From b32789ccb91bbe43e88193f68b1364a8da6d9866 Mon Sep 17 00:00:00 2001 From: Jelle Zijlstra Date: Thu, 4 Apr 2024 06:39:16 -0400 Subject: [PATCH] gh-117521: Improve typing.TypeGuard docstring (#117522) --- Lib/typing.py | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/Lib/typing.py b/Lib/typing.py index ef532f6c915..d8e4ee36359 100644 --- a/Lib/typing.py +++ b/Lib/typing.py @@ -841,22 +841,25 @@ def TypeGuard(self, parameters): 2. If the return value is ``True``, the type of its argument is the type inside ``TypeGuard``. - For example:: + For example:: - def is_str(val: Union[str, float]): - # "isinstance" type guard - if isinstance(val, str): - # Type of ``val`` is narrowed to ``str`` - ... - else: - # Else, type of ``val`` is narrowed to ``float``. - ... + def is_str_list(val: list[object]) -> TypeGuard[list[str]]: + '''Determines whether all objects in the list are strings''' + return all(isinstance(x, str) for x in val) + + def func1(val: list[object]): + if is_str_list(val): + # Type of ``val`` is narrowed to ``list[str]``. + print(" ".join(val)) + else: + # Type of ``val`` remains as ``list[object]``. + print("Not a list of strings!") Strict type narrowing is not enforced -- ``TypeB`` need not be a narrower form of ``TypeA`` (it can even be a wider form) and this may lead to type-unsafe results. The main reason is to allow for things like - narrowing ``List[object]`` to ``List[str]`` even though the latter is not - a subtype of the former, since ``List`` is invariant. The responsibility of + narrowing ``list[object]`` to ``list[str]`` even though the latter is not + a subtype of the former, since ``list`` is invariant. The responsibility of writing type-safe type guards is left to the user. ``TypeGuard`` also works with type variables. For more information, see