unnecessary-list-cast (PERF101)

Derived from the Perflint linter.

Fix is always available.

What it does

Checks for explicit casts to list on for-loop iterables.

Why is this bad?

Using a list() call to eagerly iterate over an already-iterable type (like a tuple, list, or set) is inefficient, as it forces Python to create a new list unnecessarily.

Removing the list() call will not change the behavior of the code, but may improve performance.

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

items = (1, 2, 3)
for i in list(items):
    print(i)

Use instead:

items = (1, 2, 3)
for i in items:
    print(i)

Fix safety

This rule's fix is marked as unsafe if there's comments in the list() call, as comments may be removed.

For example, the fix would be marked as unsafe in the following case:

items = (1, 2, 3)
for i in list(  # comment
    items
):
    print(i)