Keep lambda parameters on one line and parenthesize the body if it expands (#21385)

## Summary

This PR makes two changes to our formatting of `lambda` expressions:
1. We now parenthesize the body expression if it expands
2. We now try to keep the parameters on a single line

The latter of these fixes #8179:

Black formatting and this PR's formatting:

```py
def a():
    return b(
        c,
        d,
        e,
        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
            *args, **kwargs
        ),
    )
```

Stable Ruff formatting

```py
def a():
    return b(
        c,
        d,
        e,
        f=lambda self,
        *args,
        **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs),
    )
```

We don't parenthesize the body expression here because the call to
`aaaa...` has its own parentheses, but adding a binary operator shows
the new parenthesization:

```diff
@@ -3,7 +3,7 @@
         c,
         d,
         e,
-        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
-            *args, **kwargs
-        ) + 1,
+        f=lambda self, *args, **kwargs: (
+            aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1
+        ),
     )
```

This is actually a new divergence from Black, which formats this input
like this:

```py
def a():
    return b(
        c,
        d,
        e,
        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
            *args, **kwargs
        )
        + 1,
    )
```

But I think this is an improvement, unlike the case from #8179.

One other, smaller benefit is that because we now add parentheses to
lambda bodies, we also remove redundant parentheses:

```diff
 @pytest.mark.parametrize(
     "f",
     [
-        lambda x: (x.expanding(min_periods=5).cov(x, pairwise=True)),
-        lambda x: (x.expanding(min_periods=5).corr(x, pairwise=True)),
+        lambda x: x.expanding(min_periods=5).cov(x, pairwise=True),
+        lambda x: x.expanding(min_periods=5).corr(x, pairwise=True),
     ],
 )
 def test_moment_functions_zero_length_pairwise(f):
```

## Test Plan

New tests taken from #8465 and probably a few more I should grab from
the ecosystem results.

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/lambda.py

@@ -125,6 +125,13 @@ lambda a, /, c: a
     *x: x
 )
 
+(
+    lambda
+    # comment
+    *x,
+    **y: x
+)
+
 (
     lambda
     # comment 1
@@ -196,6 +203,17 @@ lambda: ( # comment
     x
 )
 
+(
+    lambda  # 1
+    # 2
+    x,  # 3
+    # 4
+    y
+    :  # 5
+    # 6
+    x
+)
+
 (
     lambda
     x,
@@ -204,6 +222,71 @@ lambda: ( # comment
     z
 )
 
+
+# Leading
+lambda x: (
+    lambda y: lambda z: x
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + y
+                        + z  # Trailing
+)  # Trailing
+
+
+# Leading
+lambda x: lambda y: lambda z: [
+    x,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    y,
+    z
+] # Trailing
+# Trailing
+
 lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs), e=1, f=2, g=2: d
 
 # Regression tests for https://github.com/astral-sh/ruff/issues/8179
@@ -228,6 +311,441 @@ def a():
         g = 10
     )
 
+def a():
+    return b(
+        c,
+        d,
+        e,
+        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
+            *args, **kwargs
+        ) + 1,
+    )
+
+# Additional ecosystem cases from https://github.com/astral-sh/ruff/pull/21385
+class C:
+    def foo():
+        mock_service.return_value.bucket.side_effect = lambda name: (
+            source_bucket
+            if name == source_bucket_name
+            else storage.Bucket(mock_service, destination_bucket_name)
+        )
+
+class C:
+	function_dict: Dict[Text, Callable[[CRFToken], Any]] = {
+        CRFEntityExtractorOptions.POS2: lambda crf_token: crf_token.pos_tag[:2]
+        if crf_token.pos_tag is not None
+        else None,
+	}
+
+name = re.sub(r"[^\x21\x23-\x5b\x5d-\x7e]...............", lambda m: f"\\{m.group(0)}", p["name"])
+
+def foo():
+    if True:
+        if True:
+            return (
+                lambda x: np.exp(cs(np.log(x.to(u.MeV).value))) * u.MeV * u.cm**2 / u.g
+            )
+
+class C:
+    _is_recognized_dtype: Callable[[DtypeObj], bool] = lambda x: lib.is_np_dtype(
+        x, "M"
+    ) or isinstance(x, DatetimeTZDtype)
+
+class C:
+    def foo():
+        if True:
+            transaction_count = self._query_txs_for_range(
+                get_count_fn=lambda from_ts, to_ts, _chain_id=chain_id: db_evmtx.count_transactions_in_range(
+                    chain_id=_chain_id,
+                    from_ts=from_ts,
+                    to_ts=to_ts,
+                ),
+            )
+
+            transaction_count = self._query_txs_for_range(
+                get_count_fn=lambda from_ts, to_ts, _chain_id=chain_id: db_evmtx.count_transactions_in_range[_chain_id, from_ts, to_ts],
+            )
+
+def ddb():
+    sql = (
+        lambda var, table, n=N: f"""
+        CREATE TABLE {table} AS
+        SELECT ROW_NUMBER() OVER () AS id, {var}
+        FROM (
+            SELECT {var}
+            FROM RANGE({n}) _ ({var})
+            ORDER BY RANDOM()
+        )
+        """
+    )
+
+long_assignment_target.with_attribute.and_a_slice[with_an_index] = ( # 1
+    # 2
+    lambda x, y, z: # 3
+    # 4
+    x + y + z # 5
+    # 6
+)
+
+long_assignment_target.with_attribute.and_a_slice[with_an_index] = (
+    lambda x, y, z: x + y + z
+)
+
+long_assignment_target.with_attribute.and_a_slice[with_an_index] = lambda x, y, z: x + y + z
+
+very_long_variable_name_x, very_long_variable_name_y = lambda a: a + some_very_long_expression, lambda b: b * another_very_long_expression_here
+
+very_long_variable_name_for_result += lambda x: very_long_function_call_that_should_definitely_be_parenthesized_now(x, more_args, additional_parameters)
+
+
+if 1:
+    if 2:
+        if 3:
+            if self.location in EVM_EVMLIKE_LOCATIONS and database is not None:
+                exported_dict["notes"] = EVM_ADDRESS_REGEX.sub(
+                    repl=lambda matched_address: self._maybe_add_label_with_address(
+                        database=database,
+                        matched_address=matched_address,
+                    ),
+                    string=exported_dict["notes"],
+                )
+
+class C:
+    def f():
+        return dict(
+            filter(
+                lambda intent_response: self.is_retrieval_intent_response(
+                    intent_response
+                ),
+                self.responses.items(),
+            )
+        )
+
+@pytest.mark.parametrize(
+    "op",
+    [
+        # Not fluent
+        param(
+            lambda left, right: (
+                ibis.timestamp("2017-04-01")
+            ),
+        ),
+        # These four are fluent and fit on one line inside the parenthesized
+        # lambda body
+        param(
+            lambda left, right: (
+                ibis.timestamp("2017-04-01").cast(dt.date)
+            ),
+        ),
+        param(
+            lambda left, right: (
+                ibis.timestamp("2017-04-01").cast(dt.date).between(left, right)
+            ),
+        ),
+        param(lambda left, right: ibis.timestamp("2017-04-01").cast(dt.date)),
+        param(lambda left, right: ibis.timestamp("2017-04-01").cast(dt.date).between(left, right)),
+        # This is too long on one line in the lambda body and gets wrapped
+        # inside the body.
+        param(
+            lambda left, right: (
+                ibis.timestamp("2017-04-01").cast(dt.date).between(left, right).between(left, right)
+            ),
+        ),
+    ],
+)
+def test_string_temporal_compare_between(con, op, left, right): ...
+
+[
+    (
+        lambda eval_df, _: MetricValue(
+            scores=eval_df["prediction"].tolist(),
+            aggregate_results={"prediction_sum": sum(eval_df["prediction"])},
+        )
+    ),
+]
+
+# reuses the list parentheses
+lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz: [xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz]
+
+# adds parentheses around the body
+lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz: xxxxxxxxxxxxxxxxxxxx + yyyyyyyyyyyyyyyyyyyy + zzzzzzzzzzzzzzzzzzzz
+
+# removes parentheses around the body
+lambda xxxxxxxxxxxxxxxxxxxx: (xxxxxxxxxxxxxxxxxxxx + 1)
+
+mapper = lambda x: dict_with_default[np.nan if isinstance(x, float) and np.isnan(x) else x]
+
+lambda x, y, z: (
+    x + y + z
+)
+
+lambda x, y, z: (
+    x + y + z
+    # trailing body
+)
+
+lambda x, y, z: (
+    x + y + z  # trailing eol body
+)
+
+lambda x, y, z: (
+    x + y + z
+) # trailing lambda
+
+lambda x, y, z: (
+    # leading body
+    x + y + z
+)
+
+lambda x, y, z: (  # leading eol body
+    x + y + z
+)
+
+(
+    lambda name:
+    source_bucket  # trailing eol comment
+    if name == source_bucket_name
+    else storage.Bucket(mock_service, destination_bucket_name)
+)
+
+(
+    lambda name:
+    # dangling header comment
+    source_bucket
+    if name == source_bucket_name
+    else storage.Bucket(mock_service, destination_bucket_name)
+)
+
+x = (
+    lambda name:
+    # dangling header comment
+    source_bucket
+    if name == source_bucket_name
+    else storage.Bucket(mock_service, destination_bucket_name)
+)
+
+(
+    lambda name: # dangling header comment
+    (
+        source_bucket
+        if name == source_bucket_name
+        else storage.Bucket(mock_service, destination_bucket_name)
+    )
+)
+
+(
+    lambda from_ts, to_ts, _chain_id=chain_id:  # dangling eol header comment
+    db_evmtx.count_transactions_in_range(
+        chain_id=_chain_id,
+        from_ts=from_ts,
+        to_ts=to_ts,
+    )
+)
+
+(
+    lambda from_ts, to_ts, _chain_id=chain_id:
+    # dangling header comment before call
+    db_evmtx.count_transactions_in_range(
+        chain_id=_chain_id,
+        from_ts=from_ts,
+        to_ts=to_ts,
+    )
+)
+
+(
+    lambda left, right:
+    # comment
+    ibis.timestamp("2017-04-01").cast(dt.date).between(left, right)
+)
+
+(
+    lambda left, right:
+    ibis.timestamp("2017-04-01")  # comment
+    .cast(dt.date)
+    .between(left, right)
+)
+
+(
+    lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy:
+    # comment
+    [xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz]
+)
+
+(
+    lambda x, y:
+    # comment
+    {
+        "key": x,
+        "another": y,
+    }
+)
+
+(
+    lambda x, y:
+    # comment
+    (
+        x,
+        y,
+        z
+    )
+)
+
+(
+    lambda x:
+    # comment
+    dict_with_default[np.nan if isinstance(x, float) and np.isnan(x) else x]
+)
+
+(
+    lambda from_ts, to_ts, _chain_id=chain_id:
+    db_evmtx.count_transactions_in_range[
+        # comment
+        _chain_id, from_ts, to_ts
+    ]
+)
+
+(
+    lambda
+    # comment
+    *args, **kwargs:
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1
+)
+
+(
+    lambda  # comment
+    *args, **kwargs:
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1
+)
+
+(
+    lambda  # comment 1
+    # comment 2
+    *args, **kwargs: # comment 3
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1
+)
+
+(
+    lambda  # comment 1
+    *args, **kwargs: # comment 3
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1
+)
+
+(
+    lambda *args, **kwargs:
+    # comment 1
+    ( # comment 2
+        # comment 3
+        aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1 # comment 4
+        # comment 5
+    ) # comment 6
+)
+
+(
+    lambda *brgs, **kwargs:
+    # comment 1
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa( # comment 2
+        # comment 3
+        *brgs, **kwargs) + 1 # comment 4
+    # comment 5
+)
+
+(
+    lambda *crgs, **kwargs: # comment 1
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*crgs, **kwargs) + 1
+)
+
+(
+    lambda *drgs, **kwargs: # comment 1
+    (
+        aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*drgs, **kwargs) + 1
+    )
+)
+
+(
+    lambda * # comment 1
+    ergs, **
+    # comment 2
+    kwargs # comment 3
+    : # comment 4
+    (
+        aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*ergs, **kwargs) + 1
+    )
+)
+
+(
+    lambda  # 1
+    # 2
+    left,  # 3
+    # 4
+    right:  # 5
+    # 6
+    ibis.timestamp("2017-04-01").cast(dt.date).between(left, right)
+)
+
+(
+    lambda x:  # outer comment 1
+    (
+        lambda y:  # inner comment 1
+        # inner comment 2
+        lambda z: (
+            # innermost comment
+            x + y + z
+        )
+    )
+)
+
+foo(
+    lambda from_ts,  # comment prevents collapsing the parameters to one line
+    to_ts, _chain_id=chain_id: db_evmtx.count_transactions_in_range(
+        chain_id=_chain_id,
+        from_ts=from_ts,
+        to_ts=to_ts,
+    )
+)
+
+foo(
+    lambda from_ts,  # but still wrap the body if it gets too long
+    to_ts,
+    _chain_id=chain_id: db_evmtx.count_transactions_in_rangeeeeeeeeeeeeeeeeeeeeeeeeeeeee(
+        chain_id=_chain_id,
+        from_ts=from_ts,
+        to_ts=to_ts,
+    )
+)
+
+transform = lambda left, right: ibis.timestamp("2017-04-01").cast(dt.date).between(left, right).between(left, right)  # trailing comment
+
+(
+    lambda:  # comment
+    1
+)
+
+(
+    lambda  # comment
+    :
+    1
+)
+
+(
+    lambda:
+    # comment
+    1
+)
+
+(
+    lambda:  # comment 1
+    # comment 2
+    1
+)
+
+(
+    lambda  # comment 1
+    # comment 2
+    :  # comment 3
+    # comment 4
+    1
+)
+
 (
     lambda
     * # comment 2
@@ -271,3 +789,18 @@ def a():
     x:
     x
 )
+
+(
+    lambda: # dangling-end-of-line
+    # dangling-own-line
+    (  # leading-body-end-of-line
+        x
+    )
+)
+
+(
+    lambda: # dangling-end-of-line
+    (  # leading-body-end-of-line
+        x
+    )
+)

crates/ruff_python_formatter/src/builders.rs

@@ -1,4 +1,4 @@
-use ruff_formatter::{Argument, Arguments, write};
+use ruff_formatter::{Argument, Arguments, format_args, write};
 use ruff_text_size::{Ranged, TextRange, TextSize};
 
 use crate::context::{NodeLevel, WithNodeLevel};
@@ -33,20 +33,27 @@ impl<'ast> Format<PyFormatContext<'ast>> for ParenthesizeIfExpands<'_, 'ast> {
         {
             let mut f = WithNodeLevel::new(NodeLevel::ParenthesizedExpression, f);
 
-            write!(
-                f,
-                [group(&format_with(|f| {
-                    if_group_breaks(&token("(")).fmt(f)?;
-
             if self.indent {
-                        soft_block_indent(&Arguments::from(&self.inner)).fmt(f)?;
+                let parens_id = f.group_id("indented_parenthesize_if_expands");
+                group(&format_args![
+                    if_group_breaks(&token("(")),
+                    indent_if_group_breaks(
+                        &format_args![soft_line_break(), &Arguments::from(&self.inner)],
+                        parens_id
+                    ),
+                    soft_line_break(),
+                    if_group_breaks(&token(")"))
+                ])
+                .with_id(Some(parens_id))
+                .fmt(&mut f)
             } else {
-                        Arguments::from(&self.inner).fmt(f)?;
+                group(&format_args![
+                    if_group_breaks(&token("(")),
+                    Arguments::from(&self.inner),
+                    if_group_breaks(&token(")")),
+                ])
+                .fmt(&mut f)
             }
-
-                    if_group_breaks(&token(")")).fmt(f)
-                }))]
-            )
         }
     }
 }

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -1,15 +1,21 @@
-use ruff_formatter::write;
+use ruff_formatter::{FormatRuleWithOptions, RemoveSoftLinesBuffer, format_args, write};
-use ruff_python_ast::AnyNodeRef;
+use ruff_python_ast::{AnyNodeRef, Expr, ExprLambda};
-use ruff_python_ast::ExprLambda;
 use ruff_text_size::Ranged;
 
-use crate::comments::dangling_comments;
+use crate::builders::parenthesize_if_expands;
-use crate::expression::parentheses::{NeedsParentheses, OptionalParentheses};
+use crate::comments::{SourceComment, dangling_comments, leading_comments, trailing_comments};
+use crate::expression::parentheses::{
+    NeedsParentheses, OptionalParentheses, Parentheses, is_expression_parenthesized,
+};
+use crate::expression::{CallChainLayout, has_own_parentheses};
 use crate::other::parameters::ParametersParentheses;
 use crate::prelude::*;
+use crate::preview::is_parenthesize_lambda_bodies_enabled;
 
 #[derive(Default)]
-pub struct FormatExprLambda;
+pub struct FormatExprLambda {
+    layout: ExprLambdaLayout,
+}
 
 impl FormatNodeRule<ExprLambda> for FormatExprLambda {
     fn fmt_fields(&self, item: &ExprLambda, f: &mut PyFormatter) -> FormatResult<()> {
@@ -20,13 +26,19 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
             body,
         } = item;
 
+        let body = &**body;
+        let parameters = parameters.as_deref();
+
         let comments = f.context().comments().clone();
         let dangling = comments.dangling(item);
+        let preview = is_parenthesize_lambda_bodies_enabled(f.context());
 
         write!(f, [token("lambda")])?;
 
-        if let Some(parameters) = parameters {
+        // Format any dangling comments before the parameters, but save any dangling comments after
-            // In this context, a dangling comment can either be a comment between the `lambda` the
+        // the parameters/after the header to be formatted with the body below.
+        let dangling_header_comments = if let Some(parameters) = parameters {
+            // In this context, a dangling comment can either be a comment between the `lambda` and the
             // parameters, or a comment between the parameters and the body.
             let (dangling_before_parameters, dangling_after_parameters) = dangling
                 .split_at(dangling.partition_point(|comment| comment.end() < parameters.start()));
@@ -86,7 +98,7 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
                 //     *x: x
                 // )
                 // ```
-                if comments.has_leading(&**parameters) {
+                if comments.has_leading(parameters) {
                     hard_line_break().fmt(f)?;
                 } else {
                     write!(f, [space()])?;
@@ -95,32 +107,90 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
                 write!(f, [dangling_comments(dangling_before_parameters)])?;
             }
 
+            // Try to keep the parameters on a single line, unless there are intervening comments.
+            if preview && !comments.contains_comments(parameters.into()) {
+                let mut buffer = RemoveSoftLinesBuffer::new(f);
+                write!(
+                    buffer,
+                    [parameters
+                        .format()
+                        .with_options(ParametersParentheses::Never)]
+                )?;
+            } else {
                 write!(
                     f,
                     [parameters
                         .format()
                         .with_options(ParametersParentheses::Never)]
                 )?;
+            }
+
+            dangling_after_parameters
+        } else {
+            dangling
+        };
 
         write!(f, [token(":")])?;
 
-            if dangling_after_parameters.is_empty() {
+        if dangling_header_comments.is_empty() {
             write!(f, [space()])?;
-            } else {
+        } else if !preview {
-                write!(f, [dangling_comments(dangling_after_parameters)])?;
+            write!(f, [dangling_comments(dangling_header_comments)])?;
         }
-        } else {
-            write!(f, [token(":")])?;
 
-            // In this context, a dangling comment is a comment between the `lambda` and the body.
+        if !preview {
-            if dangling.is_empty() {
+            return body.format().fmt(f);
-                write!(f, [space()])?;
+        }
-            } else {
+
-                write!(f, [dangling_comments(dangling)])?;
+        let fmt_body = FormatBody {
+            body,
+            dangling_header_comments,
+        };
+
+        match self.layout {
+            ExprLambdaLayout::Assignment => fits_expanded(&fmt_body).fmt(f),
+            ExprLambdaLayout::Default => fmt_body.fmt(f),
+        }
     }
 }
 
-        write!(f, [body.format()])
+#[derive(Debug, Default, Copy, Clone)]
+pub enum ExprLambdaLayout {
+    #[default]
+    Default,
+
+    /// The [`ExprLambda`] is the direct child of an assignment expression, so it needs to use
+    /// `fits_expanded` to prefer parenthesizing its own body before the assignment tries to
+    /// parenthesize the whole lambda. For example, we want this formatting:
+    ///
+    /// ```py
+    /// long_assignment_target = lambda x, y, z: (
+    ///     x + y + z
+    /// )
+    /// ```
+    ///
+    /// instead of either of these:
+    ///
+    /// ```py
+    /// long_assignment_target = (
+    ///     lambda x, y, z: (
+    ///         x + y + z
+    ///     )
+    /// )
+    ///
+    /// long_assignment_target = (
+    ///     lambda x, y, z: x + y + z
+    /// )
+    /// ```
+    Assignment,
+}
+
+impl FormatRuleWithOptions<ExprLambda, PyFormatContext<'_>> for FormatExprLambda {
+    type Options = ExprLambdaLayout;
+
+    fn with_options(mut self, options: Self::Options) -> Self {
+        self.layout = options;
+        self
     }
 }
 
@@ -137,3 +207,266 @@ impl NeedsParentheses for ExprLambda {
         }
     }
 }
+
+struct FormatBody<'a> {
+    body: &'a Expr,
+
+    /// Dangling comments attached to the lambda header that should be formatted with the body.
+    ///
+    /// These can include both own-line and end-of-line comments. For lambdas with parameters, this
+    /// means comments after the parameters:
+    ///
+    /// ```py
+    /// (
+    ///     lambda x, y  # 1
+    ///         # 2
+    ///         :  # 3
+    ///         # 4
+    ///         x + y
+    /// )
+    /// ```
+    ///
+    /// Or all dangling comments for lambdas without parameters:
+    ///
+    /// ```py
+    /// (
+    ///     lambda  # 1
+    ///         # 2
+    ///         :  # 3
+    ///         # 4
+    ///         1
+    /// )
+    /// ```
+    ///
+    /// In most cases these should formatted within the parenthesized body, as in:
+    ///
+    /// ```py
+    /// (
+    ///     lambda: (  # 1
+    ///         # 2
+    ///         # 3
+    ///         # 4
+    ///         1
+    ///     )
+    /// )
+    /// ```
+    ///
+    /// or without `# 2`:
+    ///
+    /// ```py
+    /// (
+    ///     lambda: (  # 1  # 3
+    ///         # 4
+    ///         1
+    ///     )
+    /// )
+    /// ```
+    dangling_header_comments: &'a [SourceComment],
+}
+
+impl Format<PyFormatContext<'_>> for FormatBody<'_> {
+    fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
+        let FormatBody {
+            dangling_header_comments,
+            body,
+        } = self;
+
+        let body = *body;
+        let comments = f.context().comments().clone();
+        let body_comments = comments.leading_dangling_trailing(body);
+
+        if !dangling_header_comments.is_empty() {
+            // Split the dangling header comments into trailing comments formatted with the lambda
+            // header (1) and leading comments formatted with the body (2, 3, 4).
+            //
+            // ```python
+            // (
+            //     lambda  # 1
+            //     # 2
+            //     :  # 3
+            //     # 4
+            //     y
+            // )
+            // ```
+            //
+            // Note that these are split based on their line position rather than using
+            // `partition_point` based on a range, for example.
+            let (trailing_header_comments, leading_body_comments) = dangling_header_comments
+                .split_at(
+                    dangling_header_comments
+                        .iter()
+                        .position(|comment| comment.line_position().is_own_line())
+                        .unwrap_or(dangling_header_comments.len()),
+                );
+
+            // If the body is parenthesized and has its own leading comments, preserve the
+            // separation between the dangling lambda comments and the body comments. For
+            // example, preserve this comment positioning:
+            //
+            // ```python
+            // (
+            //      lambda:  # 1
+            //      # 2
+            //      (  # 3
+            //          x
+            //      )
+            // )
+            // ```
+            //
+            // 1 and 2 are dangling on the lambda and emitted first, followed by a hard line
+            // break and the parenthesized body with its leading comments.
+            //
+            // However, when removing 2, 1 and 3 can instead be formatted on the same line:
+            //
+            // ```python
+            // (
+            //      lambda: (  # 1  # 3
+            //          x
+            //      )
+            // )
+            // ```
+            let comments = f.context().comments();
+            if is_expression_parenthesized(body.into(), comments.ranges(), f.context().source())
+                && comments.has_leading(body)
+            {
+                trailing_comments(dangling_header_comments).fmt(f)?;
+
+                // Note that `leading_body_comments` have already been formatted as part of
+                // `dangling_header_comments` above, but their presence still determines the spacing
+                // here.
+                if leading_body_comments.is_empty() {
+                    space().fmt(f)?;
+                } else {
+                    hard_line_break().fmt(f)?;
+                }
+
+                body.format().with_options(Parentheses::Always).fmt(f)
+            } else {
+                write!(
+                    f,
+                    [
+                        space(),
+                        token("("),
+                        trailing_comments(trailing_header_comments),
+                        block_indent(&format_args!(
+                            leading_comments(leading_body_comments),
+                            body.format().with_options(Parentheses::Never)
+                        )),
+                        token(")")
+                    ]
+                )
+            }
+        }
+        // If the body has comments, we always want to preserve the parentheses. This also
+        // ensures that we correctly handle parenthesized comments, and don't need to worry
+        // about them in the implementation below.
+        else if body_comments.has_leading() || body_comments.has_trailing_own_line() {
+            body.format().with_options(Parentheses::Always).fmt(f)
+        }
+        // Calls and subscripts require special formatting because they have their own
+        // parentheses, but they can also have an arbitrary amount of text before the
+        // opening parenthesis. We want to avoid cases where we keep a long callable on the
+        // same line as the lambda parameters. For example, `db_evmtx...` in:
+        //
+        // ```py
+        // transaction_count = self._query_txs_for_range(
+        //     get_count_fn=lambda from_ts, to_ts, _chain_id=chain_id: db_evmtx.count_transactions_in_range(
+        //         chain_id=_chain_id,
+        //         from_ts=from_ts,
+        //         to_ts=to_ts,
+        //     ),
+        // )
+        // ```
+        //
+        // should cause the whole lambda body to be parenthesized instead:
+        //
+        // ```py
+        // transaction_count = self._query_txs_for_range(
+        //     get_count_fn=lambda from_ts, to_ts, _chain_id=chain_id: (
+        //         db_evmtx.count_transactions_in_range(
+        //             chain_id=_chain_id,
+        //             from_ts=from_ts,
+        //             to_ts=to_ts,
+        //         )
+        //     ),
+        // )
+        // ```
+        else if matches!(body, Expr::Call(_) | Expr::Subscript(_)) {
+            let unparenthesized = body.format().with_options(Parentheses::Never);
+            if CallChainLayout::from_expression(
+                body.into(),
+                comments.ranges(),
+                f.context().source(),
+            ) == CallChainLayout::Fluent
+            {
+                parenthesize_if_expands(&unparenthesized).fmt(f)
+            } else {
+                let unparenthesized = unparenthesized.memoized();
+                if unparenthesized.inspect(f)?.will_break() {
+                    expand_parent().fmt(f)?;
+                }
+
+                best_fitting![
+                    // body all flat
+                    unparenthesized,
+                    // body expanded
+                    group(&unparenthesized).should_expand(true),
+                    // parenthesized
+                    format_args![token("("), block_indent(&unparenthesized), token(")")]
+                ]
+                .fmt(f)
+            }
+        }
+        // For other cases with their own parentheses, such as lists, sets, dicts, tuples,
+        // etc., we can just format the body directly. Their own formatting results in the
+        // lambda being formatted well too. For example:
+        //
+        // ```py
+        // lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz: [xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz]
+        // ```
+        //
+        // gets formatted as:
+        //
+        // ```py
+        // lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz: [
+        //     xxxxxxxxxxxxxxxxxxxx,
+        //     yyyyyyyyyyyyyyyyyyyy,
+        //     zzzzzzzzzzzzzzzzzzzz
+        // ]
+        // ```
+        else if has_own_parentheses(body, f.context()).is_some() {
+            body.format().fmt(f)
+        }
+        // Finally, for expressions without their own parentheses, use
+        // `parenthesize_if_expands` to add parentheses around the body, only if it expands
+        // across multiple lines. The `Parentheses::Never` here also removes unnecessary
+        // parentheses around lambda bodies that fit on one line. For example:
+        //
+        // ```py
+        // lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz: xxxxxxxxxxxxxxxxxxxx + yyyyyyyyyyyyyyyyyyyy + zzzzzzzzzzzzzzzzzzzz
+        // ```
+        //
+        // is formatted as:
+        //
+        // ```py
+        // lambda xxxxxxxxxxxxxxxxxxxx, yyyyyyyyyyyyyyyyyyyy, zzzzzzzzzzzzzzzzzzzz: (
+        //     xxxxxxxxxxxxxxxxxxxx + yyyyyyyyyyyyyyyyyyyy + zzzzzzzzzzzzzzzzzzzz
+        // )
+        // ```
+        //
+        // while
+        //
+        // ```py
+        // lambda xxxxxxxxxxxxxxxxxxxx: (xxxxxxxxxxxxxxxxxxxx + 1)
+        // ```
+        //
+        // is formatted as:
+        //
+        // ```py
+        // lambda xxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxx + 1
+        // ```
+        else {
+            parenthesize_if_expands(&body.format().with_options(Parentheses::Never)).fmt(f)
+        }
+    }
+}

crates/ruff_python_formatter/src/preview.rs

@@ -52,3 +52,10 @@ pub(crate) const fn is_avoid_parens_for_long_as_captures_enabled(
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the
+/// [`parenthesize_lambda_bodies`](https://github.com/astral-sh/ruff/pull/21385) preview style is
+/// enabled.
+pub(crate) const fn is_parenthesize_lambda_bodies_enabled(context: &PyFormatContext) -> bool {
+    context.is_preview()
+}

crates/ruff_python_formatter/src/statement/stmt_assign.rs

@@ -9,6 +9,7 @@ use crate::comments::{
     Comments, LeadingDanglingTrailingComments, SourceComment, trailing_comments,
 };
 use crate::context::{NodeLevel, WithNodeLevel};
+use crate::expression::expr_lambda::ExprLambdaLayout;
 use crate::expression::parentheses::{
     NeedsParentheses, OptionalParentheses, Parentheses, Parenthesize, is_expression_parenthesized,
     optional_parentheses,
@@ -18,6 +19,7 @@ use crate::expression::{
     maybe_parenthesize_expression,
 };
 use crate::other::interpolated_string::InterpolatedStringLayout;
+use crate::preview::is_parenthesize_lambda_bodies_enabled;
 use crate::statement::trailing_semicolon;
 use crate::string::StringLikeExtensions;
 use crate::string::implicit::{
@@ -303,12 +305,7 @@ impl Format<PyFormatContext<'_>> for FormatStatementsLastExpression<'_> {
                     && format_implicit_flat.is_none()
                     && format_interpolated_string.is_none()
                 {
-                    return maybe_parenthesize_expression(
+                    return maybe_parenthesize_value(value, *statement).fmt(f);
-                        value,
-                        *statement,
-                        Parenthesize::IfBreaks,
-                    )
-                    .fmt(f);
                 }
 
                 let comments = f.context().comments().clone();
@@ -586,11 +583,7 @@ impl Format<PyFormatContext<'_>> for FormatStatementsLastExpression<'_> {
                             space(),
                             operator,
                             space(),
-                            maybe_parenthesize_expression(
+                            maybe_parenthesize_value(value, *statement)
-                                value,
-                                *statement,
-                                Parenthesize::IfBreaks
-                            )
                         ]
                     );
                 }
@@ -1369,3 +1362,32 @@ fn is_attribute_with_parenthesized_value(target: &Expr, context: &PyFormatContex
         _ => false,
     }
 }
+
+/// Like [`maybe_parenthesize_expression`] but with special handling for lambdas in preview.
+fn maybe_parenthesize_value<'a>(
+    expression: &'a Expr,
+    parent: AnyNodeRef<'a>,
+) -> MaybeParenthesizeValue<'a> {
+    MaybeParenthesizeValue { expression, parent }
+}
+
+struct MaybeParenthesizeValue<'a> {
+    expression: &'a Expr,
+    parent: AnyNodeRef<'a>,
+}
+
+impl Format<PyFormatContext<'_>> for MaybeParenthesizeValue<'_> {
+    fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
+        let MaybeParenthesizeValue { expression, parent } = self;
+
+        if is_parenthesize_lambda_bodies_enabled(f.context())
+            && let Expr::Lambda(lambda) = expression
+            && !f.context().comments().has_leading(lambda)
+        {
+            parenthesize_if_expands(&lambda.format().with_options(ExprLambdaLayout::Assignment))
+                .fmt(f)
+        } else {
+            maybe_parenthesize_expression(expression, *parent, Parenthesize::IfBreaks).fmt(f)
+        }
+    }
+}

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_long_strings.py.snap

@@ -906,11 +906,10 @@ x = {
 -)
 +string_with_escaped_nameescape = "........................................................................... \\N{LAO KO LA}"
  
--msg = lambda x: (
+ msg = lambda x: (
 -    f"this is a very very very very long lambda value {x} that doesn't fit on a"
 -    " single line"
-+msg = (
++    f"this is a very very very very long lambda value {x} that doesn't fit on a single line"
-+    lambda x: f"this is a very very very very long lambda value {x} that doesn't fit on a single line"
  )
  
  dict_with_lambda_values = {
@@ -1403,8 +1402,8 @@ string_with_escaped_nameescape = "..............................................
 
 string_with_escaped_nameescape = "........................................................................... \\N{LAO KO LA}"
 
-msg = (
+msg = lambda x: (
-    lambda x: f"this is a very very very very long lambda value {x} that doesn't fit on a single line"
+    f"this is a very very very very long lambda value {x} that doesn't fit on a single line"
 )
 
 dict_with_lambda_values = {

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_multiline_strings.py.snap

@@ -375,7 +375,7 @@ a = b if """
  # Another use case
  data = yaml.load("""\
  a: 1
-@@ -77,19 +106,23 @@
+@@ -77,10 +106,12 @@
  b: 2
  """,
  )
@@ -390,19 +390,7 @@ a = b if """
  
  MULTILINE = """
  foo
- """.replace("\n", "")
+@@ -156,16 +187,24 @@
--generated_readme = lambda project_name: """
-+generated_readme = (
-+    lambda project_name: """
- {}
- 
- <Add content here!>
- """.strip().format(project_name)
-+)
- parser.usage += """
- Custom extra help summary.
- 
-@@ -156,16 +189,24 @@
               10 LOAD_CONST               0 (None)
               12 RETURN_VALUE
  """ % (_C.__init__.__code__.co_firstlineno + 1,)
@@ -433,7 +421,7 @@ a = b if """
  [
      """cow
  moos""",
-@@ -206,7 +247,9 @@
+@@ -206,7 +245,9 @@
      "c"
  )
  
@@ -444,7 +432,7 @@ a = b if """
  
  assert some_var == expected_result, """
  test
-@@ -224,10 +267,8 @@
+@@ -224,10 +265,8 @@
                  """Sxxxxxxx xxxxxxxx, xxxxxxx xx xxxxxxxxx
                  xxxxxxxxxxxxx xxxxxxx xxxxxxxxx xxx-xxxxxxxxxx xxxxxx xx xxx-xxxxxx"""
              ),
@@ -457,7 +445,7 @@ a = b if """
          },
      }
  
-@@ -246,14 +287,12 @@
+@@ -246,14 +285,12 @@
  a
  a"""
      ),
@@ -597,13 +585,11 @@ data = yaml.load(
 MULTILINE = """
 foo
 """.replace("\n", "")
-generated_readme = (
+generated_readme = lambda project_name: """
-    lambda project_name: """
 {}
 
 <Add content here!>
 """.strip().format(project_name)
-)
 parser.usage += """
 Custom extra help summary.
 

crates/ruff_python_formatter/tests/snapshots/format@multiline_string_deviations.py.snap

@@ -1,7 +1,6 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/multiline_string_deviations.py
-snapshot_kind: text
 ---
 ## Input
 ```python
@@ -106,3 +105,22 @@ generated_readme = (
 """.strip().format(project_name)
 )
 ```
+
+
+## Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -44,10 +44,8 @@
+ # this by changing `Lambda::needs_parentheses` to return `BestFit` but it causes
+ # issues when the lambda has comments.
+ # Let's keep this as a known deviation for now.
+-generated_readme = (
+-    lambda project_name: """
++generated_readme = lambda project_name: """
+ {}
+ 
+ <Add content here!>
+ """.strip().format(project_name)
+-)
+```