Prefer splitting right hand side of assignments

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/assignment_split_value_first.options.json

@@ -0,0 +1,5 @@
+[
+  {
+    "preview": "enabled"
+  }
+]

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/assignment_split_value_first.py

@@ -0,0 +1,218 @@
+#######
+# Unsplittable target and value
+
+# Only parenthesize the value if it makes it fit, otherwise avoid parentheses.
+b = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvee
+
+bbbbbbbbbbbbbbbb = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvv
+
+# Avoid parenthesizing the value even if the target exceeds the configured width
+bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb = bbb
+
+
+############
+# Splittable targets
+
+# Does not double-parenthesize tuples
+(
+    first_item,
+    second_item,
+) = some_looooooooong_module.some_loooooog_function_name(
+    first_argument, second_argument, third_argument
+)
+
+
+# Preserve parentheses around the first target
+(
+    req["ticket"]["steps"]["step"][0]["tasks"]["task"]["fields"]["field"][
+        "access_request"
+    ]["destinations"]["destination"][0]["ip_address"]
+) = dst
+
+# Augmented assignment
+req["ticket"]["steps"]["step"][0]["tasks"]["task"]["fields"]["field"][
+    "access_request"
+] += dst
+
+# Always parenthesize the value if it avoids splitting the target, regardless of the value's width.
+_a: a[aaaa] = (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvv
+)
+
+#####
+# Avoid parenthesizing the value if the expression right before the `=` splits to avoid an unnecessary pair of parentheses
+
+# The type annotation is guaranteed to split because it is too long.
+_a: a[
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvv
+] = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvv
+
+# The target is too long
+(
+    aaaaaaaaaaa,
+    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb,
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvv
+
+# The target splits because of a magic trailing comma
+(
+    a,
+    b,
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvvvv
+
+# The targets split because of a comment
+(
+    # leading
+    a
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvvvv
+
+(
+    a
+    # trailing
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvvvv
+
+(
+    a,  # nested
+    b
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvvvv
+
+#######
+# Multi targets
+
+# Black always parenthesizes the right if using multiple targets regardless if the parenthesized value exceeds the
+# the configured line width or not
+aaaa = bbbbbbbbbbbbbbbb = (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvee
+)
+
+# Black does parenthesize the target if the target itself exceeds the line width and only parenthesizes
+# the values if it makes it fit.
+# The second target is too long to ever fit into the configured line width.
+aaaa = (
+    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbdddd
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbvvvvvvvvvvvvvvvvvee
+
+# Does also apply for other multi target assignments, as soon as a single target exceeds the configured
+# width
+aaaaaa = a["aaa"] = bbbbb[aa, bbb, cccc] = dddddddddd = eeeeee = (
+    fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+) = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+
+
+######################
+# Call expressions:
+# For unsplittable targets: Parenthesize the call expression if it makes it fit.
+#
+# For splittable targets:
+# Only parenthesize a call expression if the parens of the call don't fit on the same line
+# as the target. Don't parenthesize the call expression if the target (or annotation) right before
+# splits.
+
+# Don't parenthesize the function call if the left is unsplittable.
+this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it = a.b.function(
+    arg1, arg2, arg3
+)
+this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it = function(
+    [1, 2, 3], arg1, [1, 2, 3], arg2, [1, 2, 3], arg3
+)
+this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it = function(
+    [1, 2, 3],
+    arg1,
+    [1, 2, 3],
+    arg2,
+    [1, 2, 3],
+    arg3,
+    dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd,
+    eeeeeeeeeeeeee,
+)
+
+this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it = (
+    function()
+)
+this_is_a_ridiculously_long_name_and_nobodyddddddddddddddddddddddddddddddd = (
+    a.b.function(arg1, arg2, arg3)
+)
+this_is_a_ridiculously_long_name_and_nobodyddddddddddddddddddddddddddddddd = function()
+this_is_a_ridiculously_long_name_and_nobodyddddddddddddddddddddddddddddddd = function(
+    [1, 2, 3], arg1, [1, 2, 3], arg2, [1, 2, 3], arg3
+)
+this_is_a_ridiculously_long_name_and_nobodyddddddddddddddddddddddddddddddd = function(
+    [1, 2, 3],
+    arg1,
+    [1, 2, 3],
+    arg2,
+    [1, 2, 3],
+    arg3,
+    dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd,
+    eeeeeeeeeeeeee,
+)
+
+####### Fluent call expressions
+# Uses the regular `Multiline` layout where the entire `value` gets parenthesized
+# if it doesn't fit on the line.
+this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use = (
+    function().b().c([1, 2, 3], arg1, [1, 2, 3], arg2, [1, 2, 3], arg3)
+)
+
+
+#######
+# Test comment inlining
+value.__dict__[key] = (
+    "test"  # set some Thrift field to non-None in the struct aa bb cc dd ee
+)
+value.__dict__.keye = (
+    "test"  # set some Thrift field to non-None in the struct aa bb cc dd ee
+)
+value.__dict__.keye = (
+    "test"  # set some Thrift field to non-None in the struct aa bb cc dd ee
+)
+
+
+# Don't parenthesize the value because the target's trailing comma forces it to split.
+a[
+    aaaaaaa,
+    b,
+] = cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc  # comment
+
+# Parenthesize the value, but don't duplicate the comment.
+a[aaaaaaa, b] = (
+    cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc  # comment
+)
+
+# Format both as flat, but don't loos the comment.
+a[aaaaaaa, b] = bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb  # comment
+
+#######################################################
+# Test the case where a parenthesized value now fits:
+a[
+    aaaaaaa,
+    b
+] = (
+    cccccccc # comment
+)
+
+# Splits the target but not the value because of the magic trailing comma.
+a[
+    aaaaaaa,
+    b,
+] = (
+    cccccccc # comment
+)
+
+# Splits the second target because of the comment and the first target because of the trailing comma.
+a[
+    aaaaaaa,
+    b,
+] = (
+    # leading comment
+    b
+) = (
+    cccccccc # comment
+)
+
+
+########
+# Type Alias Statement
+type A[str, int, number] = VeryLongTypeNameThatShouldBreakFirstToTheRightBeforeSplitngtin
+
+type A[VeryLongTypeNameThatShouldBreakFirstToTheRightBeforeSplitngtinthatExceedsTheWidth] = str
+

crates/ruff_python_formatter/src/expression/mod.rs

@@ -952,7 +952,7 @@ impl OwnParentheses {
 /// Differs from [`has_own_parentheses`] in that it returns [`OwnParentheses::NonEmpty`] for
 /// parenthesized expressions, like `(1)` or `([1])`, regardless of whether those expression have
 /// their _own_ parentheses.
-fn has_parentheses(expr: &Expr, context: &PyFormatContext) -> Option<OwnParentheses> {
+pub(crate) fn has_parentheses(expr: &Expr, context: &PyFormatContext) -> Option<OwnParentheses> {
     let own_parentheses = has_own_parentheses(expr, context);
 
     // If the node has its own non-empty parentheses, we don't need to check for surrounding

crates/ruff_python_formatter/src/lib.rs

@@ -181,7 +181,7 @@ mod tests {
 
     use ruff_python_parser::{parse_ok_tokens, AsMode};
 
-    use crate::{format_module_ast, format_module_source, PyFormatOptions};
+    use crate::{format_module_ast, format_module_source, PreviewMode, PyFormatOptions};
 
     /// Very basic test intentionally kept very similar to the CLI
     #[test]
@@ -209,13 +209,9 @@ if True:
     #[test]
     fn quick_test() {
         let source = r#"
-def main() -> None:
-    if True:
-        some_very_long_variable_name_abcdefghijk = Foo()
-        some_very_long_variable_name_abcdefghijk = some_very_long_variable_name_abcdefghijk[
-            some_very_long_variable_name_abcdefghijk.some_very_long_attribute_name
-            == "This is a very long string abcdefghijk"
-        ]
+this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it = (
+    function()
+)
 
 "#;
         let source_type = PySourceType::Python;
@@ -224,7 +220,8 @@ def main() -> None:
         // Parse the AST.
         let source_path = "code_inline.py";
         let module = parse_ok_tokens(tokens, source, source_type.as_mode(), source_path).unwrap();
-        let options = PyFormatOptions::from_extension(Path::new(source_path));
+        let options = PyFormatOptions::from_extension(Path::new(source_path))
+            .with_preview(PreviewMode::Enabled);
         let formatted = format_module_ast(&module, &comment_ranges, source, options).unwrap();
 
         // Uncomment the `dbg` to print the IR.

crates/ruff_python_formatter/src/preview.rs

@@ -17,3 +17,10 @@ pub(crate) const fn is_hug_parens_with_braces_and_square_brackets_enabled(
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the [`prefer_splitting_right_hand_side_of_assignments`](https://github.com/astral-sh/ruff/issues/6975) preview style is enabled.
+pub(crate) const fn is_prefer_splitting_right_hand_side_of_assignments_enabled(
+    context: &PyFormatContext,
+) -> bool {
+    context.is_preview()
+}

crates/ruff_python_formatter/src/statement/stmt_ann_assign.rs

@@ -2,8 +2,12 @@ use ruff_formatter::write;
 use ruff_python_ast::StmtAnnAssign;
 
 use crate::comments::{SourceComment, SuppressionKind};
+use crate::expression::has_parentheses;
 use crate::prelude::*;
-use crate::statement::stmt_assign::FormatStatementsLastExpression;
+use crate::preview::is_prefer_splitting_right_hand_side_of_assignments_enabled;
+use crate::statement::stmt_assign::{
+    AnyAssignmentOperator, AnyBeforeOperator, FormatStatementsLastExpression,
+};
 use crate::statement::trailing_semicolon;
 
 #[derive(Default)]
@@ -19,21 +23,33 @@ impl FormatNodeRule<StmtAnnAssign> for FormatStmtAnnAssign {
             simple: _,
         } = item;
 
-        write!(
-            f,
-            [target.format(), token(":"), space(), annotation.format(),]
-        )?;
+        write!(f, [target.format(), token(":"), space()])?;
 
         if let Some(value) = value {
-            write!(
-                f,
-                [
-                    space(),
-                    token("="),
-                    space(),
-                    FormatStatementsLastExpression::new(value, item)
-                ]
-            )?;
+            if is_prefer_splitting_right_hand_side_of_assignments_enabled(f.context())
+                && has_parentheses(annotation, f.context()).is_some()
+            {
+                FormatStatementsLastExpression::RightToLeft {
+                    before_operator: AnyBeforeOperator::Expression(annotation),
+                    operator: AnyAssignmentOperator::Assign,
+                    value,
+                    statement: item.into(),
+                }
+                .fmt(f)?;
+            } else {
+                write!(
+                    f,
+                    [
+                        annotation.format(),
+                        space(),
+                        token("="),
+                        space(),
+                        FormatStatementsLastExpression::left_to_right(value, item)
+                    ]
+                )?;
+            }
+        } else {
+            annotation.format().fmt(f)?;
         }
 
         if f.options().source_type().is_ipynb()