I don't see filepath actually used outside this assert.  What's it for ?
You actually open `CF_PROMISE_SUMMARY_FILE` instead.
We can use it as default for filepath, if that makes sense.
Also: you don't need it to be a unicode.  Any string should be OK for open(); both normal string and unicode are based on basestring, so that you can test against that.
Suggestion (based on guessing your intent, so possibly wildly wrong):
```
def get_last_line_of_file(filepath=CF_PROMISE_SUMMARY_FILE):
    assert(isinstance(filepath, basestring))
```