At attempt at an atomic and bounded cache incr/decr function which handles key existence.

incrdecr.py

from django.core.cache import cache

def _incr_decr(key, incr=True, min=None, max=None, initial=None):
    '''Incr/decr function which handles key creation as needed'''
    # FIXME: min/max have rampant race conditions which could be fixed using
    # memcache's "cas" feature, but current pylibmc + libmemcached causes a
    # segfault when used.

    initial = initial or min or 0

    if incr:
        action = cache.incr
    else:
        action = cache.decr

    try:
        value = action(key)
    except ValueError: # the key doesn't exist, add it
        cache.add(key, initial)
        value = action(key)
    if min is not None and value < min:
        print 'min reached, resetting'
        value = min
        cache.set(key, value)
    if max is not None and value > max:
        print 'max reached, resetting'
        value = max
        cache.set(key, value)
    return value

def _incr(key, max=None, initial=None):
    return _incr_decr(key, max=max, initial=initial)

def _decr(key, min=None, initial=None):
    return _incr_decr(key, incr=False, min=min, initial=initial)