[New-bugs-announce] [issue26075] typing.Union unifies types too broadly

[Alex Grönholm]

New submission from Alex Grönholm:

>>> from typing import Union, Iterable
>>> Union[str, Iterable[int]]
typing.Iterable[int]

The union loses the "str" parameter because issubclass(str, collections.abc.Iterable) returns True and the check completely disregards generics.

Guido mentioned that issubclass() support for generic types should be going away. In the mean time, maybe the subclass check in typing.GenericMeta should be modified not to do it with types from Typing, but how can we accurately identify them?

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components: Extension Modules
messages: 257910
nosy: alex.gronholm, gvanrossum, lukasz.langa
priority: normal
severity: normal
status: open
title: typing.Union unifies types too broadly
type: behavior
versions: Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26075>
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