https://textual.textualize.io/blog/2023/02/11/the-heisenbug-lurking-in-your-async-code/

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The Heisenbug lurking in your async code
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Will McGugan Will McGugan
CEO / code-monkey

  * Metadata
  * February 11, 2023
  * in DevLog
  * 2 min read

The Heisenbug lurking in your async codeP

I'm taking a brief break from blogging about Textual to bring you
this brief PSA for Python developers who work with async code. I
wanted to expand a little on this tweet.

If you have ever used asyncio.create_task you may have created a bug
for yourself that is challenging (read almost impossible) to
reproduce. If it occurs, your code will likely fail in unpredictable
ways.

The root cause of this Heisenbug is that if you don't hold a
reference to the task object returned by create_task then the task
may disappear without warning when Python runs garbage collection. In
other words the code in your task will stop running with no obvious
indication why.

This behavior is well documented, as you can see from this excerpt
(emphasis mine):

create task

But who reads all the docs? And who has perfect recall if they do? A
search on GitHub indicates that there are a lot of projects where
this bug is waiting for just the right moment to ruin somebody's day.

I suspect the reason this mistake is so common is that tasks are a
lot like threads (conceptually at least). With threads you can just
launch them and forget. Unless you mark them as "daemon" threads they
will exist for the lifetime of your app. Not so with Tasks.

The solution recommended in the docs is to keep a reference to the
task for as long as you need it to live. On modern Python you could
use TaskGroups which will keep references to your tasks. As long as
all the tasks you spin up are in TaskGroups, you should be fine.

 
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