crosswind

About

Crosswind is a tool and library that is looking to enable easy conversion of a codebase from Python 2 to 3 and back. The intention is to enable teams to write modern Python 3 even if they have to support older versions of their software still using Python 2. Crosswind also aims to provide a means to write arbitrary large-scale refactoring that can be applied to a codebase in much the same way as the 2to3 or 3to2 migration.

Crosswind is a fork of the lib3to2 project that can be found on Bitbucket and PyPi. The source history has been ported from Mercurial to Git using hg-fast-export.

lib2to3 was also ported over from the Python project.

Objectives/Goals

The following are the overarching objectives of crosswind. They are subject to change as a game plan is established. For now, they are:

• Support only Python 2.7 for backporting 3 to 2
• Support upgrading to 3.6 and above whem migrating 2 to 3
• Running all tooling is only supported on 3.6 and up
• Reach and maintain parity between the two_to_three and three_to_two fixer suites
• Consistently pull in upstream updates for both lib2to3 and lib3to2 when it aligns with the above goals
• Investigate/resolve open issues on lib3to2

Contributing

If you're looking for ways to contribute there are a few options to choose from:

• Helping write wiki pages with more knowledge of all facets of writing fixers or tests or understanding the pgen/lib2to3 libraries/APIs
• Try out Crosswind on your codebase and open issues for things you suspect aren't coming out quite right (please include code samples!)
• Submit a pull request with a fix or enhancement (it would be ideal to open an issue first for discussion)

You can start by taking a look at the Writing a Fixer wiki page to learn about what goes into the fixer and its tests.

Development

Crosswind uses the Poetry tool for managing dependencies and virtual environments. Common development tasks have aliases that have been collected in a Makefile at the root of the project.

To get started with crosswind development, run make install. This will use Poetry to create the virtualenv and install the dependencies (both runtime and dev-time). Note that you need your currently active python to be compatible with the versions in the pyproject.toml file. If that isn't the case, you'll have to manage the virtualenv creation yourself. For example, by executing the following in your checkout of Crosswind:

python3 -m virtualenv .venv

If you don't yet have virtualenv available you can get it via pip: python3 -m pip install virtualenv. Once you have the virtualenv created you will need to activate it before continuing.

WIth the virtualenv created you can then proceed to make install to have Poetry download and install dependencies into your activated venv.

Then you can lint the code and run the tests with make lint and make tests or combine them (since they're separate targets and make allows specifying many targets in a single invocation) with make lint tests and so on.

Currently, running crosswind requires a manual invocation of poetry such as:

poetry run python crosswind/crosswind --help

This will show the help output containing the currently supported options. It should be noted that the current state of the crosswind tool is that it only has access to the 2to3 fixers. Further efforts are needed to allow it to combine arbitrary fixers and fixer suites.

Configuration

In addition to passing flags at the command line (issue --help to crosswind for available flags) you can configure crosswind by defining such configuration in pyproject.toml under a tool.crosswind heading such as:

[tool.crosswind]
output_dir = "path/to/output"

Additionally, presets can be defined by adding .preset.<name> as an additional heading like:

[tool.crosswind.preset.foo]
output_dir = "foo/specific/path"

If a preset is specified it will exclusively use that preset's configuration and will not merge it with the default configuration. Command line flags are still merged into the preset configuration however.

Thanks/Inspiration

Thanks to Joseph Amenta for developing lib3to2 and making it open source. This effort wouldn't be possible without it!

Also a huge thank you to everyone who has contribued to Python, pgen, and lib2to3. Having such a foundation to start from has made things immeasurably easier and far more productive!

And finally a shoutout to contributors of both futurize and drop2 which together served as the basis of the defuturize fixer suite.