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manual-list-comprehension (PERF401)#

Derived from the Perflint linter.

What it does#

Checks for for loops that can be replaced by a list comprehension.

Why is this bad?#

When creating a transformed list from an existing list using a for-loop, prefer a list comprehension. List comprehensions are more readable and more performant.

Using the below as an example, the list comprehension is ~10% faster on Python 3.11, and ~25% faster on Python 3.10.

Note that, as with all perflint rules, this is only intended as a micro-optimization, and will have a negligible impact on performance in most cases.


original = list(range(10000))
filtered = []
for i in original:
    if i % 2:

Use instead:

original = list(range(10000))
filtered = [x for x in original if x % 2]

If you're appending to an existing list, use the extend method instead:

original = list(range(10000))
filtered.extend(x for x in original if x % 2)