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.