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literal-membership (PLR6201)

Preview (since v0.1.1) · Related issues · View source

Derived from the Pylint linter.

Fix is always available.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for membership tests on list and tuple literals.

Why is this bad?

When testing for membership in a static sequence, prefer a set literal over a list or tuple, as Python optimizes set membership tests.

Example

1 in [1, 2, 3]

Use instead:

1 in {1, 2, 3}

Fix safety

This rule's fix is marked as unsafe, as the use of a set literal will error at runtime if either the element being tested for membership (the left-hand side) or any element of the sequence (the right-hand side) is unhashable (like lists or dictionaries). While Ruff will attempt to infer the hashability of both sides and skip the fix when it can determine that either side is unhashable, it may not always be able to do so.

References