How to cache result of function depending on the value returned

Question

I am aware of functools.lru_cache and functools.cache (since Python 3.9) but I struggle with caching these arguments of a function which do not return None (or any other specific value):

from functools import lru_cache

@lru_cache
def my_fun(link):
    res = fetch_data(link)
    return res

res is None when fetch_data is encountering an intermittent error. And this is when I do not want the result to be cached.

score 0 · Accepted Answer · answered Aug 05 '21 at 11:10

Cache for functions with positional arguments

I figured that I can implement the cache on my own using a dictionary and store the result only when the return value is not None:

from functools import wraps

def my_cache(no_cache_result=tuple()):
    if no_cache_result is None:
        no_cache_result = tuple()

    cache = dict()

    def decorator(fun):
        @wraps(fun)
        def wrapper(*args, **kwargs):
            assert len(kwargs) == 0
            if args in cache:
                print('cache: taken from cache')
                return cache[args]
            else:
                res = fun(*args, **kwargs)
                if res not in no_cache_result:
                    print('cache: stored in cache')
                    cache[args] = res
                else:
                    print('cache: NOT stored')
                return res

        return wrapper
    return decorator


@my_cache(no_cache_result=[None])
def my_fun(a):
    print(f'my_fun: called with {a}')
    if a <= 1:
        return a
    else:
        return None


my_fun(0)
my_fun(1)
my_fun(2)

my_fun(0)
my_fun(1)
my_fun(2)

Which prints (as expected):

my_fun: called with 0
cache: stored in cache
my_fun: called with 1
cache: stored in cache
my_fun: called with 2
cache: NOT stored
cache: taken from cache
cache: taken from cache
my_fun: called with 2
cache: NOT stored

Cache for functions with positional and keyword arguments

The solution above limits the functions that can be decorated to those with only positional arguments and not keyword arguments.

At the expense of small slow-down it can be improved in the following way:

def my_cache(no_cache_result=tuple()):
    if no_cache_result is None:
        no_cache_result = tuple()

    cache = dict()

    def decorator(fun):
        @wraps(fun)
        def wrapper(*args, **kwargs):
            _kwargs = tuple(kwargs.items())
            if (args, _kwargs) in cache:
                print('cache: taken from cache')
                return cache[(args, _kwargs)]
            else:
                res = fun(*args, **kwargs)
                if res not in no_cache_result:
                    print('cache: stored in cache')
                    cache[(args, _kwargs)] = res
                else:
                    print('cache: NOT stored')
                return res

        return wrapper
    return decorator

Which works as expected:

@my_cache(no_cache_result=[None, ])
def my_fun2(a, b=7):
    print(f'my_fun2: called with {a}, {b}')
    if a <= 1:
        return a
    else:
        return None


my_fun2(0, b=2)
my_fun2(1)
my_fun2(2)

my_fun2(0, b=2)
my_fun2(1)
my_fun2(2)

Printing:

my_fun2: called with 0, 2
cache: stored in cache
my_fun2: called with 1, 7
cache: stored in cache
my_fun2: called with 2, 7
cache: NOT stored
cache: taken from cache
cache: taken from cache
my_fun2: called with 2, 7
cache: NOT stored

How it works?

Decorators

The details on the implementation of wrappers (also, with arguments) you can find amply discussed in answers to Decorators with parameters?.

Forbidden return values - performance

The performance of the cache depends on the type of passed no_cache_result argument. In case you wish to restrict caching for more than a few return values, it is recommended to pass a set, instead of typically used list because if x in no_cache_result operation is much quicker for sets than for lists.