How can I type-check variables in Python?

Question

I have a Python function that takes a numeric argument that must be an integer in order for it behave correctly. What is the preferred way of verifying this

Answer 1

isinstance(n, int)

If you need to know whether it's definitely an actual int and not a subclass of int (generally you shouldn't need to do this):

type(n) is int

this:

return int(n) == n

isn't such a good idea, as cross-type comparisons can be true - notably int(3.0)==3.0

Answer 2

how about:

def ip(string):
    subs = string.split('.')
    if len(subs) != 4:
        raise ValueError("incorrect input")
    out = tuple(int(v) for v in subs if 0 <= int(v) <= 255)
    if len(out) != 4:
        raise ValueError("incorrect input")
    return out

ofcourse there is the standard isinstance(3, int) function ...

Answer 3

Don't type check. The whole point of duck typing is that you shouldn't have to. For instance, what if someone did something like this:

class MyInt(int):
    # ... extra stuff ...