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collection-literal-concatenation (RUF005)

Added in v0.0.227 · Related issues · View source

Fix is sometimes available.

What it does

Checks for uses of the + operator to concatenate collections.

Why is this bad?

In Python, the + operator can be used to concatenate collections (e.g., x + y to concatenate the lists x and y).

However, collections can be concatenated more efficiently using the unpacking operator (e.g., [*x, *y] to concatenate x and y).

Prefer the unpacking operator to concatenate collections, as it is more readable and flexible. The * operator can unpack any iterable, whereas + operates only on particular sequences which, in many cases, must be of the same type.

Example

foo = [2, 3, 4]
bar = [1] + foo + [5, 6]

Use instead:

foo = [2, 3, 4]
bar = [1, *foo, 5, 6]

Fix safety

The fix is always marked as unsafe because the + operator uses the __add__ magic method and *-unpacking uses the __iter__ magic method. Both of these could have custom implementations, causing the fix to change program behaviour.

References