method-cache-max-size-none / W1518#

Message emitted:

'lru_cache(maxsize=None)' or 'cache' will keep all method args alive indefinitely, including 'self'

Description:

By decorating a method with lru_cache or cache the 'self' argument will be linked to the function and therefore never garbage collected. Unless your instance will never need to be garbage collected (singleton) it is recommended to refactor code to avoid this pattern or add a maxsize to the cache. The default value for maxsize is 128.

Correct code:

import functools


@functools.cache
def cached_fibonacci(n):
    if n in {0, 1}:
        return n
    return cached_fibonacci(n - 1) + cached_fibonacci(n - 2)


class Fibonnaci:
    def fibonacci(self, n):
        return cached_fibonacci(n)

Problematic code:

import functools


class Fibonnaci:
    @functools.lru_cache(maxsize=None)  # [method-cache-max-size-none]
    def fibonacci(self, n):
        if n in {0, 1}:
            return n
        return self.fibonacci(n - 1) + self.fibonacci(n - 2)

Created by the stdlib checker.