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CLAs look good, thanks!

This is not backwards-compatible with Python 2. Please check against file if available. Some options are mentioned here:
https://stackoverflow.com/questions/25738602/how-to-judge-a-file-type-both-work-in-python-2-and-python-3

Which method do you think is better in this case, using the tuple and try/except or checking hasattr(f, 'fileno') / hasattr(f, 'write')?

I think fileno() is a good proxy because it supports file-like objects better (like sockets, etc)

Coverage increased (+0.3%) to 43.893% when pulling 2ead78a on martin2250:master into d9b94b2 on google:master.

Ok, the PR should be good now