From: Mathias Rav <mathiasrav@gmail.com>
Date: Wed, 26 Apr 2017 10:49:45 +0000 (-0400)
Subject: bpo-29974: Improve typing.TYPE_CHECKING example (GH-982)
X-Git-Tag: v3.7.0a1~900
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=87c07fe9d908d0a2143fcc8369255c6ff3241503;p=python

bpo-29974: Improve typing.TYPE_CHECKING example (GH-982)

* Fix PEP 8 (SomeType instead of some_type)
* Add a function parameter annotation
* Explain, using wording from PEP 484 and PEP 526,
  why one annotation is in quotes and another is not.

Suggested by Ivan Levkevskyi.
---

diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index 2cfc37f695..1780739ad1 100644
--- a/Doc/library/typing.rst
+++ b/Doc/library/typing.rst
@@ -1041,5 +1041,10 @@ The module defines the following classes, functions and decorators:
       if TYPE_CHECKING:
           import expensive_mod
 
-      def fun():
-          local_var: expensive_mod.some_type = other_fun()
+      def fun(arg: 'expensive_mod.SomeType') -> None:
+          local_var: expensive_mod.AnotherType = other_fun()
+
+   Note that the first type annotation must be enclosed in quotes, making it a
+   "forward reference", to hide the ``expensive_mod`` reference from the
+   interpreter runtime.  Type annotations for local variables are not
+   evaluated, so the second annotation does not need to be enclosed in quotes.