gh-131837: Clarify context variable lifetime in closures

[limadog9]

Explain why creating context variables in closures can extend their lifetimes: a reachable Context holds strong references to variables with values set in it. Repeated creation and binding in a long-lived context can accumulate variables.

Keep the recommendation to create variables at module level while describing the actual retention condition instead of saying they cannot be properly garbage collected.

Fixes #131837.

Validation:

• Used a weak-referenceable default value to confirm retention by a reachable context, collection after discarding the context, and collection after resetting the variable to its unset state.
• Full HTML documentation build with warnings treated as errors.
• Sphinx lint and patchcheck.

• Issue: Improve ContextVar in closures documentation #131837

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #35258704 | 📁 Comparing 20a4450 against main (6044c62)

  🔍 Preview build  

1 file changed

± library/contextvars.html