unsorted-dunder-slots (RUF023)

Added in 0.8.0 · Related issues · View source

Fix is sometimes available.

What it does

Checks for __slots__ definitions that are not ordered according to a natural sort.

Why is this bad?

Consistency is good. Use a common convention for this special variable to make your code more readable and idiomatic.

Example

class Dog:
    __slots__ = "name", "breed"

Use instead:

class Dog:
    __slots__ = "breed", "name"

Fix safety

This rule's fix is marked as unsafe in three situations.

Firstly, the fix is unsafe if there are any comments that take up a whole line by themselves inside the __slots__ definition, for example:

class Foo:
    __slots__ = [
        # eggy things
        "duck_eggs",
        "chicken_eggs",
        # hammy things
        "country_ham",
        "parma_ham",
    ]

This is a common pattern used to delimit categories within a class's slots, but it would be out of the scope of this rule to attempt to maintain these categories when applying a natural sort to the items of __slots__.

Secondly, the fix is also marked as unsafe if there are more than two __slots__ items on a single line and that line also has a trailing comment, since here it is impossible to accurately gauge which item the comment should be moved with when sorting __slots__:

class Bar:
    __slots__ = [
        "a", "c", "e",  # a comment
        "b", "d", "f",  # a second  comment
    ]

Lastly, this rule's fix is marked as unsafe whenever Ruff can detect that code elsewhere in the same file reads the __slots__ variable in some way and the __slots__ variable is not assigned to a set. This is because the order of the items in __slots__ may have semantic significance if the __slots__ of a class is being iterated over, or being assigned to another value.

In the vast majority of other cases, this rule's fix is unlikely to cause breakage; as such, Ruff will otherwise mark this rule's fix as safe. However, note that (although it's rare) the value of __slots__ could still be read by code outside of the module in which the __slots__ definition occurs, in which case this rule's fix could theoretically cause breakage.