Since Python is a dynamically typed language, it doesn't offer type checking out of the box. There are workarounds around this though. One that I use in my projects is the following:

def T(val, t):
  assert type(val) == t, '(%s) %s != %s' % (str(val), type(val), t)

Here's an example of how to use this:

def sum(a, b):
  T(a, int)
  T(b, int)
  return a + b

Let's say that you call the function incorrectly, like this:

a = "hello"
b = 5
s = sum(a, b)

You'll get the following exception:

>>> sum(a, b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in sum
  File "<stdin>", line 2, in T
AssertionError: (hello) <type 'str'> != <type 'int'>

So that's it - pretty simple. I use the T function every once in a while, to guard against type errors.