Promises is a very basic type checking library for Python.

from numbers import Number
from promises import promise

@promise.returns(str)
def add(a, b):
    return a + b

@promise.accepts(Number, Number)
@promise.returns(Number)
def sub(a, b):
    return a - b

sub(5, 2)        # works
sub('5', '2')    # raises TypeError

add('Hel', 'lo') # works
add(1, 2)        # raises ReturnError