Probably slightly nicer to write this like this:
```
return self.map(lambda v: (v, None)) \
    .cogroup(other.map(lambda v: (v, None))) \
    .filter(lambda x: (len(x[1][0]) != 0) and (len(x[1][1]) != 0)) \
    .keys()
```
Or put parens around the whole thing to avoid Python thinking lines ended (but we used the backslash style before).

Other than that it looks good.