test.py

class Counter:
    def __init__(self, high):
        self.current = 0
        self.highest = high 

    # This class is now considered and iterable because of this special function
    def __iter__(self):
        return self

    def next(self):
        if self.current > self.highest:
            raise StopIteration
        else:
            self.current += 1
            return self.current - 1


def add_all(items):
    a = 0
    for i in items: # This implicitly turns the argument into an iterable
        a = a + i
        
    return i
    
print add_all([1, 2, 3, 4, 5, 6]) # using a list
print add_all((1, 2, 3, 4, 5, 6)) # using a tuple
print add_all(Counter(6)) # Using an object

# The add_all function doesn't care, so long as it's argument is an iterable of some sort, it will do the right thing.