Fix inspect.signature() of operator.{attrgetter,itemgetter,methodcaller} instances

[dgilland]

Bug report

Bug description:

On Python 3.12.3 and 3.11.9, it looks like the inspect.Signature object for operator's attrgetter, itemgetter, and methodcaller classes doesn't match their __call__ method signature:

>>> import inspect
>>> import operator
>>> inspect.signature(operator.attrgetter("spam"))
<Signature (*args, **kwargs)>
>>> inspect.signature(operator.itemgetter("spam"))
<Signature (*args, **kwargs)>
>>> inspect.signature(operator.methodcaller("spam"))
<Signature (*args, **kwargs)>

but their __call__ methods only accept a single argument:

# attrgetter / itemgetter
    def __call__(self, obj):
        return self._call(obj)

# methodcaller
    def __call__(self, obj):
        return getattr(obj, self._name)(*self._args, **self._kwargs)

CPython versions tested on:

3.11, 3.12

Operating systems tested on:

macOS

Linked PRs

• gh-118285: Fix signatures of operator.{attrgetter,itemgetter,methodcaller} instances #118316

[terryjreedy]

The signatures of the getter/caller classes are the signature of their .__init__ methods, and inspect.signature returns these properly. The signature of callable instances should, it seems, match that of the .__call__ methods, but, as you tested, it does not. However, while the operator callables (functions and these 3 classes) are all initially defined in python, they are all replaced with C implementations, when possible, with try: from _operator import *. After this, itemgetter.__call__, for instance, is a 'wrapper_descriptor' and itemgetter(s).__call__ is a 'method_wrapper'. Perhaps the C code for the call methods was never revised to use argument-clinic.

[serhiy-storchaka]

Actually, the operator module was initially implemented in C. The Python implementation was added later, partially as a fallback for alternative Python implementations (although Jython, PyPy, and other already had their own implementations), partially as experiment (which failed in my opinion).

Now inspect.signature() returns <Signature (*args, **kwargs)> for custom callable objects implemented in C instead of raising a ValueError. It was perhaps not intentional change, but it may be too later to change this again, some code can already rely on this. At least not in 3.11 (which is in the security fixes mode) or 3.12.

In any case, I added a simple way to specify signature for callables implemented in C and implemented it for the operator callables. Whether make inspect.signature() raising a ValueError by default is an open question.