How to ignore a parameter in functools. lru_cache?

Question

This is a skeleton of the function I want to enhance with a cache, because doing RPC (remote procedure call) involves a TCP connection to other host.

def rpc(rpc_server, rpc_func, arg):
    return rpc_server.do_rpc(rpc_func, arg)

However, the most convenient way of simply decorating it with:

@functools.lru_cache()

does not work well, beacuse rpc_server objects come and go and this parameter should be ignored by the cache.

I can write a simple memoizing code myself. No problem with that. Actually, I see no other solution.

I am unable to rewrite this function in such way that @lru_cache() decorator can be applied and rpc_server will be passed as an argument (i.e. I don't want to make rpc_server a global variable). Is it possible?

Answer 1

I'm posting this just for completness. Comments are welcome, but please do not vote.

---

I have found a way how to satisfy the conditions from my question. I'm not going to use this code. But it shows how flexible Python is.

import functools

class BlackBox:
    """All BlackBoxes are the same."""
    def __init__(self, contents):
        # TODO: use a weak reference for contents
        self._contents = contents

    @property
    def contents(self):
        return self._contents

    def __eq__(self, other):
        return isinstance(other, type(self))

    def __hash__(self):
        return hash(type(self))

@functools.lru_cache()
def _cached_func(blackbox, real_arg):
    print("called with args:", blackbox.contents, real_arg)
    return real_arg + 1000

def cached_func(ignored_arg, real_arg):
    # ignored means ignored by the cache
    return _cached_func(BlackBox(ignored_arg), real_arg)

cached_func("foo", 1) # cache miss
cached_func("bar", 1) # cache hit

cached_func("bar", 2) # cache miss
cached_func("foo", 2) # cache hit