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quadratic-list-summation (RUF017)#

Fix is always available.

What it does#

Checks for the use of sum() to flatten lists of lists, which has quadratic complexity.

Why is this bad?#

The use of sum() to flatten lists of lists is quadratic in the number of lists, as sum() creates a new list for each element in the summation.

Instead, consider using another method of flattening lists to avoid quadratic complexity. The following methods are all linear in the number of lists:

  • functools.reduce(operator.iadd, lists, [])
  • list(itertools.chain.from_iterable(lists))
  • [item for sublist in lists for item in sublist]

When fixing relevant violations, Ruff defaults to the functools.reduce form, which outperforms the other methods in microbenchmarks.


lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
joined = sum(lists, [])

Use instead:

import functools
import operator

lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
functools.reduce(operator.iadd, lists, [])