def unregister_sizer(self, attr_name):
if attr_name not in self._sizedimage_registry:<EOL><INDENT>raise NotRegistered(<EOL>'<STR_LIT>' % attr_name<EOL>)<EOL><DEDENT>else:<EOL><INDENT>del self._sizedimage_registry[attr_name]<EOL><DEDENT>
Unregister the SizedImage subclass currently assigned to `attr_name`.

If a SizedImage subclass isn't already registered to `attr_name`
NotRegistered will raise.