[Python-checkins] bpo-31564: Update typing documentation (GH-3696) (GH-3715)
Mariatta
webhook-mailer at python.org
Sat Sep 23 22:46:27 EDT 2017
https://github.com/python/cpython/commit/018e6b9f696311a65f47f33ea04b7baef3f3664f
commit: 018e6b9f696311a65f47f33ea04b7baef3f3664f
branch: 3.6
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: Mariatta <Mariatta at users.noreply.github.com>
date: 2017-09-23T19:46:24-07:00
summary:
bpo-31564: Update typing documentation (GH-3696) (GH-3715)
Mention that ``NewType`` can derive from another ``NewType``.
(cherry picked from commit 039b25d8fd21f8d5d9e3cb536402d952cf068dc1)
files:
M Doc/library/typing.rst
diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index 1e48fecdbf7..bd04f731a12 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.
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