Wrapping generator functions in Python

Question

I am writing some code that traverses a structure that may have cyclic references. Rather than explicitly doing checks at the beginning of the recursive functions I thought that I would create a decorator that didn't allow a function to be called more than once with the same arguments.

Below is what I came up with. As it is written, this will try to iterate over Nonetype and raise an exception. I know that I could fix it by returning say an empty list, but I wanted to be more elegant. Is there a way to tell from within the decorator whether the function being decorated is a generator function or not? This way I could conditionally raise StopIteration if it is a generator or just return None otherwise.

previous = set()
def NO_DUPLICATE_CALLS(func):
    def wrapped(*args, **kwargs):
        if args in previous:
            print 'skipping previous call to %s with args %s %s' % (func.func_name, repr(args), repr(kwargs))
            return
        else:
            ret = func(*args, **kwargs)
            previous.add(args)
            return ret
    return wrapped

@NO_DUPLICATE_CALLS
def foo(x):
    for y in x:
        yield y

for f in foo('Hello'):
    print f

for f in foo('Hello'):
    print f

Answer 1

Okay, check this out:

>>> from inspect import isgeneratorfunction
>>> def foo(x):
...    for y in x:
...        yield y
...
>>> isgeneratorfunction(foo)
True

This requires Python 2.6 or higher, though.