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Safe code refactoring for modern Python projects.

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Current Development Status

Bowler 0.x is based on fissix (a fork of lib2to3) which was never intended to be a stable api. We've pretty much reached the limit of being able to add new language features, so while we can support 3.8's walrus, that's basically the last it's going to get.

See discussion for f(**not x) for one example -- a proper fix for things like this would mean invalidating all current patterns since there's no way to match against a specific revision of the grammar.

If you need to do codemods today, we recommend looking at LibCST codemods which are a bit more verbose, but work well on modern python grammars. We have contributed support for Python 3.0-3.3 grammars and have a draft PR going even further back to 2.3.

The longer term plan for Bowler is to build Bowler 2.x on top of libcst's parser, while still supporting a very simple fluent api. That's unlikely to materialize in a final release during 2021.


Bowler is a refactoring tool for manipulating Python at the syntax tree level. It enables safe, large scale code modifications while guaranteeing that the resulting code compiles and runs. It provides both a simple command line interface and a fluent API in Python for generating complex code modifications in code.

Bowler uses a "fluent" Query API to build refactoring scripts through a series of selectors, filters, and modifiers. Many simple modifications are already possible using the existing API, but you can also provide custom selectors, filters, and modifiers as needed to build more complex or custom refactorings. See the Query Reference for more details.

Using the query API to rename a single function, and generate an interactive diff from the results, would look something like this:

query = (
    Query(<paths to modify>)

For more details or documentation, check out

Installing Bowler

Bowler supports modifications to code from any version of Python 2 or 3, but it requires Python 3.6 or higher to run. Bowler can be easily installed using most common Python packaging tools. We recommend installing the latest stable release from PyPI with pip:

pip install bowler

You can also install a development version from source by checking out the Git repo:

git clone
cd bowler
python install


Bowler is MIT licensed, as found in the LICENSE file.