Add some constants here such as:

```python
INSERTION = 'Insertion'
DELETION = 'Deletion'
NOCHANGE = 'No Change'
```

And then, throughout the code dealing with change ttypes, you can reference these constants rather than using the strings themselves so that, if these every change, they only need to change in this single spot rather than replacing all the occurrences of the strings and worrying about case mismatches, missed occurrences, etc.