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modified-iterating-set (PLE4703)

Preview (since v0.3.5) · 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 loops in which a set is modified during iteration.

Why is this bad?

If a set is modified during iteration, it will cause a RuntimeError.

If you need to modify a set within a loop, consider iterating over a copy of the set instead.

Known problems

This rule favors false negatives over false positives. Specifically, it will only detect variables that can be inferred to be a set type based on local type inference, and will only detect modifications that are made directly on the variable itself (e.g., set.add()), as opposed to modifications within other function calls (e.g., some_function(set)).

Example

nums = {1, 2, 3}
for num in nums:
    nums.add(num + 5)

Use instead:

nums = {1, 2, 3}
for num in nums.copy():
    nums.add(num + 5)

Fix safety

This fix is always unsafe because it changes the program’s behavior. Replacing the original set with a copy during iteration allows code that would previously raise a RuntimeError to run without error.

References