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gh-131837: Clarify context variable lifetime in closures #159849

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gh-131837: Clarify context variable lifetime in closures #159849

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11 changes: 7 additions & 4 deletions Doc/library/contextvars.rst
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Expand Up @@ -37,10 +37,13 @@ Context Variables
:meth:`ContextVar.get` when no value for the variable is found
in the current context.

**Important:** Context Variables should be created at the top module
level and never in closures. :class:`Context` objects hold strong
references to context variables which prevents context variables
from being properly garbage collected.
**Important:** Prefer creating context variables at module level rather
than in closures. A :class:`Context` holds strong references to the
variables that have values set in it. A variable created in a closure can
therefore remain alive after the closure returns, for as long as it has a
value in a reachable context. Repeatedly creating and setting new variables
in a long-lived context can accumulate variables that would otherwise be
garbage collected.

:class:`!ContextVar`\s are :ref:`generic <generics>` over the type of
their contained value.
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