`inspect.Parameter` checks that `name` is an identifier, but does not check for keywords

[Zac-HD]

Consider the following code:

def send(from):
    ...

This parses, but it doesn't compile: from is a keyword and so the code is syntactially invalid.

The problem is that inspect.Parameter checks that the given name is an identifier, but doesn't check that it's not a keyword. You can thus create invalid signatures, which cause considerable confusion on downstream introspection:

def f(source):
    pass

from inspect import Signature, Parameter
f.__signature__ = Signature([Parameter("from", P.KEYWORD_ONLY)], return_annotation=None)

# Or, as the real-world motivation,
class Model(pydantic.BaseModel):
    source: None = pydantic.Field(None, alias="from")

We're fixing this downstream in HypothesisWorks/hypothesis#3317 and pydantic/pydantic#4012, but I think that inspect.Parameter("from", ...) should also raise an error, just as if the provided name was not an identifier at all.

[JelleZijlstra]

I can see a scenario where this will break things: we try to get the signature for a function implemented in C that takes a positional-only argument with a name that happens to be a keyword. I can't find any instances of that pattern in typeshed or CPython, but they may exist elsewhere.

[Zac-HD]

Hmm, that makes sense. I'll tighten the check to only apply if the kind is not P.POSITIONAL_ONLY, since I don't expect this to weaken it as a check on Python code 👍

[Zac-HD]

I think we're fine to continue ignoring unicode names which normalize to keywords, since that's the existing behaviour of keyword.iskeyword and does actually work at runtime:

>>> def f(dｅf):  # non-ASCII "e" in argument name
...     print(locals())
... f(1)
{'def': 1}