ruff/docs/rules/unnecessary-map.md

unnecessary-map (C417)

Derived from the flake8-comprehensions linter.

Autofix is sometimes available.

What it does

Checks for unnecessary map calls with lambda functions.

Why is this bad?

Using map(func, iterable) when func is a lambda is slower than using a generator expression or a comprehension, as the latter approach avoids the function call overhead, in addition to being more readable.

Examples

map(lambda x: x + 1, iterable)

Use instead:

(x + 1 for x in iterable)

This rule also applies to map calls within list, set, and dict calls. For example:

• Instead of list(map(lambda num: num * 2, nums)), use [num * 2 for num in nums].
• Instead of set(map(lambda num: num % 2 == 0, nums)), use {num % 2 == 0 for num in nums}.
• Instead of dict(map(lambda v: (v, v ** 2), values)), use {v: v ** 2 for v in values}.