problem with curring in python

[Antoon Pardon]

Using python 3.5

I have been experimenting with curried functions. A bit like in Haskell.
So I can write the following function:

def sum4(a, b, c, d):
    return a + b + c + d

summing = curry(sum4)

print summing(1)(2)(3)(4) # this prints 10.

The problem is I need the signature of the original function in order to
know when to finally call the function and return the actual result.
However buildin functions don't have a signature. Here below is my
current experimental implementation. Any ideas for an other approach?

    def curry(func, *args):
        arg_len = len(signature(func).parameters)
        if arg_len <= len(args):
            return func(*args)
        else:
            return CurryType(func, arg_len, args)

    class CurryType:
        def __init__(self, func, arg_len, args):
            self.func = func
            self.arg_len = arg_len
            self.args = list(args)

        def __call__(self, *args):
            args = self.args + list(args)
            if self.arg_len <= len(args):
                return self.func(*args)
            else:
                return CurryType(self.func, self.arg_len, args)