"Base Cache class."

from django.core.exceptions import ImproperlyConfigured

class InvalidCacheBackendError(ImproperlyConfigured):

    pass

class BaseCache(object):

    def __init__(self, params):

        timeout = params.get('timeout', 300)

        try:

            timeout = int(timeout)

        except (ValueError, TypeError):

            timeout = 300

        self.default_timeout = timeout

    def add(self, key, value, timeout=None):

        """

        Set a value in the cache if the key does not already exist. If

        timeout is given, that timeout will be used for the key; otherwise

        the default cache timeout will be used.

        Returns True if the value was stored, False otherwise.

        """

        raise NotImplementedError

    def get(self, key, default=None):

        """

        Fetch a given key from the cache. If the key does not exist, return

        default, which itself defaults to None.

        """

        raise NotImplementedError

    def set(self, key, value, timeout=None):

        """

        Set a value in the cache. If timeout is given, that timeout will be

        used for the key; otherwise the default cache timeout will be used.

        """

        raise NotImplementedError

    def delete(self, key):

        """

        Delete a key from the cache, failing silently.

        """

        raise NotImplementedError

    def get_many(self, keys):

        """

        Fetch a bunch of keys from the cache. For certain backends (memcached,

        pgsql) this can be *much* faster when fetching multiple values.

        Returns a dict mapping each key in keys to its value. If the given

        key is missing, it will be missing from the response dict.

        """

        d = {}

        for k in keys:

            val = self.get(k)

            if val is not None:

                d[k] = val

        return d

    def has_key(self, key):

        """

        Returns True if the key is in the cache and has not expired.

        """

        return self.get(key) is not None

    def incr(self, key, delta=1):

        """

        Add delta to value in the cache. If the key does not exist, raise a

        ValueError exception.

        """

        if key not in self:

            raise ValueError, "Key '%s' not found" % key

        new_value = self.get(key) + delta

        self.set(key, new_value)

        return new_value

    def decr(self, key, delta=1):

        """

        Subtract delta from value in the cache. If the key does not exist, raise

        a ValueError exception.

        """

        return self.incr(key, -delta)

    def __contains__(self, key):

        """

        Returns True if the key is in the cache and has not expired.

        """

        # This is a separate method, rather than just a copy of has_key(),

        # so that it always has the same functionality as has_key(), even

        # if a subclass overrides it.

        return self.has_key(key)