Missing error: "parameter without a default follows parameter with a default" with a function that uses `*args`

[adrien-lsh]

Bug report

Bug description:

Hey there,

I don't know if that's a bug or if that's expected behavior, but since it's weird I might as well get this checked out.

>>> def f(*args, a = 0, b): pass
... 
>>> def f(a = 0, b): pass
  File "<python-input-1>", line 1
    def f(a = 0, b): pass
                 ^
SyntaxError: parameter without a default follows parameter with a default

I clearly understand the error here as it is not the first time I'm using Python, the thing that eludes me is: why is there no error with the first function ?

Thank you for your help !

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[sergey-miryanov]

I believe this is a desired behavior according https://peps.python.org/pep-3102/:

Keyword-only arguments are not required to have a default value. Since Python requires that all arguments be bound to a value, and since the only way to bind a value to a keyword-only argument is via keyword, such arguments are therefore ‘required keyword’ arguments. Such arguments must be supplied by the caller, and they must be supplied via keyword.

So a and b are keyword-only arguments - so user should supply them by keyword. But a has default value and can be omitted from the call.

[sergey-miryanov]

Maybe we should extend the test_positional_only_and_kwonlyargs_invalid_calls test case and add a keyword-only argument with a default value as an example?

[adrien-lsh]

Oh ok I get it. It's a bit confusing because we could simply think that b should become a keyword-only argument.
But since there is no varargs, b is just a positional argument with no default.

Thanks for the explanation !

[terryjreedy]

I don't know if that's a bug or if that's expected behavior,

Questions like this should go to https://discuss.python.org/c/help/7, where they are more likely to be found by others.