How to enforce automatic parameter casting in function definitions?

Question

Consider this example of a function (and its use) that receives one parameter which is only meaningful as int:

def fun(index):
    index = int(index)
    # do something with index

def main():
    print fun(1e6)

I find that I keep repeating this pattern on several functions that deal with integers since it ensures that I really receive some integer number (note that 1e6 is a floating point instance).

This is a bit cumbersome and if forgotten once it could produce problems (there is code that reacts differently to floating-point/integer/string/... values). So could this be somehow encoded in the function signature?

E.g. something along the lines of this pseudo-code:

def fun(int index):
    # do something with index

Answer 1

First of all, you shouldn't really be doing this (even though some days I kind of want to as well…). Python is not statically-typed, so you shouldn't really be introducing cludgy static-typing.

Buuut, to answer you question. Decorators:

def ensure_int(fn):
    def wrapper(num):
        return fn(int(num))
    return wrapper

def test(num):
    return num + 2

@ensure_int
def test2(num):
    return num + 2

Works like:

> test("2") 
ERROR!!!

>test2("2")
4

Answer 2

In Python 3 you could use function annotations and then write a decorator that converts (or tries to convert) arguments to the types you specify.

In Python 2 you can't do this in the way you seem to want. Python 2's syntax doesn't support putting anything in the function definition except the arguments themselves, and you can't alter Python's syntax to let you put types in there as well.