/Doc/library/typing.rst +++ b/Doc/library/typing.rst @@ -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:: -- cgit v0.12