Introduction

What is Bowler?

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 is built on lib2to3 from the standard Python library, and requires only a few third party dependencies, like click, for common components.

Why lib2to3

lib2to3 provides a concrete syntax tree (CST) implementation that recognizes and supports the grammar of all Python versions back to 2.6. By nature of being a CST, lib2to3 enables modifications to the syntax tree while maintaining all formatting and comments, preventing modifications from destroying valuable information.

By building on lib2to3, Bowler is capable of reading and modifying source files written for both Python 2 and 3. That said, Bowler requires Python 3.6 or newer to run, as it uses a large number of modern features of the language to enable more readable and maintainable code.

Technical detail: Bowler actually uses fissix, a backport of lib2to3 that includes a few minor improvements and features that have not yet been upstreamed to CPython. This allows Bowler to parse grammars and utilize features from newer versions of Python than might otherwise be supported by the local runtime.