From ca2f3ddadc97f871bc3d69eb023afb4be361e629 Mon Sep 17 00:00:00 2001
From: HoraDomu <dommcpro@gmail.com>
Date: Tue, 21 Apr 2026 17:17:05 -0500
Subject: [PATCH]  Fix comment before binary op causing unnecessary line split
 (#3713)               A standalone comment before an arithmetic/comparator
 expression was inside brackets and caused delimiter_split to use the operator
 as a split point. It was breaking the expression across lines. The fix was
 when the first leaf is a standalone comment and the delimter priority is
 below LOGIC_PRIORITY, raise CannotSplit to defer to standalone_comment_split.
 This peels off the comment first, and then lets the expression stay on one
 line if it fits.

---
 src/black/linegen.py                         | 16 ++++++++
 tests/data/cases/comment_before_binary_op.py | 42 ++++++++++++++++++++
 tests/data/cases/function_trailing_comma.py  |  4 +-
 3 files changed, 59 insertions(+), 3 deletions(-)
 create mode 100644 tests/data/cases/comment_before_binary_op.py

diff --git a/src/black/linegen.py b/src/black/linegen.py
index 72f7366489d..1abf0c76e6d 100644
--- a/src/black/linegen.py
+++ b/src/black/linegen.py
@@ -13,6 +13,7 @@
 from black.brackets import (
     COMMA_PRIORITY,
     DOT_PRIORITY,
+    LOGIC_PRIORITY,
     STRING_PRIORITY,
     get_leaves_inside_matching_brackets,
     max_delimiter_priority_in_atom,
@@ -1384,6 +1385,21 @@ def delimiter_split(
     ):
         raise CannotSplit("Splitting a single attribute from its owner looks wrong")
 
+    # When a standalone comment is the first token on the line and precedes an
+    # arithmetic/comparator expression, let standalone_comment_split handle it so
+    # the expression can stay on one line if it fits (issue #3713). Only applies
+    # when the comment leads the line (not trailing) and the delimiter has lower
+    # priority than logical operators to avoid changing `or`/`and` behavior.
+    if (
+        line.leaves
+        and line.leaves[0].type == STANDALONE_COMMENT
+        and delimiter_priority < LOGIC_PRIORITY
+    ):
+        raise CannotSplit(
+            "Standalone comment leads line before arithmetic delimiter; deferring to"
+            " standalone_comment_split"
+        )
+
     current_line = Line(
         mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
     )
diff --git a/tests/data/cases/comment_before_binary_op.py b/tests/data/cases/comment_before_binary_op.py
new file mode 100644
index 00000000000..b61b1888618
--- /dev/null
+++ b/tests/data/cases/comment_before_binary_op.py
@@ -0,0 +1,42 @@
+my_func(
+    arg1=1,
+    arg2=[
+        func_call(
+            # Comment.
+            [MyClass(arg1, arg2)] * 10000
+        ),
+    ],
+)
+
+result = func_call(
+    # Comment.
+    value * 10000
+)
+
+result = func_call(
+    # Comment.
+    value + other_value
+)
+
+
+# output
+
+my_func(
+    arg1=1,
+    arg2=[
+        func_call(
+            # Comment.
+            [MyClass(arg1, arg2)] * 10000
+        ),
+    ],
+)
+
+result = func_call(
+    # Comment.
+    value * 10000
+)
+
+result = func_call(
+    # Comment.
+    value + other_value
+)
diff --git a/tests/data/cases/function_trailing_comma.py b/tests/data/cases/function_trailing_comma.py
index 63cf3999c2e..c4b0f3f4fb8 100644
--- a/tests/data/cases/function_trailing_comma.py
+++ b/tests/data/cases/function_trailing_comma.py
@@ -252,9 +252,7 @@ def foo() -> (
 
 def foo() -> (
     # comment inside parenthesised new union return type
-    int
-    | str
-    | bytes
+    int | str | bytes
 ): ...