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.
- Also run isort on the output -
python-modernizetends 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. Runningisorton 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). - Incorrect cPickle imports -
python-modernizereplaces "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". - Disable pylint warnings for some six.moves imports - The
six.movespackage is structured in a way that makes it difficult forpylintto understand how it works, so it will often triggerimport-errorwarnings on imports from it. Rather than rewrite the imports thatpython-modernizesuggests, 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. - Use of six.string_types outside of isinstance -
python-modernizeblindly replaces occurrences of "basestring" with "six.string_types". This is fine for its most common use as the second argument ofisinstance(), 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.