The right approach to memoization: keep it separate

example.py

import time


def memoize(f):
    memo = {}
    def wrapper(*args):
        if args not in memo:
            memo[args] = f(*args)
        return memo[args]
    return wrapper


def _identity(x):
    time.sleep(0.05)  # simulate expensive operation
    return x

identity = memoize(_identity)


if __name__ == '__main__':
    import timeit
    for desc, name in (('original', '_identity'), ('memoized', 'identity')):
        print('{}: {}'.format(desc, timeit.timeit('{}(42)'.format(name),
                setup='from __main__ import {}'.format(name), number=100)))

$ python example.py
original: 5.08163619041
memoized: 0.0512199401855