Python 2 support

Note

This document is relevant for packages that are either direct dependencies of Zope version 4 or popular add-ons for Zope version 4 and their dependencies.

Zope 4 will retain full Python 2 (and Python 3.5) compatibility throughout its lifetime. That means all its direct dependencies and (ideally) many popular add-on packages should also continue supporting Python 2 and Python 3.5 until Zope 4 reaches end-of-life status.

When to drop support

Now it the time to drop support for Python 2.7 up to 3.6 as they are all no longer maintained by the Python community and the maintencance burden has become too heavy.

How to drop support

When dropping Python 2 support you should also “upgrade” the code to support more modern practices, like using f-strings.

Prerequisites

  • Installed packages the code examples below expect it to be on $PATH.

    • pyupgrade

    • check-python-versions

    • zest.releaser

  • Repository is under control of meta/config thus it has a .meta.toml file.

Typical steps

The following steps are necessary to remove support for Python 2.7 up to 3.6 from a package:

  • Update version number to next major version:

    $ bumpversion --breaking
$ addchangelogentry "Drop support for Python 2.7, 3.5, 3.6."

  • Remove Python 2 support from the Trove classifiers in setup.py and update python_requires:

    $ check-python-versions --drop 2.7,3.5,3.6

  • .meta.toml

    • Remove Python 2 specific settings

  • Run meta/config to remove Python 2 support from other configuration files and create a branch:

    $ bin/python config-package.py <PATH-TO-REPOS> --without-legacy-python --no-commit --branch=py3-only

  • setup.py

    • Remove six from the list of dependencies

    • Remove other things pointing to Python 2 or PyPy2.

  • Remove Python 2.7 up to 3.6 support code:

    • Update the code to Python 3 only and update usage of six:

      $ find src -name "*.py" -exec pyupgrade --py3-plus --py37-plus {} \;

    • Replace all remaining six mentions, find it by running:

      $ grep -rn six src

    • Find any remaining code that may support Python 2:

      $ egrep -rn "2.7|3.5|3.6|sys.version|PY2|PY3|Py2|Py3|Python 2|Python 3|__unicode__|ImportError" src

    • Run the tests with tox and fix any problems you encounter.

  • Commit the changes to the branch.

  • Create pull request against master with your changes.