[New-bugs-announce] [issue46981] Empty typing.Tuple

[Serhiy Storchaka]

New submission from Serhiy Storchaka <storchaka+cpython at gmail.com>:

There are two empty typing.Tuple. They have the same repr but are not equal.

>>> from typing import *
>>> t1 = Tuple[()]
>>> t2 = t1.copy_with(())
>>> t1
typing.Tuple[()]
>>> t2
typing.Tuple[()]
>>> t1 == t2
False
>>> t1.__args__
((),)
>>> t2.__args__
()

The only differences is that one has empty __args__, while other has __args__ containing an empty tuple. There is a code purposed to make __args__ containing an empty tuple in this case. What is the purpose?

It is not pure theoretical question. This affects unpacked TypeVarTuple substitution. With natural implementation Tuple[Unpack[Ts]][()] is not equal to Tuple[()] and I still have not figured which and where code should be added to handle this special case. It would be easier if such special case did not exist.

Built-in tuple does not have a special case:

>>> tuple[()].__args__
()

----------
components: Library (Lib)
messages: 414892
nosy: gvanrossum, kj, serhiy.storchaka
priority: normal
severity: normal
status: open
title: Empty typing.Tuple

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46981>
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