methodic_cache

functools.cache() for methods, done correctly.

methodic_cache.cached_method is a decorator that caches the return value of a method, based on the arguments passed to it.

The peculiarity of this library is that it does not store anything on objects themselves, but rather on a separate WeakKeyDictionary where the lifetime of the cache matches the lifetime of the object.

An advantage of this approach over storing the cache on the object itself when needed is that objects will keep their memory footprint smaller thanks to shared key dictionaries. See PEP 412 and The Dictionary Even Mightier - Brandon Rhodes at PyCon 2017, 00:21:02 for more details.

Features

• Simple to use
• Extendable with custom cache backends (e.g. LRUCache, LFUCache, etc.)
• Works with non-hashable objects
• Works with frozen/slotted classes
• Tested for memory leaks

Installation

pip install methodic_cache

Usage

from methodic_cache import cached_method


class MyClass:
    @cached_method
    def my_method(self, arg1, arg2):
        return arg1 + arg2


my_obj = MyClass()
my_obj.my_method(1, 2)  # returns 3
my_obj.my_method(1, 2)  # returns 3 from the cache

Using classes with __slots__

Classes that define __slots__ need to have a __weakref__ slot to be able to be weakly referenced:

from methodic_cache import cached_method


class MyClass:
    __slots__ = ("my_attr", "__weakref__")  # <-- __weakref__ is required

    def __init__(self, my_attr):
        self.my_attr = my_attr

    @cached_method
    def my_method(self, arg1, arg2):
        print(f"Computing {self.my_attr} + {arg1} + {arg2}...")
        return self.my_attr + arg1 + arg2

my_obj = MyClass(1)
my_obj.my_method(2, 3)
# prints "Computing 1 + 2 + 3..."
# returns 6
my_obj.my_method(2, 3)
# returns 6

Custom cache backends

You can use any cache backend that implements the MutableMapping interface (e.g. dict, lru_cache, functools.lru_cache, etc.). The default cache backend is cachetools.Cache(maxsize=math.inf), which will keep the cache bounded to the lifetime of the self object.

You can use a different cache backend by passing it as the cache_factory argument to cached_method:

from methodic_cache import cached_method
from cachetools import LRUCache


class MyClass:
    @cached_method(cache_factory=lambda: LRUCache(maxsize=1))
    def my_method(self, arg1, arg2):
        print(f"Computing {arg1} + {arg2}...")
        return arg1 + arg2


my_obj = MyClass()
my_obj.my_method(1, 1)
# prints Computing 1 + 1...
# returns 2
my_obj.my_method(1, 1)
# returns 2
my_obj.my_method(2, 2)
# prints Computing 2 + 2...
# returns 4
my_obj.my_method(1, 1)  # <-- this will be recomputed because the cache is full
# prints Computing 1 + 1...
# returns 2