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Module 02: Coroutines and the Event Loop

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Stub module — Full content to be generated. This file defines the scope and prerequisites so the topic roadmap is complete.

Overview

This module is a deep dive into the internals of coroutines and the asyncio event loop. Module 01 introduced coroutines as "functions that can pause." This module explains the mechanism behind that: how coroutines are implemented on top of Python's generator protocol, how the event loop works as a scheduler, and how asyncio.run() relates to the lower-level loop.run_until_complete().

Understanding these internals makes you a better async programmer: you know what is happening under the hood when a coroutine suspends, why certain patterns are safe or unsafe, and how to diagnose problems that are invisible without this knowledge.

Table of Contents

  1. Overview
  2. Prerequisites
  3. Objectives
  4. Theory
  5. Key Concepts
  6. Examples
  7. Common Pitfalls
  8. Cross-Links
  9. Summary

Prerequisites

  • Module 01: Introduction to Async Python — coroutines, await, asyncio.run()
  • Python generators — yield and the generator protocol (send, throw, StopIteration)

Objectives

By the end of this module, you will be able to:

  1. Explain how coroutines are implemented using Python's generator protocol
  2. Describe the lifecycle of a coroutine: created → scheduled → running → suspended → done
  3. Use asyncio.get_event_loop(), loop.run_until_complete(), and explain when to use each vs. asyncio.run()
  4. Implement a custom awaitable by defining __await__
  5. Use asyncio.to_thread() to safely call blocking functions from async code
  6. Inspect the event loop using asyncio.current_task() and asyncio.all_tasks()

Theory

Full content to be generated. Topics to cover:

  • How coroutines extend the generator protocol (send(), throw(), close())
  • The coroutine object lifecycle: created, suspended, running, done
  • asyncio.run() vs loop.run_until_complete() vs loop.run_forever()
  • How the event loop uses selectors (epoll/kqueue/IOCP) to monitor I/O readiness
  • The asyncio.AbstractEventLoop interface and why there are multiple implementations
  • How asyncio.sleep(0) works as a yield-to-scheduler primitive
  • Custom awaitables: implementing __await__
  • Thread integration: run_in_executor() and asyncio.to_thread()

Key Concepts

To be filled in when this module is fully generated.

Examples

To be filled in when this module is fully generated.

Common Pitfalls

To be filled in when this module is fully generated.

  • [[async-python/modules/01_introduction]] — prerequisite; coroutines and asyncio.run()
  • [[async-python/modules/03_tasks-and-futures]] — next module; builds on event loop knowledge
  • [[async-python/modules/06_structured-concurrency]] — TaskGroup requires understanding the event loop lifecycle covered here

Summary

To be filled in when this module is fully generated.