what's a good/structured way to prevent memcached CacheKeyWarning's?

Question

I was wondering if anyone knew of a handy way or method to make sure the keys you pass to django.core.cache.set() or cache.get() are ok.

From https://docs.djangoproject.com/en/1.3/topics/cache/#cache-key-warnings:

Memcached, the most commonly-used production cache backend, does not allow cache keys longer than 250 characters or containing whitespace or control characters, and using such keys will cause an exception.

I found this md5_constructor() function here: https://github.com/django/django/blob/master/django/utils/hashcompat.py,

maybe one way is to md5-ifiy the key you use always? Not 100% sure if that's safe or not.

Answer 1

You might want to use a custom key function https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-CACHES-KEY_FUNCTION

Set it in your settings:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'KEY_FUNCTION': 'path.to.my.make_key',
        'LOCATION': [
            '127.0.0.1:11211',
        ]
    }
}

I would use something like:

from django.utils.encoding import smart_str

def _smart_key(key):
    return smart_str(''.join([c for c in key if ord(c) > 32 and ord(c) != 127]))

def make_key(key, key_prefix, version):
    "Truncate all keys to 250 or less and remove control characters"
    return ':'.join([key_prefix, str(version), _smart_key(key)])[:250]