Validate and return a value.

def validate(self, name, value):
        """Validate and return a value."""

        if self.valid_set and value not in self.valid_set:
            raise ImproperlyConfigured(
                "%s: \"%s\" is not a valid setting (choose between %s)." %
                (name, value, ", ".join("\"%s\"" % x for x in self.valid_set)))

        return value