decorating a function that yields

A generator function, when called, returns an iterator object. If your decorator is itself a generator too, you'll need to loop over the wrapped result:

def foobar_creator(func):
    def wrapped(**kwargs):
        gen = func(**kwargs)
        flag = True
        for k, v in kwargs:
            if res % v == 0:
                flag = False
                yield k
        if flag:
            for res in gen:
                yield res
    return wrapped

If you are using Python 3.3 or up, you can use delegation to hand control the wrapped generator, by using yield from:

if flag:
    yield from gen

Instead of yielding every potential return value, why not yield only those that actually exist? Something like

def wrap(f, arg):
    for x in f(arg):
        yield x

(actual decorator syntax, handling of positional and keyword arguments, etc. is omitted for clarity.)

For the case in comment42684128, the solution is as simple as:

(v for v in f(<args>) if filter_condition(v))

As a decorator:

def yfilter(filter_condition):
   def yfilter_p(f):
       def wrapped(*args,**kwargs):
           return (v for v in f(*args,**kwargs) if filter_condition(v))
       return wrapped
   return yfilter_p