Foo.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}

import Prelude hiding ((*))
import qualified Prelude as P

class Mul a b c | a b -> c where
  (*) :: a -> b -> c

instance Mul Int Int Int where
  x * y = x P.* y

instance Mul String Int String where
  x * 1 = x
  x * n | n <= 0 = ""
        | n >  0 = x ++ x * (n - 1)

-- The type can be inferred.
-- foo :: Mul a Int c => a -> c
foo x = x * (2::Int)

main = do
  print $ foo (3::Int)
  print $ foo "hi"

Foo.py

def foo(x):
  return x * 2

print foo(3)
print foo("hi")