From 4eadfb1c1a035970905b00263db8d9bd277098fc Mon Sep 17 00:00:00 2001 From: "Miss Islington (bot)" <31488909+miss-islington@users.noreply.github.com> Date: Wed, 7 Jun 2023 11:11:48 -0700 Subject: [3.12] gh-90015: Document that PEP-604 unions do not support forward references (GH-105366) (#105460) gh-90015: Document that PEP-604 unions do not support forward references (GH-105366) (cherry picked from commit fbdee000de47ae96fbf53ce8908e8efbb23cfba4) Co-authored-by: Alex Waygood --- Doc/library/stdtypes.rst | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst index fdef531..0caa725 100644 --- a/Doc/library/stdtypes.rst +++ b/Doc/library/stdtypes.rst @@ -5162,6 +5162,14 @@ enables cleaner type hinting syntax compared to :data:`typing.Union`. def square(number: int | float) -> int | float: return number ** 2 + .. note:: + + The ``|`` operand cannot be used at runtime to define unions where one or + more members is a forward reference. For example, ``int | "Foo"``, where + ``"Foo"`` is a reference to a class not yet defined, will fail at + runtime. For unions which include forward references, present the + whole expression as a string, e.g. ``"int | Foo"``. + .. describe:: union_object == other Union objects can be tested for equality with other union objects. Details: -- cgit v0.12