Dynamically cache a method of an object once

cache_method_once_dynamically_add.py

import functools
import random
import time
import types

class A:
    def f(self):
        time.sleep(1)
        return random.random()


def cache_once(obj):
    """Caches a method once."""
    cache = {}
    done = set([])  # args which already have been cached

    @functools.wraps(obj)
    def memoizer(*args, **kwargs):
        self, *args = args
        key = str(args) + str(kwargs)
        if key in cache and key not in done:
            # this is the only time we return a cached result
            done.add(key)
            return cache.pop(key)
        else:
            cache[key] = obj(*args, **kwargs)
            return cache[key]

    memoizer._cache = cache
    memoizer._done = done
    return memoizer


a = A()
a.f = types.MethodType(cache_once(a.f), a)

a.f()  # takes 1s
a.f()  # is cached
a.f()  # takes 1s
a.f()  # takes 1s