How to make underlying cache methods available in timed LRU cache in Python?

We know that cache can greatly speed up a load of frequently used data (for example, S3 objects). Python 3 has a built-in implementation of simple unbound and LRU caches in functools module called cache and lru_cache respectively (functools documentation). Both cache and lru_cache expose some useful methods like cache_clear or cache_info. Clearing cache is especially important between your code tests because you can get strange results (a good article that describes this problem).

If you expect your data to change during the program execution, it makes sense to implement a timed cache. Unfortunately, Python 3 doesn’t provide any built-in functionality for timed cache so we need to implement it by ourselves.

Searching on the Internet, you can find a lot of different implementations but all of them are pretty much the same. Let’s take an example from great article about cache by the Real Python website.

Implementation of the timed cache from the article:

from functools import lru_cache, wraps
from datetime import datetime, timedelta

def timed_lru_cache(seconds: int, maxsize: int = 128):
    def wrapper_cache(func):
        func = lru_cache(maxsize=maxsize)(func)
        func.lifetime = timedelta(seconds=seconds)
        func.expiration = datetime.utcnow() + func.lifetime

        @wraps(func)
        def wrapped_func(*args, **kwargs):
            if datetime.utcnow() >= func.expiration:
                func.cache_clear()
                func.expiration = datetime.utcnow() + func.lifetime

            return func(*args, **kwargs)

        return wrapped_func

    return wrapper_cache

Note: a cache is cleared only when the timed_lru_cache function is called and the condition is checked. Do not expect cache to be cleared right on the expiration time.

And usage:

@timed_lru_cache()
def load_key_from_s3(key: str):
    # ...

Warning: brackets are important. @timed_lru_cache will not work, use @timed_lru_cache()

But there is an issue, the wraps method from functools is only preserving the original function’s name and docstring but not original methods (documentation on wraps). Calling, for example, load_key_from_s3.cache_clear() will fail. What if we want to expose missing methods for test and statistics purposes? The simplest fix to the above implementation is the following:

def timed_lru_cache(seconds: int, maxsize: int = 128):
    def wrapper_cache(func):
        func = lru_cache(maxsize=maxsize)(func)
        func.lifetime = timedelta(seconds=seconds)
        func.expiration = datetime.utcnow() + func.lifetime

        @wraps(func)
        def wrapped_func(*args, **kwargs):
            if datetime.utcnow() >= func.expiration:
                func.cache_clear()
                func.expiration = datetime.utcnow() + func.lifetime

            return func(*args, **kwargs)
        
        # add missing methods to wrapped function
        wrapped_func.cache_clear = func.cache_clear
        wrapped_func.cache_info = func.cache_info

        return wrapped_func

    return wrapper_cache

Now both cache_clear and cache_info methods are exposed for load_key_from_s3 function. Thank you for reading.