Signature.bind allows certain positional-only parameters as keywords

[dfremont]

Bug report

Bug description:

Prior to 3.13, the following code correctly raised a TypeError (I've tested 3.8-3.12, but see the last paragraph below):

def fun(x, /, **kwargs):
    pass

import inspect
sig = inspect.signature(fun)
sig.bind(x=1)

In 3.13, the binding succeeds even though the positional-only parameter was passed as a keyword argument (of course, fun(x=1) fails with a TypeError).

As far as I can tell this bug hasn't been previously reported, although issue #107831 reports inspect.getcallargs as having the same problem (prior to 3.13, unlike this bug).

I'm guessing the bug was introduced by #103404. If I try a version of 3.12 containing that patch (rather than the older versions I had installed previously), e.g. 3.12.9, it also exhibits the bug. It's possible that fixing the case where the positional-only parameter has a default value accidentally broke this case. Mentioning @jacobtylerwalls as the author of that PR in case they would like to take a look. Thanks!

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-130164: Fix inspect.Signature.bind() handling of positional-only args without defaults #130192

[jacobtylerwalls]

Thanks, bisected to #103404 and testing a fix. Affects 3.12.4+

[picnixz]

Just to be clear, the issue is not that

In 3.13, the binding succeeds even though the positional-only parameter was passed as a keyword argument

Indeed, consider

def fun(x, /, **kwargs): pass

fun(1, x=2)

So

import inspect
sig = inspect.signature(fun)
sig.bind(x=1)

means that you're binding it with a keyword argument x but it doesn't affect the positional-only parameter x itself. However, bind actually expects to specify all required arguments, in which case here the positional x is missing. So it's not because the positional-only parameter was passed as a keyword, it's more that we're actually thinking it was correctly specified.

[picnixz]

A fix for that could be to first check if we have a **kwargs in the signature. If we do, we need to first check if another argument should be bound before we fill the kwargs mapping. If a positional-only cannot be filled (for instance, x should first be checked as a non-keyword argument, if it's not possible to bind it, as in this case, we treat it as a kwarg), we store it as a keyword argument. Once we processed everything, we need to check if a positional-only is still missing because it wasn't treated in the first pass.

EDIT: ok my paragraph is unclear but TL;DR we probably need a second pass to check that the arguments were properly bound (namely, a positional-only argument should only be bound from the *args of bind and not from anything **kwargs)