manual-dict-comprehension (PERF403)

Derived from the Perflint linter.

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

What it does

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

Why is this bad?

When creating or extending a dictionary in a for-loop, prefer a dictionary comprehension. Comprehensions are more readable and more performant.

For example, when comparing {x: x for x in list(range(1000))} to the for loop version, the comprehension is ~10% faster on Python 3.11.

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.

Example

pairs = (("a", 1), ("b", 2))
result = {}
for x, y in pairs:
    if y % 2:
        result[x] = y

Use instead:

pairs = (("a", 1), ("b", 2))
result = {x: y for x, y in pairs if y % 2}

If you're appending to an existing dictionary, use the update method instead:

pairs = (("a", 1), ("b", 2))
result.update({x: y for x, y in pairs if y % 2})