How to find out the arity of a method in Python

Question

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):
  retur         


        

Answer 1

This is the only way that I can think of that should be 100% effective (at least with regard to whether the function is user-defined or written in C) at determining a function's (minimum) arity. However, you should be sure that this function won't cause any side-effects and that it won't throw a TypeError:

from functools import partial

def arity(func):
    pfunc = func
    i = 0
    while True:
        try:
            pfunc()
        except TypeError:
            pfunc = partial(pfunc, '')
            i += 1
        else:
            return i

def foo(x, y, z):
    pass

def varfoo(*args):
    pass

class klass(object):
    def klassfoo(self):
        pass

print arity(foo)
print arity(varfoo)

x = klass()
print arity(x.klassfoo)

# output
# 3
# 0
# 0

As you can see, this will determine the minimum arity if a function takes a variable amount of arguments. It also won't take into account the self or cls argument of a class or instance method.

To be totally honest though, I wouldn't use this function in a production environment unless I knew exactly which functions would be called though as there is a lot of room for stupid errors. This may defeat the purpose.