Understanding and Resolving Execution Hangs with `asyncio.Queue` and `TaskGroup` in Pytest

What will you learn?

In this comprehensive guide, you will delve into strategies to diagnose and resolve execution hangs or deadlocks encountered while utilizing asyncio.Queue and TaskGroup in pytest. By understanding the intricacies of async queues and task groups, you will be equipped to create reliable and deadlock-free asyncio-based tests.

Introduction to the Problem and Solution

When working with asynchronous Python code, particularly in pytest tests, encountering indefinite test suite hangs is not uncommon. These issues often stem from improper handling of async queues (asyncio.Queue) or task groups (TaskGroup). Tasks waiting endlessly for data that is never queued or mismanagement of async tasks within task groups can lead to deadlocks or execution hangs.

To tackle this challenge effectively, a dual-pronged approach is essential. Firstly, ensuring correct usage and management of asyncio.Queue objects within tests prevents tasks from being stuck indefinitely. Secondly, employing TaskGroups alongside async queues guarantees successful completion of all tasks without causing deadlocks. By mastering these concepts and implementing best practices, you can enhance the reliability of your asyncio-based tests significantly.


import asyncio
import pytest

async def test_async_queue_with_task_group():
    queue = asyncio.Queue()
    results = []

    async def producer():
        await queue.put("data")
        await queue.put(None)  # Sentinel value indicating completion

    async def consumer():
        while True:
            item = await queue.get()
            if item is None:
                break  # Exit loop if sentinel value is encountered

    async with asyncio.TaskGroup() as tg:

    assert results == ["data"]

# Copyright PHD


The provided solution demonstrates a straightforward pattern for integrating asyncio.Queue with TaskGroup in a pytest context. Here’s a breakdown:

  • Creating Producer-Consumer Tasks: Defining asynchronous functions for producer (putting data into the queue) and consumer (retrieving data). The producer includes a special “sentinel” value (None) to signal completion.
  • Using Task Groups: Utilizing an asyncio.TaskGroup to concurrently run producer and consumer tasks.
  • Handling Sentinel Values: The consumer loop breaks upon encountering the sentinel value, allowing both tasks to finish gracefully.
  • Testing Asynchronous Operations: The pytest function marked with @pytest.mark.asyncio manages Queue instantiation, task execution via Task Group within an ‘async with’ block to prevent deadlocks.

This approach ensures proper synchronization of asynchronous operations, preventing deadlocks caused by unhandled scenarios like empty queues awaiting entries indefinitely.

  1. How does an asyncio Queue facilitate communication between concurrent parts of an application?

  2. An asyncio Queue enables safe passing of items among different sections of an application running concurrently using coroutines.

  3. What role does a Task Group play in managing multiple concurrent async tasks?

  4. A Task Group simplifies handling multiple async tasks collectively, ensuring either all succeed or fail together while streamlining error management.

  5. Why are sentinel values necessary when working with Queues?

  6. Sentinel values serve as markers for completion or special states between asynchronous producers/consumers, crucial for avoiding infinite wait scenarios on queues.

  7. Can I have multiple consumers/producers working simultaneously?

  8. Yes! Depending on workload requirements, employing multiple consumers/producers is common practice; ensure proper synchronization using Queues/Events/etc., as needed.

  9. What typically causes deadlocks in asyncio applications?

  10. Deadlocks often result from incorrect task synchronization where one coroutine awaits resources held by another that also awaits resources held by the first one.

  11. Is leveraging the pytest-mark-asyncio plugin essential for testing asynchronous code?

  12. While not mandatory for all scenarios, the plugin offers valuable decorators simplifying testing of asynchronous code by automating event loop setup/teardown among other conveniences.


Mastering the integration of features like asyncio.Queue and Task Groups empowers developers creating modern Python applications´┐Żespecially those heavily reliant on concurrency´┐Żto sidestep common pitfalls such as deadlocks/hangs during testing phases. By focusing on synchronization mechanisms along strategic placement of sentinels in communication patterns even intricate workflows become manageable and resilient against typical concurrency challenges.

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