Is this really a required that? From the code above it does not seem so. Or you remove this check. Or you put in `except` above something like:

```
self.pending_backprocess = False
```

How could this work in tests anyway? Am I missing something? So we didn't made `derived_from` and `contributes_to` required. Maybe we can make `pending_backprocess` required because it is only a boolean value, but I would be then more explicit (and assure that it is really a boolean value).