<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def evaluate(self, name, variables=None):
 """ Evaluate a named rule. :param name: The name of the rule to evaluate. :param variables: An optional dictionary of variables to make available during evaluation of the rule. :returns: An instance of ``policies.authorization.Authorization`` with the result of the rule evaluation. This will include any authorization attributes. """


        # Get the rule and predeclaration
        rule = self._rules.get(name)
        default = self._defaults.get(name)

        # Short-circuit if we don't have either
        if rule is None and default is None:
            return authorization.Authorization(False)

        # Marry the attribute defaults
        attrs = {}
        if default:
            attrs.update(default.attrs)
        if rule:
            attrs.update(rule.attrs)

        # Select the rule we'll actually use
        if rule is None:
            rule = default

        # Construct the context
        ctxt = self.context_class(self, attrs, variables or {})

        # Execute the rule
        try:
            with ctxt.push_rule(name):
                rule.instructions(ctxt)
        except Exception as exc:
            # Fail closed
            return authorization.Authorization(False, attrs)

        # Return the authorization result
        return ctxt.authz