Adding trailing commas to dictionaries when comments are present

at the end

CHANGES.md

@@ -21,6 +21,8 @@
   parentheses (#3307)
 - Correctly handle trailing commas that are inside a line's leading non-nested parens
   (#3370)
+- Correctly handle trailing commas inside dictionaries where a comment is after the
+  final entry (#3393)
 
 ### Configuration
 

src/black/linegen.py

@@ -830,6 +830,25 @@ 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.
@@ -871,7 +890,8 @@ 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):
@@ -888,6 +908,11 @@ 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
@@ -896,14 +921,9 @@ 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 +1,11 @@
 def f(a,):
     d = {'key': 'value',}
+    e = {
+        "a": fun(msg, "ts"),
+        "longggggggggggggggid": ...,
+        "longgggggggggggggggggggkey": ..., "created": ...
+        # "longkey": ...
+    }
     tup = (1,)
 
 def f2(a,b,):
@@ -68,6 +74,13 @@ def f(
     d = {
         "key": "value",
     }
+    e = {
+        "a": fun(msg, "ts"),
+        "longggggggggggggggid": ...,
+        "longgggggggggggggggggggkey": ...,
+        "created": ...,
+        # "longkey": ...
+    }
     tup = (1,)