The python-modernize utility from the modernize package is very useful for quickly updating Python 2 code to also support Python 3, but the changes it makes aren't always ideal (or even correct). Here are some tips on how to improve its output.
python-modernize tends to add new imports after all existing imports, and when adding "from __future__ import absolute_imports" fails to combine it on the same line with any existing similar imports. Running isort on the same files should fix this, just make sure there aren't any comments near reordered imports giving a good reason why they need to stay in an unusual order (we've caused a bug or two in the past by failing to notice this).python-modernize replaces "cPickle" in any imports with "six.moves.cPickle"; this works sometimes, but not if the original code was "import cPickle as pickle". In this case, the rewritten import will fail and needs to be updated to "from six.moves import cPickle as pickle".six.moves package is structured in a way that makes it difficult for pylint to understand how it works, so it will often trigger import-error warnings on imports from it. Rather than rewrite the imports that python-modernize suggests, it's usually better to append " # pylint: disable=import-error" (two spaces before the comment) to the end of such lines when pylint complains about them.python-modernize blindly replaces occurrences of "basestring" with "six.string_types". This is fine for its most common use as the second argument of isinstance(), but can fail in other contexts where the switch from a type to a tuple containing a single type causes problems. In such cases, you may need to replace "six.string_types" with "six.string_types[0]".If you have to make other changes to the code generated by python-modernize, please note them here.