Store the cache to a file functools.lru_cache in Python >= 3.2

Question

I'm using @functools.lru_cache in Python 3.3. I would like to save the cache to a file, in order to restore it when the program will be restarted. How could I do?

Edit 1 Possible solution: We need to pickle any sort of callable

Problem pickling __closure__:

_pickle.PicklingError: Can't pickle <class 'cell'>: attribute lookup builtins.cell failed

If I try to restore the function without it, I get:

TypeError: arg 5 (closure) must be tuple

Answer 1

You can't do what you want using lru_cache, since it doesn't provide an API to access the cache, and it might be rewritten in C in future releases. If you really want to save the cache you have to use a different solution that gives you access to the cache.

It's simple enough to write a cache yourself. For example:

from functools import wraps

def cached(func):
    func.cache = {}
    @wraps(func)
    def wrapper(*args):
        try:
            return func.cache[args]
        except KeyError:
            func.cache[args] = result = func(*args)
            return result   
    return wrapper

You can then apply it as a decorator:

>>> @cached
... def fibonacci(n):
...     if n < 2:
...             return n
...     return fibonacci(n-1) + fibonacci(n-2)
... 
>>> fibonacci(100)
354224848179261915075L

And retrieve the cache:

>>> fibonacci.cache
{(32,): 2178309, (23,): 28657, ... }

You can then pickle/unpickle the cache as you please and load it with:

fibonacci.cache = pickle.load(cache_file_object)

---

I found a feature request in python's issue tracker to add dumps/loads to lru_cache, but it wasn't accepted/implemented. Maybe in the future it will be possible to have built-in support for these operations via lru_cache.