Function decorator for lazy evaluation. Only for argument-less functions. Shouldn't be used for functions with side-effects.

lazy.py

def lazy(fun):
    class Memoizer(object):
        def __init__(self, fun):
            self.fun = fun
            self.set = False
            self.res = None

        def __call__(self):
            if not self.set:
                self.set = True
                self.res = self.fun()
            return self.res

    return Memoizer(fun)

# Demo (with side-effect function)

i = 0

def fi():
    global i
    i += 1
    print i

@lazy
def fj():
    global i
    i += 1
    print i

fi()
fi()
fi()
fj()
fj()
fj()

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