ruff/crates/ruff_python_formatter

Rust Python Formatter

The goal of our formatter is to be compatible with Black except for rare edge cases (mostly involving comment placement).

Implementing a node

Formatting each node follows roughly the same structure. We start with a Format{{Node}} struct that implements Default (and AsFormat/IntoFormat impls in generated.rs, see orphan rules below).

#[derive(Default)]
pub struct FormatStmtReturn;

We implement FormatNodeRule<{{Node}}> for Format{{Node}}. Inside, we destructure the item to make sure we're not missing any field. If we want to write multiple items, we use an efficient write! call, for single items .format().fmt(f) or .fmt(f) is sufficient.

impl FormatNodeRule<StmtReturn> for FormatStmtReturn {
    fn fmt_fields(&self, item: &StmtReturn, f: &mut PyFormatter) -> FormatResult<()> {
        // Here we destructure item and make sure each field is listed.
        // We generally don't need range is it's underscore-ignored
        let StmtReturn { range: _, value } = item;
        // Implement some formatting logic, in this case no space (and no value) after a return with
        // no value
        if let Some(value) = value {
            write!(
                f,
                [
                    text("return"),
                    // There are multiple different space and newline types (e.g.
                    // `soft_line_break_or_space()`, check the builders module), this one will
                    // always be translate to a normal ascii whitespace character
                    space(),
                    // `return a, b` is valid, but if it wraps we'd need parentheses.
                    // This is different from `(a, b).count(1)` where the parentheses around the
                    // tuple are mandatory
                    value.format().with_options(Parenthesize::IfBreaks)
                ]
            )
        } else {
            text("return").fmt(f)
        }
    }
}

Check the builders module for the primitives that you can use.

If something such as list or a tuple can break into multiple lines if it is too long for a single line, wrap it into a group. Ignoring comments, we could format a tuple with two items like this:

write!(
    f,
    [group(&format_args![
        text("("),
        soft_block_indent(&format_args![
            item1.format()
            text(","),
            soft_line_break_or_space(),
            item2.format(),
            if_group_breaks(&text(","))
        ]),
        text(")")
    ])]
)

If everything fits on a single line, the group doesn't break and we get something like ("a", "b"). If it doesn't, we get something like

(
    "a",
    "b",
)

For a list of expression, you don't need to format it manually but can use the JoinBuilder util, accessible through .join_with. Finish will write to the formatter internally.

f.join_with(&format_args!(text(","), soft_line_break_or_space()))
    .entries(self.elts.iter().formatted())
    .finish()?;
// Here we need a trailing comma on the last entry of an expanded group since we have more
// than one element
write!(f, [if_group_breaks(&text(","))])

If you need avoid second mutable borrows with a builder, you can use format_with(|f| { ... }) as a formattable element similar to text() or group().

Comments

Comments can either be own line or end-of-line and can be marked as Leading, Trailing and Dangling.

# Leading comment (always own line)
print("hello world")  # Trailing comment (end-of-line)
# Trailing comment (own line)

Comments are automatically attached as Leading or Trailing to a node close to them, or Dangling if there are only tokens and no nodes surrounding it. Categorization is automatic but sometimes needs to be overridden in place_comment in placement.rs, which this section is about.

[
    # This needs to be handled as a dangling comment
]

Here, the comment is dangling because it is preceded by [, which is a non-trivia token but not a node, and followed by ], which is also a non-trivia token but not a node. In the FormatExprList implementation, we have to call dangling_comments manually and stub out the fmt_dangling_comments default from FormatNodeRule.

impl FormatNodeRule<ExprList> for FormatExprList {
    fn fmt_fields(&self, item: &ExprList, f: &mut PyFormatter) -> FormatResult<()> {
        // ...

        write!(
            f,
            [group(&format_args![
                text("["),
                dangling_comments(dangling), // Gets all the comments marked as dangling for the node
                soft_block_indent(&items),
                text("]")
            ])]
        )
    }

    fn fmt_dangling_comments(&self, _node: &ExprList, _f: &mut PyFormatter) -> FormatResult<()> {
        // Handled as part of `fmt_fields`
        Ok(())
    }
}

A related common challenge is that we want to attach comments to tokens (think keywords and syntactically meaningful characters such as :) that have no node on their own. A slightly simplified version of the while node in our AST looks like the following:

pub struct StmtWhile {
    pub range: TextRange,
    pub test: Box<Expr<TextRange>>,
    pub body: Vec<Stmt<TextRange>>,
    pub orelse: Vec<Stmt<TextRange>>,
}

That means in

while True:  # Trailing condition comment
    if f():
        break
    # trailing while comment
# leading else comment
else:
    print("while-else")

the else has no node, we're just getting the statements in its body.

The preceding token of the leading else comment is the break, which has a node, the following token is the else, which lacks a node, so by default the comment would be marked as trailing the break and wrongly formatted as such. We can identify these cases by looking for comments between two bodies that have the same indentation level as the keyword, e.g. in our case the leading else comment is inside the while node (which spans the entire snippet) and on the same level as the else. We identify those case in handle_in_between_bodies_own_line_comment and mark them as dangling for manual formatting later. Similarly, we find and mark comment after the colon(s) in handle_trailing_end_of_line_condition_comment .

The comments don't carry any extra information such as why we marked the comment as trailing, instead they are sorted into one list of leading, one list of trailing and one list of dangling comments per node. In FormatStmtWhile, we can have multiple types of dangling comments, so we have to split the dangling list into after-colon-comments, before-else-comments, etc. by some element separating them (e.g. all comments trailing the colon come before the first statement in the body) and manually insert them in the right position.

A simplified implementation with only those two kinds of comments:

fn fmt_fields(&self, item: &StmtWhile, f: &mut PyFormatter) -> FormatResult<()> {

    // ...

    // See FormatStmtWhile for the real, more complex implementation
    let first_while_body_stmt = item.body.first().unwrap().end();
    let trailing_condition_comments_end =
        dangling_comments.partition_point(|comment| comment.slice().end() < first_while_body_stmt);
    let (trailing_condition_comments, or_else_comments) =
        dangling_comments.split_at(trailing_condition_comments_end);

    write!(
        f,
        [
            text("while"),
            space(),
            test.format(),
            text(":"),
            trailing_comments(trailing_condition_comments),
            block_indent(&body.format())
            leading_comments(or_else_comments),
            text("else:"),
            block_indent(&orelse.format())
        ]
    )?;
}

Development notes

Handling parentheses and comments are two major challenges in a Python formatter.

We have copied the majority of tests over from Black and use insta for snapshot testing with the diff between Ruff and Black, Black output and Ruff output. We put additional test cases in resources/test/fixtures/ruff.

The full Ruff test suite is slow, cargo test -p ruff_python_formatter is a lot faster.

There is a ruff_python_formatter binary that avoid building and linking the main ruff crate.

You can use scratch.py as a playground, e.g. cargo run --bin ruff_python_formatter -- --emit stdout scratch.py, which additional --print-ir and --print-comments options.

The origin of Ruff's formatter is the Rome formatter, e.g. the ruff_formatter crate is forked from the rome_formatter crate. The Rome repository can be a helpful reference when implementing something in the Ruff formatter

Checking formatter stability and panics

There are tree common problems with the formatter: The second formatting pass looks different than the first (formatter instability or lack of idempotency), we print invalid syntax (e.g. missing parentheses around multiline expressions) and panics (mostly in debug assertions). We test for all of these using the check-formatter-stability subcommand in ruff_dev

The easiest is to check CPython:

git clone --branch 3.10 https://github.com/python/cpython.git crates/ruff/resources/test/cpython
cargo run --bin ruff_dev -- check-formatter-stability crates/ruff/resources/test/cpython

It is also possible large number of repositories using ruff. This dataset is large (~60GB), so we only do this occasionally:

curl https://raw.githubusercontent.com/akx/ruff-usage-aggregate/master/data/known-github-tomls.jsonl > github_search.jsonl
python scripts/check_ecosystem.py --checkouts target/checkouts --projects github_search.jsonl -v $(which true) $(which true)
cargo run --bin ruff_dev -- check-formatter-stability --multi-project target/checkouts

The orphan rules and trait structure

For the formatter, we would like to implement Format from the rust_formatter crate for all AST nodes, defined in the rustpython_parser crate. This violates Rust's orphan rules. We therefore generate in generate.py a newtype for each AST node with implementations of FormatNodeRule, FormatRule, AsFormat and IntoFormat on it.