I'm having trouble changing the signature of unbound methods.

[benfogelson]

Below is a MWE of my attempt to change the signature of MyClass.func from (self, a, b, c) to (self, x, y, z). As you can see from the outputs, the change_sig decorator works on functions and also works on the bound method MyClass().func, but fails on the unbound method. Is there some sort of magic going on behind the scenes when I assign a callable attribute to a class that keeps this from working?

MWE:

import wrapt
import inspect

def change_sig(func):
    adapter = '(self, x, y, z)'
    @wrapt.decorator(adapter=adapter)
    def wrapper(wrapped, instance, args, kwargs):
        pass
    wrapped_func = wrapper(func)
    return wrapped_func

@change_sig
def func(self, a, b, c):
    pass

class MyClass:
    @change_sig
    def func(self, a, b, c):
        pass

print('inspect.signature(func):', inspect.signature(func))
print('inspect.signature(MyClass.func):', inspect.signature(MyClass.func))
print('inspect.signature(MyClass().func):', inspect.signature(MyClass().func))

print('inspect.signature(change_sig(MyClass.func)):', inspect.signature(change_sig(MyClass.func)))
MyClass.wrapped_func = change_sig(MyClass.func)
print('inspect.signature(MyClass.wrapped_func):', inspect.signature(MyClass.wrapped_func))

Outputs:

inspect.signature(func): (self, x, y, z)
inspect.signature(MyClass.func): (self, a, b, c)
inspect.signature(MyClass().func): (x, y, z)
inspect.signature(change_sig(MyClass.func)): (self, x, y, z)
inspect.signature(MyClass.wrapped_func): (self, a, b, c)

I also posted about this on stackoverflow (https://stackoverflow.com/questions/60199457/how-can-i-use-wrapt-decorator-adapters-to-change-the-signature-of-unbound-method) but figured it would be good to ask here as well.

[GrahamDumpleton]

Possibly solved by wrapt adding an appropriate __signature__ property.

• https://www.python.org/dev/peps/pep-0362/

This didn't exist when I first wrote wrapt for Python 2.X. Python 3.X wasn't really usable back then from what I recollect.

So I didn't realise they had added this attribute. Up till now had been relying on how introspection relied upon working down chain of __wrapped__ properties.

I can't remember if I ever validated if signature worked when changing it on unbound method.

I'll look into it some more and add some unit tests so can work out a change.

[GrahamDumpleton]

What version of Python are you using?

[GrahamDumpleton]

Can you try:

pip https://github.com/GrahamDumpleton/wrapt/archive/develop.zip

and test with that.

[benfogelson]

Works great! Thanks for coming up with a fix so quickly. (Python 3.6.10 to answer your earlier question)

[GrahamDumpleton]

This has been released now in 1.12.0.