Linter tweaks.

[rpgoldman]

I'm not sure if these will be interesting to you. I was reading over the Popper code in preparation for some extensions, and fixed some issues raised by pylint and the IDE. These should have no effect on Popper's performance, but might make it easier to review and extend.

Update: I believe that the fixes in the second commit are actually important and would fix incorrect behavior under some circumstances. So perhaps this is more worthwhile than I at first thought.

Code formatting issues from the linter.

Pruned imports per linter

Provide more information in error-handler.

Replace error print statements with logging messages at error level.

Tweak error handling.

[rpgoldman]

The second commit contains a couple of corrections that might actually be important. I fixed two linter errors that I think could cause actual incorrectness:

1. Comparison against None using equality instead of is and
2. Mutable function default values.

[andrewcropper]

Thanks @rpgoldman ! I will review them all early next week and will hopefully merge afterwards. Thanks again.

[rpgoldman]

I hope these are helpful. My IDE (PyCharm) pushes these out while I'm touring the code to figure out the innards. Mostly they are stylistic or benign, but I have found a small number of errors that are real (all software is fallible...)

[andrewcropper]

I get this error when trying to run the code:

NameError: name 'Optional' is not defined

Do you not have this error?

[rpgoldman]

My apologies. Made a mistake in the push. Will push an update that fixes this, and also prunes some print statements that I added and that crept into the branch.

[rpgoldman]

Had to force-push, sorry.