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mutable-contextvar-default (B039)

Derived from the flake8-bugbear linter.

What it does

Checks for uses of mutable objects as ContextVar defaults.

Why is this bad?

The ContextVar default is evaluated once, when the ContextVar is defined.

The same mutable object is then shared across all .get() method calls to the ContextVar. If the object is modified, those modifications will persist across calls, which can lead to unexpected behavior.

Instead, prefer to use immutable data structures. Alternatively, take None as a default, and initialize a new mutable object inside for each call using the .set() method.

Types outside the standard library can be marked as immutable with the lint.flake8-bugbear.extend-immutable-calls configuration option.

Example

from contextvars import ContextVar


cv: ContextVar[list] = ContextVar("cv", default=[])

Use instead:

from contextvars import ContextVar


cv: ContextVar[list | None] = ContextVar("cv", default=None)

...

if cv.get() is None:
    cv.set([])

Options

References