Actually add trailing commas to collection literals even if there are terminating comments

CHANGES.md

@@ -16,6 +16,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Correctly handle trailing commas inside dictionaries where a comment is after the
+  final entry (#3393)
 - Improve the performance on large expressions that contain many strings (#3467)
 - Fix a crash in preview style with assert + parenthesized string (#3415)
 - Fix crashes in preview style with walrus operators used in function return annotations

src/black/linegen.py

@@ -938,6 +938,25 @@ def split_wrapper(line: Line, features: Collection[Feature] = ()) -> Iterator[Li
     return split_wrapper
 
 
+def _get_last_non_comment_leaf(line: Line) -> Optional[int]:
+    for leaf_idx in range(len(line.leaves) - 1, 0, -1):
+        if line.leaves[leaf_idx].type != STANDALONE_COMMENT:
+            return leaf_idx
+    return None
+
+
+def _safe_add_trailing_comma(safe: bool, delimiter_priority: int, line: Line) -> Line:
+    if (
+        safe
+        and delimiter_priority == COMMA_PRIORITY
+        and line.leaves[-1].type != token.COMMA
+        and line.leaves[-1].type != STANDALONE_COMMENT
+    ):
+        new_comma = Leaf(token.COMMA, ",")
+        line.append(new_comma)
+    return line
+
+
 @dont_increase_indentation
 def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:
     """Split according to delimiters of the highest priority.
@@ -979,7 +998,8 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
             )
             current_line.append(leaf)
 
-    for leaf in line.leaves:
+    last_non_comment_leaf = _get_last_non_comment_leaf(line)
+    for leaf_idx, leaf in enumerate(line.leaves):
         yield from append_to_line(leaf)
 
         for comment_after in line.comments_after(leaf):
@@ -996,6 +1016,11 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
                     trailing_comma_safe and Feature.TRAILING_COMMA_IN_CALL in features
                 )
 
+        if last_leaf.type == STANDALONE_COMMENT and leaf_idx == last_non_comment_leaf:
+            current_line = _safe_add_trailing_comma(
+                trailing_comma_safe, delimiter_priority, current_line
+            )
+
         leaf_priority = bt.delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
             yield current_line
@@ -1004,14 +1029,9 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
                 mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
             )
     if current_line:
-        if (
-            trailing_comma_safe
-            and delimiter_priority == COMMA_PRIORITY
-            and current_line.leaves[-1].type != token.COMMA
-            and current_line.leaves[-1].type != STANDALONE_COMMENT
-        ):
-            new_comma = Leaf(token.COMMA, ",")
-            current_line.append(new_comma)
+        current_line = _safe_add_trailing_comma(
+            trailing_comma_safe, delimiter_priority, current_line
+        )
         yield current_line
 
 

tests/data/simple_cases/function_trailing_comma.py

@@ -1,5 +1,29 @@
 def f(a,):
     d = {'key': 'value',}
+    e = {
+        "a": fun(msg, "ts"),
+        "longggggggggggggggid": ...,
+        "longgggggggggggggggggggkey": ..., "created": ...
+        # "longkey": ...
+    }
+    f = [
+        arg1,
+        arg2,
+        arg3, arg4
+        # comment
+    ]
+    g = (
+        arg1,
+        arg2,
+        arg3, arg4
+        # comment
+    )
+    h = {
+        arg1,
+        arg2,
+        arg3, arg4
+        # comment
+    }
     tup = (1,)
 
 def f2(a,b,):
@@ -68,6 +92,34 @@ def f(
     d = {
         "key": "value",
     }
+    e = {
+        "a": fun(msg, "ts"),
+        "longggggggggggggggid": ...,
+        "longgggggggggggggggggggkey": ...,
+        "created": ...,
+        # "longkey": ...
+    }
+    f = [
+        arg1,
+        arg2,
+        arg3,
+        arg4,
+        # comment
+    ]
+    g = (
+        arg1,
+        arg2,
+        arg3,
+        arg4,
+        # comment
+    )
+    h = {
+        arg1,
+        arg2,
+        arg3,
+        arg4,
+        # comment
+    }
     tup = (1,)