from django.template import Library, Node, TemplateSyntaxError

from django.template import resolve_variable

from django.core.cache import cache

from django.utils.encoding import force_unicode

register = Library()

class CacheNode(Node):

    def __init__(self, nodelist, expire_time, fragment_name, vary_on):

        self.nodelist = nodelist

        self.expire_time = expire_time

        self.fragment_name = fragment_name

        self.vary_on = vary_on

    def render(self, context):

        # Build a unicode key for this fragment and all vary-on's.

        cache_key = u':'.join([self.fragment_name] + \

            [force_unicode(resolve_variable(var, context)) for var in self.vary_on])

        value = cache.get(cache_key)

        if value is None:

            value = self.nodelist.render(context)

            cache.set(cache_key, value, self.expire_time)

        return value

def do_cache(parser, token):

    """

    This will cache the contents of a template fragment for a given amount

    of time.

    Usage::

        {% load cache %}

        {% cache [expire_time] [fragment_name] %}

            .. some expensive processing ..

        {% endcache %}

    This tag also supports varying by a list of arguments::

        {% load cache %}

        {% cache [expire_time] [fragment_name] [var1] [var2] .. %}

            .. some expensive processing ..

        {% endcache %}

    Each unique set of arguments will result in a unique cache entry.

    """

    nodelist = parser.parse(('endcache',))

    parser.delete_first_token()

    tokens = token.contents.split()

    if len(tokens) < 3:

        raise TemplateSyntaxError(u"'%r' tag requires at least 2 arguments." % tokens[0])

    try:

        expire_time = int(tokens[1])

    except ValueError:

        raise TemplateSyntaxError(u"First argument to '%r' must be an integer (got '%s')." % (tokens[0], tokens[1]))

    return CacheNode(nodelist, expire_time, tokens[2], tokens[3:])

register.tag('cache', do_cache)