'''
def add(a,b):
    return a+b

oneAddition = once(add)

oneAddition(2,2) # 4
oneAddition(2,2) # None
oneAddition(12,200) # None
'''

def once(fn):
    fn.is_called = False
    def inner(*args):
        if not(fn.is_called):
            fn.is_called = True
            return fn(*args)
    return inner