From: Guido van Rossum <guido@python.org>
Date: Wed, 10 Feb 2016 17:48:58 +0000 (-0800)
Subject: Hopefully clarify the difference between Optional[t] and an optional argument.
X-Git-Tag: v3.6.0a1~620^2
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=e93b06a0a3255026802be19de249b3fdfe955b96;p=python

Hopefully clarify the difference between Optional[t] and an optional argument.
---

diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index 41f594ecc4..12b5490b66 100644
--- a/Doc/library/typing.rst
+++ b/Doc/library/typing.rst
@@ -286,6 +286,13 @@ The module defines the following classes, functions and decorators:
 
    ``Optional[X]`` is equivalent to ``Union[X, type(None)]``.
 
+   Note that this is not the same concept as an optional argument,
+   which is one that has a default.  An optional argument with a
+   default needn't use the ``Optional`` qualifier on its type
+   annotation (although it is inferred if the default is ``None``).
+   A mandatory argument may still have an ``Optional`` type if an
+   explicit value of ``None`` is allowed.
+
 .. class:: Tuple
 
   Tuple type; ``Tuple[X, Y]`` is the is the type of a tuple of two items