Maybe you are more interested on a Differ object. These are the results from your example using '\n'.join(difflib.Differ().compare(a,b))

- This is my old file, containing only one line.
+ This is my new file.
+ It contains multiple lines.
+ SequenceMatcher should see two blocks as a result.

and 

+ This is my new file, containing only one line.
- This is my old file.
- It contains multiple lines.
- SequenceMatcher should see two blocks as a result.