Expression

Expression is a practical, typed functional-programming library for Python 3.10+. It adds small, composable tools for common Python problems:

• transform data in readable pipelines;

• represent a value that may be absent with Option;

• return expected failures explicitly with Result;

• work with immutable collections and tagged unions; and

• compose optional, fallible, and asynchronous workflows.

You do not need prior functional-programming experience. Start with Getting started, then choose a workflow in Which type should I use?.

Important

Expression complements ordinary Python; it does not require a purely functional style. Use the parts that make a workflow clearer, and keep using familiar Python types and control flow where they are the best fit.

Start here

• Getting started
• Which type should I use?

Core workflows

• Compose data transformations
• Handle optional values with Option
• Model expected failures with Result
• Work with collections
• Model domain alternatives with tagged unions

Tutorials

• Tutorial: build a small data workflow
• Tutorial: choose a collection
• Tutorial: make absence explicit
• Tutorial: compose expected failures
• Tutorial: model domain alternatives
• Tutorial: write a linear fallible workflow

Advanced

• Use effect builders for short-circuiting workflows
• Concepts and terminology
• Advanced concept: functions and composition
• Advanced concept: callbacks and async code

API reference

• API reference

About this documentation

• Maintaining documentation