Python functools lru_cache with class methods: release object

Question

How can I use functools' lru_cache inside classes without leaking memory? In the following minimal example the foo instance won't be released although going out of scope and having no referrer (other than the lru_cache).

from functools import lru_cache
class BigClass:
    pass
class Foo:
    def __init__(self):
        self.big = BigClass()
    @lru_cache(maxsize=16)
    def cached_method(self, x):
        return x + 5

def fun():
    foo = Foo()
    print(foo.cached_method(10))
    print(foo.cached_method(10)) # use cache
    return 'something'

fun()

But foo and hence foo.big (a BigClass) are still alive

import gc; gc.collect()  # collect garbage
len([obj for obj in gc.get_objects() if isinstance(obj, Foo)]) # is 1

That means that Foo/BigClass instances are still residing in memory. Even deleting Foo (del Foo) will not release them.

Why is lru_cache holding on to the instance at all? Doesn't the cache use some hash and not the actual object?

What is the recommended way use lru_caches inside classes?

I know of two workarounds: Use per instance caches or make the cache ignore object (which might lead to wrong results, though)

Answer 1

This is not the cleanest solution, but it's entirely transparent to the programmer:

import functools
import weakref

def memoized_method(*lru_args, **lru_kwargs):
    def decorator(func):
        @functools.wraps(func)
        def wrapped_func(self, *args, **kwargs):
            # We're storing the wrapped method inside the instance. If we had
            # a strong reference to self the instance would never die.
            self_weak = weakref.ref(self)
            @functools.wraps(func)
            @functools.lru_cache(*lru_args, **lru_kwargs)
            def cached_method(*args, **kwargs):
                return func(self_weak(), *args, **kwargs)
            setattr(self, func.__name__, cached_method)
            return cached_method(*args, **kwargs)
        return wrapped_func
    return decorator

It takes the exact same parameters as lru_cache, and works exactly the same. However it never passes self to lru_cache and instead uses a per-instance lru_cache.