cache_util.py

from django.utils.encoding import smart_str
from django.utils.hashcompat import md5_constructor


def short_key(key, key_prefix, version):
    """
    Like the default `KEY_FUNCTION`, but hashes strings longer
    than 250 characters for safety with memcached. Uses python hash()
    only when key is too long for memcached. Does not do anything
    about memcached's restricted character set, only the key
    length restriction. Due to keeping as much of the original
    key as possible, keys are somewhat introspectable.
    """
    k = ":".join([key_prefix, str(version), smart_str(key)])

    if len(k) <= 250:
        return k
    else:
        # hash() isn't necessarily cryptographically secure, but
        # the values are unique enough for this use. 
        # see http://effbot.org/zone/python-hash.htm for algorithm
        return k[:220] + ".trunc." + str(hash(k[220:]))


def md5_key(key, key_prefix, version):
    """
    Like the default `KEY_FUNCTION`, but md5 to hash the input key.
    Causes output key to be within memcached's key length restriction
    and restricted ASCII character set (provided the `key_prefix` and
    `version` are). Keys are no longer introspectable.
    """
    return ":".join([
        key_prefix,
        str(version),
        md5_constructor(smart_str(key)).hexdigest()
    ])