Boost Python Readability: Inside the Powerful PrettyPrinter Library

PrettyPrinter is a powerful, syntax‑highlighting pretty‑print package for Python 3.6 and newer, built on an improved Wadler‑Leijen layout algorithm and inspired by Haskell’s prettyprinter, JavaScript’s Prettier, Ruby’s prettypreinter, and IPython’s pretty module.

Why Python Needs an Extra Pretty‑Print Package

Printing data to the console is the most basic interface for developers, and improving it enhances development experience and productivity. Python’s built‑in __repr__ and __str__ methods return plain strings without formatting. The standard pprint module formats built‑in containers but uses a greedy layout algorithm and cannot handle custom types well. Third‑party pprintpp improves on pprint but shares the same limitations. IPython’s IPython.lib.pretty offers a more advanced alternative, yet it requires understanding the layout algorithm and can be disrupted by invalid syntax.

Key Improvements Introduced by PrettyPrinter

An algorithm that maximizes pretty‑printing coverage, even at a modest performance cost; a tenth of a second spent formatting can save seconds of manual data searching.

A super‑simple, descriptive API for user‑defined pretty‑printing, avoiding the need to rewrite __repr__ for most classes.

Robust syntax highlighting that gracefully handles invalid Python syntax without breaking.

Simple Descriptive API

The core functions are pretty_call for describing function‑call style output and register_pretty for attaching custom formatters to classes. Below is an example of a pretty_call invocation:

The processing flow of PrettyPrinter can be illustrated as:

Using register_pretty, a custom formatter for MyClass can be defined:

The resulting output demonstrates clear, colored formatting.

Stateful Instance Representation

PrettyPrinter supports explanatory comments to embed state information directly in the output, allowing developers to display additional context for objects such as a Connection class with “connected” and “disconnected” states.

Defining the formatter yields the illustrated output:

Conclusion

PrettyPrinter greatly enhances the developer experience by providing fast, high‑quality pretty‑printing with an easy‑to‑use API, especially within IPython where all results are automatically formatted. Documentation and source code are available on GitHub and Read the Docs, and the library includes ready‑made definitions for Django models, QuerySets, and attrs‑based classes.

Original English article: https://tommikaikkonen.github.io/introducing-prettyprinter-for-python/