Support for get_by_key and set_by_key operations

[dterei]

Libmemcached support memcached_get_by_key and memcached_set_by_key. These can be useful for implementing some specific caching and usage strategies with memcached.

It would be awesome if pylibmc supported these.

[lericson]

There is an earlier discussion about this and how the API would look. Do you have any suggestions? How do we keep the API from becoming too crowded?

[dterei]

It seems to me there are three approaches. I don't properly know python, so not sure if all three are achievable in the language:

1. More functions

Simply add-in new functions. This is perhaps the least attractive, since you'd basically nearly double the size of the API. i.e.,

set_by_key(hash_key, key, value, options)
get_by_key(hash_key, key, value)
touch_by_key(hash_key, key, value)
...

2. New option argument

Extend all functions to take an optional argument, hash_key that indicates what value should be used to choose the server to speak to.

set(key, value, hash_key=k)

3. Subclass or lambda

Say an API like:

mc_k = mc.with_hash_key(hkey)
mc_k.set(key, value) // set using hkey as the hashing key

mc.set(key, value) // set using default hashing, (i.e., key as the hash key)

This is nice since it composes well, doesn't complicate the API much.

A similar idea would take a lambda instead:

mc.with_hash_key(hkey, lambda mck:
        mck.set(key, value)
)

I think the subclass idea seems the nicest to me. Haven't thought it through completely if there are any issues with it.