I came up with a caching decorator for pure functions. Is it ok? Could it be better/simpler/faster?

def cached(f):    
    def _(*args):
        if args in _._cache:
            return _._cache[args]
            result = f(*args)
            _._cache[args] = result
            return result     
    _._cache = {}
    return _
  • \$\begingroup\$ I mean, not an improvement or anything, but they do have a lru_cache function already built into the library. I would suspect it to be faster than this... In other words, if this is part of a bigger scheme I would consider using lru_cache instead. \$\endgroup\$
    – Dair
    Commented Feb 11, 2016 at 9:51
  • \$\begingroup\$ @Dair It might not be available to everyone since lru_cache has only been added in Python 3.2. \$\endgroup\$ Commented Feb 11, 2016 at 10:10
  • \$\begingroup\$ @MathiasEttinger: Good point. I still think he should be aware, especially since he hasn't specified his python version. \$\endgroup\$
    – Dair
    Commented Feb 11, 2016 at 10:12
  • \$\begingroup\$ @Dair Yes, it's just that, by the look of things, he seems to be using Python 2. But your point remains valid anyway. \$\endgroup\$ Commented Feb 11, 2016 at 10:20
  • \$\begingroup\$ I'm using Python 2.6 and LRU cache as I understand it stores only a limited set of latest used results and I need a full cache. \$\endgroup\$
    – Eugene
    Commented Feb 11, 2016 at 10:31

1 Answer 1


What you are trying to achieve is called memoization and has already a number of recipe available on the Python wiki.

Your implementation matches the one using nested functions:

# note that this decorator ignores **kwargs
def memoize(obj):
    cache = obj.cache = {}

    def memoizer(*args, **kwargs):
        if args not in cache:
            cache[args] = obj(*args, **kwargs)
        return cache[args]
    return memoizer

A few things to note here on top of yours:

  • usage of a local variable which resolves faster than an attribute lookup
  • usage of functools.wraps which keeps some properties of the original function intact (name and docstring, mainly)
  • explicit names for variables
  • \$\begingroup\$ What you mean by explicit names for variables? \$\endgroup\$
    – Eugene
    Commented Feb 11, 2016 at 10:35
  • \$\begingroup\$ @Eugene _ has no meaning; memoizer has an explicit name which convey meaning. \$\endgroup\$ Commented Feb 11, 2016 at 10:37

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