wip: parenthesize long lambda bodies

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};
@@ -31,20 +31,32 @@ impl ParenthesizeIfExpands<'_, '_> {
 impl<'ast> Format<PyFormatContext<'ast>> for ParenthesizeIfExpands<'_, 'ast> {
     fn fmt(&self, f: &mut Formatter<PyFormatContext<'ast>>) -> FormatResult<()> {
         {
+            let parens_id = f.group_id("optional_parentheses");
+
             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)?;
+                        write!(
+                            f,
+                            [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))]
+                        )
                     } else {
+                        if_group_breaks(&token("(")).fmt(f)?;
                         Arguments::from(&self.inner).fmt(f)?;
+                        if_group_breaks(&token(")")).fmt(f)
                     }
-
-                    if_group_breaks(&token(")")).fmt(f)
                 }))]
             )
         }

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -4,10 +4,14 @@ use ruff_python_ast::ExprLambda;
 use ruff_text_size::Ranged;
 
 use crate::comments::dangling_comments;
-use crate::expression::parentheses::{NeedsParentheses, OptionalParentheses};
+use crate::expression::has_own_parentheses;
+use crate::expression::parentheses::{
+    NeedsParentheses, OptionalParentheses, is_expression_parenthesized, optional_parentheses,
+};
 use crate::other::parameters::ParametersParentheses;
 use crate::prelude::*;
 use crate::preview::is_force_single_line_lambda_parameters_enabled;
+use crate::preview::is_parenthesize_lambda_bodies_enabled;
 
 #[derive(Default)]
 pub struct FormatExprLambda;
@@ -27,7 +31,7 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
         write!(f, [token("lambda")])?;
 
         if let Some(parameters) = parameters {
-            // In this context, a dangling comment can either be a comment between the `lambda` the
+            // 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()));
@@ -76,7 +80,15 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
             }
         }
 
-        write!(f, [body.format()])
+        // Avoid parenthesizing lists, dictionaries, etc.
+        if is_parenthesize_lambda_bodies_enabled(f.context())
+            && has_own_parentheses(body, f.context()).is_none()
+            && !is_expression_parenthesized(body.into(), comments.ranges(), f.context().source())
+        {
+            fits_expanded(&optional_parentheses(&body.format())).fmt(f)
+        } else {
+            body.format().fmt(f)
+        }
     }
 }
 

crates/ruff_python_formatter/src/preview.rs

@@ -53,6 +53,13 @@ pub(crate) const fn is_avoid_parens_for_long_as_captures_enabled(
     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()
+}
+
 /// Returns `true` if the
 /// [`force_single_line_lambda_parameters`](https://github.com/astral-sh/ruff/pull/21385) preview
 /// style is enabled.

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/format@expression__lambda.py.snap

@@ -682,18 +682,86 @@ class C:
 ```diff
 --- Stable
 +++ Preview
-@@ -280,9 +280,7 @@
+@@ -74,7 +74,9 @@
+ 
+ # lambda arguments don't have parentheses, so we never add a magic trailing comma ...
+ def f(
+-    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb = lambda x: y,
++    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb = lambda x: (
++        y
++    ),
+ ):
+     pass
+ 
+@@ -218,29 +220,31 @@
+ 
+ # 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
++    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
+ 
+ 
+@@ -280,9 +284,9 @@
  ]  # Trailing
  # Trailing
  
 -lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
 -    *args, **kwargs
 -), e=1, f=2, g=2: d
-+lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs), e=1, f=2, g=2: d
++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
-@@ -291,9 +289,9 @@
+@@ -291,9 +295,9 @@
          c,
          d,
          e,
@@ -706,7 +774,7 @@ class C:
      )
  
  
-@@ -302,15 +300,7 @@
+@@ -302,15 +306,9 @@
          c,
          d,
          e,
@@ -719,21 +787,22 @@ class C:
 -        e=1,
 -        f=2,
 -        g=2: d,
-+        f=lambda self, araa, kkkwargs, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, args, kwargs, e=1, f=2, g=2: d,
++        f=lambda self, araa, kkkwargs, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, args, kwargs, e=1, f=2, g=2: (
++            d
++        ),
          g=10,
      )
  
-@@ -320,9 +310,10 @@
+@@ -320,9 +318,9 @@
          c,
          d,
          e,
 -        f=lambda self,
 -        *args,
 -        **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1,
-+        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
++        f=lambda self, *args, **kwargs: (
-+            *args, **kwargs
++            aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1
-+        )
++        ),
-+        + 1,
      )