def incr(self, key, delta=1):
        try:
            val = self._cache.incr(key, delta)

        # python-memcache responds to incr on non-existent keys by
        # raising a ValueError. Cmemcache returns None. In both
        # cases, we should raise a ValueError though.
        except __HOLE__:
            val = None
        if val is None:
            raise ValueError("Key '%s' not found" % key)

        return val
dataset/ETHPy150Open AppScale/appscale/AppServer/google/appengine/_internal/django/core/cache/backends/memcached.py/CacheClass.incr