How to set the signature of a function?

Question

I tried changing the signature of a function using the inspect module:

import inspect

def some_func(a, b):
    return

sig = inspect.signature(some_func)
new_params = list(sig.parameters.values()) + [inspect.Parameter('c', inspect._ParameterKind.POSITIONAL_OR_KEYWORD)]
new_sig = sig.replace(parameters=new_params)
some_func.__signature__ = new_sig

When I inspect the function's signature, it shows the new signature:

>>> inspect.signature(some_func)
>>> <Signature (a, b, c)>

But when I try to call the function according to the new signature, I get a TypeError:

>>> some_func(1, 2, 3)
>>> TypeError: some_func() takes 2 positional arguments but 3 were given

How can I set the signature so that the interpreter checks the arguments against the new signature instead of the original one?

Answer 1

Reassigning function signatures is not something Python supports.

Most functions do something with their arguments. If you somehow managed to change a function's signature, the function body wouldn't have the right arguments to work with. It's not a thing that makes sense to do.

If you're absolutely adamant about doing this anyway, you'd have to do something like mess with the function's __code__ . This is a low-level implementation detail, and messing with it this way is likely to crash Python or worse.

Answer 2

I think this is a category error. You can create a Signature object by inspecting an existing function, but you can't assign an existing function a new signature the way you're trying to do.

That is, hasattr(some_func, '__signature__') returns False . You assigned it in your script, but you can assign arbitrary attributes to functions.

To actually create a function that modifies an existing function you'll have to wrap it.