Static type checking decorator for Python

import inspect

def typed(__return_type__ = None, __accept_subclasses__ = True, **argument_types):
    """
    typed - run-time simulation of static typing in python

    A decorator that will throw a TypeError if the type of an argument value passed to the decorated function
    does not match the type defined by this decorator.
    Usage example below. Would throw a TypeError if the "string" argument is not of type str and if the type of the
    returned value is not int:
        @typed(int, string = str)
        def getStringLength(string):
            return len(string)
    """
    def decorator(wrappee): # The decorator function that will be returned by typed()
        def wrapper(*args, **kwargs):
            """
            The wrapper function that will be returned by decorator when the decorated function is called
            This is the actual function that will be run before the wrapped function runs
            """
            def checkType(arg, expected_type, found_value, return_value = False):
                """
                Raise an exception if the found value doesn't match the expected type
                Adapt exception text to return value/argument
                """

                # Will be set to True if the found_value doesn't match the expected type
                bad_type = False

                # If the option accept subclasses is true, check if the found value is an instance of the expected
                # type and set bad_type = True if not
                if __accept_subclasses__:
                    if not isinstance(found_value, expected_type):
                        bad_type = True

                # If the option accept subclasses is false, check that the class of the found_value matches the
                # expected type exactly, and set bad_type = True if not
                else:
                    if not found_value.__class__ == expected_type: #isinstance(found_value, expected_type):
                        bad_type = True

                # Raise a TypeError if a bad type was found and adapt the text depending on if its a return type
                # or argument type that was bad
                if bad_type:
                    if return_value:
                        raise TypeError(u"Return value of %s() expected type %s but found type %s" % (wrappee.__name__, __return_type__, found_value.__class__))
                    else:
                        raise TypeError(u"Argument %s of %s() expected type %s but found type %s" % (arg, wrappee.__name__, expected_type, found_value.__class__))


            # Fetch an ordered list of the arguments defined for the wrapped function
            wrappee_args,_,_,_ = inspect.getargspec(wrappee)

            # Check all arguments defined by the wrapped function against the types defined
            for i, arg in enumerate(wrappee_args):
                if i >= len(args):
                    break
                checkType(arg, argument_types[arg], args[i])

            # Check all keywords arguments defined by the wrapped function against the types defined
            for arg in argument_types.keys():
                if kwargs.has_key(arg):
                    checkType(arg, argument_types[arg], kwargs[arg])

            # Run the wrapped function
            result = wrappee(*args, **kwargs)

            # Check the return value against the type defined
            if __return_type__ is not None:
                checkType("", __return_type__, result, return_value = True)
            return result
        return wrapper
    return decorator

###
# Usage example
###
@typed(int, string = str)
def getStringLength(string):
    return len(string)

#####
# Simple tests below
#####
def raisesTypeError(func, *args, **kwargs):
    try:
        func(*args, **kwargs)
        return False
    except TypeError:
        return True

assert getStringLength("four") == 4
assert raisesTypeError(getStringLength, 4)

@typed(int, string = str)
def getStringLengthFloat(string):
    return float(len(string))

assert raisesTypeError(getStringLengthFloat, "four")