OK - so I wasn't thinking clearly, but the reason you shouldn't do this is because using `=` calls `__set__`, which ought to mark everything, mutable or not, as dirty. What you want is something that only calls `__get__`.  Might I suggest `mutable_test.list_field.count(1)` or something that will access the list (via `__get__`) without explicitly calling `__set__`.

Also write a comment that explains that :)