[issue43413] tuple subclasses allow kwargs

[Jason R. Coombs]

New submission from Jason R. Coombs <jaraco at jaraco.com>:

While troubleshooting a strange problem (https://github.com/jaraco/keyring/issues/498) where a program worked on Python 3.7+ but failed on Python 3.6, I discovered a somewhat unintuitive behavior. On Python 3.7+, keyword arguments to tuple subclasses are allowed and ignored:

>>> class Foo(tuple): pass
>>> tuple(name='xyz')
TypeError: tuple() takes no keyword arguments
>>> Foo(name='xyz')
()

But on Python 3.6, the keyword parameter causes an error:

$ python3.6 -c "type('Foo', (tuple,), {})(name='xyz')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: 'name' is an invalid keyword argument for this function

I checked out the What's new in Python 3.7 and I see this notice:

Functions bool(), float(), list() and tuple() no longer take keyword arguments. The first argument of int() can now be passed only as positional argument.

Hmm, but in my experience, tuple on Python 3.6 doesn't take keyword arguments either:

importlib_metadata main $ python3.6 -c "tuple(name='xyz')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: 'name' is an invalid keyword argument for this function


So that change may be related, but I'm not sure where or how.

The main place my expectation is violated is in the subclass. Why should a subclass of tuple allow keyword arguments when the parent class does not? I'd expect that the subclass should reject keyword arguments as well.

Less importantly, the What's New doc implies that keyword arguments were accepted in Python 3.6; why aren't they?

----------
components: Interpreter Core
keywords: 3.7regression
messages: 388183
nosy: jaraco
priority: normal
severity: normal
status: open
title: tuple subclasses allow kwargs
versions: Python 3.10, Python 3.7, Python 3.8, Python 3.9

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<https://bugs.python.org/issue43413>
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