Add trailing comma for single as imports, too

black.py

@@ -2329,8 +2329,9 @@ def bracket_split_build_line(
             # Since body is a new indent level, remove spurious leading whitespace.
             normalize_prefix(leaves[0], inside_brackets=True)
             # Ensure a trailing comma when expected.
-            if original.is_import and len(leaves) == 1:
+            if original.is_import:
-                leaves.append(Leaf(token.COMMA, ","))
+                if leaves[-1].type != token.COMMA:
+                    leaves.append(Leaf(token.COMMA, ","))
     # Populate the line
     for leaf in leaves:
         result.append(leaf, preformatted=True)

docs/reference/reference_functions.rst

@@ -79,6 +79,8 @@ Parsing
 Split functions
 ---------------
 
+.. autofunction:: black.bracket_split_build_line
+
 .. autofunction:: black.bracket_split_succeeded_or_raise
 
 .. autofunction:: black.delimiter_split

tests/data/comments2.py

@@ -1,3 +1,10 @@
+from com.my_lovely_company.my_lovely_team.my_lovely_project.my_lovely_component import (
+    MyLovelyCompanyTeamProjectComponent  # NOT DRY
+)
+from com.my_lovely_company.my_lovely_team.my_lovely_project.my_lovely_component import (
+    MyLovelyCompanyTeamProjectComponent as component  # DRY
+)
+
 # Please keep __all__ alphabetized within each category.
 
 __all__ = [
@@ -148,6 +155,13 @@ def inline_comments_in_brackets_ruin_everything():
 # output
 
 
+from com.my_lovely_company.my_lovely_team.my_lovely_project.my_lovely_component import (
+    MyLovelyCompanyTeamProjectComponent,  # NOT DRY
+)
+from com.my_lovely_company.my_lovely_team.my_lovely_project.my_lovely_component import (
+    MyLovelyCompanyTeamProjectComponent as component,  # DRY
+)
+
 # Please keep __all__ alphabetized within each category.
 
 __all__ = [

tests/data/comments4.py

@@ -1,3 +1,11 @@
+from com.my_lovely_company.my_lovely_team.my_lovely_project.my_lovely_component import (
+    MyLovelyCompanyTeamProjectComponent,  # NOT DRY
+)
+from com.my_lovely_company.my_lovely_team.my_lovely_project.my_lovely_component import (
+    MyLovelyCompanyTeamProjectComponent as component,  # DRY
+)
+
+
 class C:
     @pytest.mark.parametrize(
         ("post_data", "message"),