<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_filter(self, filter_):
 """ Add an image filter for post-processing """

        assert has_pil, _("Cannot add filters without python PIL")
        self.cache.basename += filter_.basename
        self._filters.append(filter_)