class ParanoidKeyError(Exception):
    """
    This can't be a KeyError subclass or Django template
    rendering will swallow it.
    """

class ParanoidContextProxy(object):
    """
    This is a poor-man's proxy for a context instance.

    Make sure template rendering stops immediately on a KeyError.
    """
    def __init__(self, context):
        self.context = context
        self.seen_keys = set()

    def __getitem__(self, key):
        self.seen_keys.add(key)
        try:
            return self.context[key]
        except KeyError:
            raise ParanoidKeyError('ParanoidKeyError: %r' % (key,))

    def __getattr__(self, name):
        return getattr(self.context, name)
    def __setitem__(self, key, value):
        self.context[key] = value
    def __delitem__(self, key):
        del self.context[key]

@contextlib.contextmanager
def paranoid_context_manager(context, expected_keys=None):
    """
    use ParanoidContextProxy *and* make sure keys in expected_keys iterable
    are all referenced inside the with statement.
    """
    yield ParanoidContextProxy(context)

    if expected_keys:
        unref = set(expected_keys).difference(context.seen_keys)
        if unref:
            raise ParanoidKeyError('Keys not referenced: %r' % (unref,))

def paranoid_render_to_response(template_name, dictionary=None,
        context_instance=None, *args, **kwargs):
    """
    use ParanoidContextProxy *and* require that all variables passed
    in the data dictionary are used by templates at least once.
    """
    if dictionary is None:
        dictionary = {}
    with paranoid_context_manager(context_instance, dictionary.keys()) as c:
        return render_to_response(template_name, dictionary, c, *args, **kwargs)