[Python] How to find out the arity of a method in Python

I'd like to find out the arity of a method in Python (the number of parameters that it receives). Right now I'm doing this:

def arity(obj, method):
return getattr(obj.__class__, method).func_code.co_argcount - 1 # remove self

class Foo:
def bar(self, bla):
pass

arity(Foo(), "bar") # => 1


I'd like to be able to achieve this:

Foo().bar.arity() # => 1


Update: Right now the above function fails with built-in types, any help on this would also be appreciated:

# Traceback (most recent call last):
# File "bla.py", line 10, in <module>
# print arity('foo', 'split') # =>
# File "bla.py", line 3, in arity
# return getattr(obj.__class__, method).func_code.co_argcount - 1 # remove self
# AttributeError: 'method_descriptor' object has no attribute 'func_co

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