Simple type specification in Python

signature.py

#                                                                                                                                                                             
# Simpla and Naive type checker ...                                                                                                                                           
#                                                                                                                                                                             

import inspect

class TypeCheckError(Exception):
    pass


def verifyType(value, aType):
    # Whole type checking rule must be done here. Simple one is done here                                                                                                     
    if inspect.isclass(aType) and isinstance(value,aType):
	return value
    if type(value) == aType:
        return value
    else:
        raise TypeCheckError("%s is not a %s" % (value, aType))


def verifyTypes(i, values, types):
    if len(values) == i:
        pass
    else:
        verifyType(values[i], types[i])
        verifyTypes(i + 1, values, types)


def verifyCoherence(values, types):
    if len(values) != len(types):
        raise TypeCheckError("takes exactly %s arguments (%s given)" %
                             (len(types), len(values)))


def sig(typeIn, typeOut):
    def validate(f):
        def __apply(*args, **kwargs):
            verifyCoherence(args, typeIn)
            verifyTypes(0, args, typeIn)
            return verifyType(f(*args, **kwargs), typeOut)

        return __apply

    return validate

#                                                                                                                                                                             
# Example ...                                                                                                                                                                 
# Must be improved for class type and subtyping :-)                                                                                                                           
#                                                                                                                                                                             

class A:
    def __init__(self):
        pass

    def m(self):
        return 42

class C(A):
    pass

@sig([A], int)
def tt(a):
    return a.m()

@sig([int, int], int)
def add(a, b):
    return a + b


if __name__ == '__main__':
    print add(1, 3)
    print tt(12)