simple function signature checking in python

funcsig.py

from itertools import izip

def isiterable(o):
    try:
        it = iter(o)
    except TypeError:
        return False
    return True

class Args(object):
    def __init__(s, *args):
        s.args = args

    def __call__(s, f):
        def wrapper(*args, **kwargs):
            if len(args) != len(s.args):
                raise TypeError, "wrong arguments"
            for n in izip(args, s.args):
                if type(n[0]) != n[1]:
                    msg = repr(n[0]) + " is not in type " + repr(n[1])
                    raise TypeError, msg
            r = f(*args, **kwargs)
            return r
        return wrapper

class Ret(object):
    def __init__(s, *args):
        s.args = args

    def __call__(s, f):
        def wrapper(*args, **kwargs):
            r = f(*args, **kwargs)
            if not isiterable(r):
                r = (r,)
            if len(r) != len(s.args):
                raise TypeError, "wrong return values"
            for n in izip(r, s.args):
                if type(n[0]) != n[1]:
                    msg = repr(n[0]) + " is not in type " + repr(n[1])
                    raise TypeError, msg
            return r
        return wrapper


###
### test
###

@Args(int, int)
@Ret(int)
def foo(a, b):
    print a, b
    return a + b

@Args(int, int)
@Ret(int, int)
def bar(a, b):
    print a, b
    return a, b

def test(f, args):
    try:
        apply(f, args)
    except TypeError, msg:
        print 'Error:', msg

test(foo, (5, 6))
test(foo, (4, "abc"))
test(foo, (4, None))
test(foo, (7,))
test(foo, ())
test(bar, (100, 200))
test(bar, (100, "string"))