Automatically Enforce Software Structures With Powerful Code Modifications Powered By LibCST


April 25th, 2022

56 mins 47 secs

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About this Episode


Programmers love to automate tedious processes, including refactoring your code. In order to support the creation of code modifications for your Python projects Jimmy Lai created LibCST. It provides a richly typed and high level API for creating and manipulating concrete syntax trees of your source code. In this episode Jimmy Lai and Zsolt Dollenstein explain how it works, some of the linting and automatic code modification utilities that you can build with it and how to get started with using it to maintain your own Python projects.


  • Hello and welcome to Podcast.__init__, the podcast about Python’s role in data and science.
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  • Your host as usual is Tobias Macey and today I’m interviewing Zsolt Dollenstein and Jimmy Lai about LibCST, a concrete syntax tree parser and serializer library for Python


  • Introductions
  • How did you get introduced to Python?
  • Can you describe what LibCST is and the story behind it?
  • How does a concrete syntax tree differ from an abstract syntax tree?
    • What are some of the situations where the preservation of the exact structure is necessary?
  • There are a few other libraries in Python for creating concrete syntax trees. What was missing in the available options that made it necessary to create LibCST?
  • What are the use cases that LibCST is focused on supporting
  • Can you describe how LibCST is implemented?
    • How have the design and goals of the project changed or evolved since you started working on it?
  • How might I use LibCST for something like restructuring a set of modules to move a function definition while maintaining proper imports?
    • How do the capabilities of LibCST for codemodding compare to the Rope framework?
  • What are some other workflows that someone might build with LibCST?
  • What are some of the ways that LibCST is being used in your own work?
  • What are the most interesting, innovative, or unexpected ways that you have seen LibCST used?
  • What are the most interesting, unexpected, or challenging lessons that you have learned while working on LibCST?
  • When is LibCST the wrong choice?
  • What do you have planned for the future of LibCST?

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The intro and outro music is from Requiem for a Fish The Freak Fandango Orchestra / CC BY-SA