New submission from Dominik V. <dominik.vilsmeier1...@gmail.com>:
Due to caching of `__getitem__` for generic types, the order of arguments as returned by `get_args` might be different for Union: ```python >>> from typing import List, Union, get_args >>> get_args(get_args(List[Union[int, str]])[0]) (<class 'int'>, <class 'str'>) >>> get_args(get_args(List[Union[str, int]])[0]) (<class 'int'>, <class 'str'>) ``` This is because `List[Union[int, str]] is List[Union[str, int]]`. I understand that caching is useful to reduce the memory footprint of type hints, so I suggest to update the documentation of `get_args`. At the moment it reads: > For a typing object of the form X[Y, Z, ...] these functions return X and (Y, > Z, ...). This seems to imply that the returned objects are identical to the ones in the form `X[Y, Z, ...]`. However that's not the case: ```python >>> U1 = Union[int, str] >>> U2 = Union[str, int] >>> get_args(List[U1])[0] is U1 True >>> get_args(List[U2])[0] is U2 False ``` I'm not so much concerned about the identity, but the fact that a subsequent call to `get_args` on the Union returns a different type seems to be relevant. So I propose to add the following sentence to the `get_args` docs: > [...], it gets normalized to the original class. > If `X` is a `Union`, the order of `(Y, Z, ...)` can be different from the one > of the original arguments `[Y, Z, ...]`. Or alternatively: > [...], it gets normalized to the original class. > If `X` is a `Union`, the order of `(Y, Z, ...)` is arbitrary. The second version is shorter but it's not completely accurate (since the order is actually not arbitrary). ---------- assignee: docs@python components: Documentation messages: 380699 nosy: Dominik V., docs@python priority: normal severity: normal status: open title: Docs of `typing.get_args`: Mention that due to caching of typing generics the order of arguments for Unions can be different from the one of the returned tuple type: enhancement versions: Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue42317> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
