List comprehension, literal comparison, import ordering, staticmethod

[dericke]

• Reorder imports to PEP8
• Replace a create-then-append pattern with a list comprehension
• Use != instead of is not to compare strings
• Reduce indenting by raising early in a guard condition
• Change a @classmethod that did not reference its class to a @staticmethod

If it would be helpful/appropriate, I would be happy to break these up into multiple small PRs instead.