Remove redundant pass-es.
Remove unused variable (idx).
Enforce new-style capturing of exception object (PEP 3110).
Invoke join on separator.
Remove unused import.
Replace calls to map with list comprehensions.

In Python 2, a call to `map` builds an actual list,
so it behaves exactly the same as the comprehension syntax.
The latter reads better.
Replace one-off lists with generators.

As the entire list doesn't get built, less space is needed
and performance may improve.
Drop unnecessary slice.
Fix typos.
Remove unused import.
Merge pull request #534 from Renelvon/style_fixes

Style fixes