caching decorator

Question

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]
        else:
            result = f(*args)
            _._cache[args] = result
            return result     
    _._cache = {}
    return _

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 = {}

    @functools.wraps(obj)
    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