[MNT] - Minor updates from linting

[TomDonoghue]

Small set of code updates from running linting.

Main changes:

• Errors that are raised from an exception are now 'raised from' (https://stackoverflow.com/questions/24752395/python-raise-from-usage)
• some list comprehensions switched to be generator comprehension (https://stackoverflow.com/questions/47789/generator-expressions-vs-list-comprehensions)