No validation on erroneous creation of 2 or more parameter variables (VAR_KEYWORD and VAR_POSITIONAL) in inspect.Signature

[ApostolFet]

Bug report

Bug description:

We can create inspect.Signature with two or more variable parameters, but this will be syntactically incorrect when defining the function manually:

import inspect

parameters = [
    inspect.Parameter("args", kind=inspect.Parameter.VAR_POSITIONAL),
    inspect.Parameter("other_args", kind=inspect.Parameter.VAR_POSITIONAL),
    inspect.Parameter("kwargs", kind=inspect.Parameter.VAR_KEYWORD),
    inspect.Parameter("other_kwargs", kind=inspect.Parameter.VAR_KEYWORD),
]

signature = inspect.Signature(parameters)
print(signature)

OUTPUT:

(*args, *other_args, **kwargs, **other_kwargs)

And naturally we get a syntax error when defining a function with such a signature:

def func(*args, *other_args, **kwargs, **other_kwargs):
    ...

OUTPUT:

  File "/home/apostol_fet/inspect_signature.py", line 13
    def func(*args, *other_args, **kwargs, **other_kwargs):
                    ^
SyntaxError: * argument may appear only once

Can we add validations for this case in inspect.Signature.__init__?
I would like to send a PR to fix this if it is not intended behavior.

CPython versions tested on:

3.12

Operating systems tested on:

Linux, Windows

Linked PRs

• gh-127610: Added validation for more than one var positional and var keyword parameters in inspect.Signature #127657

[Eclips4]

cc @serhiy-storchaka

I do think that this check would be useful. We already have a check for situations where user is trying to create a signature with a variadic keyword parameter before a variadic positional parameter:

>>> import inspect
>>> parameters = [
...     inspect.Parameter("args", kind=inspect.Parameter.VAR_KEYWORD),
...     inspect.Parameter("kwargs", kind=inspect.Parameter.VAR_POSITIONAL),
... ]
>>> inspect.Signature(parameters)
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    inspect.Signature(parameters)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/home/eclips4/programming/programming-languages/cpython/Lib/inspect.py", line 2958, in __init__
    raise ValueError(msg)
ValueError: wrong parameter order: variadic keyword parameter before variadic positional parameter
>>>

[skirpichev]

I would like to send a PR to fix this if it is not intended behavior.

Definitely is a bug. Why do we want to produce syntactically incorrect Signature objects?!

[serhiy-storchaka]

I agree. There are other checks for the order of parameters, for example positional or keyword parameter is not allowed before positional-only parameter. We should simply add more checks: for var-positional parameter before var-positional parameter and var-keyword parameter before any parameter.

[serhiy-storchaka]

Thank you for your contribution, @ApostolFet.