Strip trailing commas in subscripts with -C (psf#3209)

Fixes psf#2296, psf#3204

CHANGES.md

@@ -24,6 +24,8 @@
   this is invalid. This was a bug introduced in version 22.6.0. (#3166)
 - `--skip-string-normalization` / `-S` now prevents docstring prefixes from being
   normalized as expected (#3168)
+- When using `--skip-magic-trailing-comma` or `-C`, trailing commas are stripped from
+  subscript expressions with more than 1 element (#3209)
 
 ### _Blackd_
 

src/black/lines.py

@@ -273,6 +273,8 @@ def has_magic_trailing_comma(
         - it's not a single-element subscript
         Additionally, if ensure_removable:
         - it's not from square bracket indexing
+        (specifically, single-element square bracket indexing with
+        Preview.skip_magic_trailing_comma_in_subscript)
         """
         if not (
             closing.type in CLOSING_BRACKETS
@@ -301,8 +303,22 @@ def has_magic_trailing_comma(
 
             if not ensure_removable:
                 return True
+
             comma = self.leaves[-1]
-            return bool(comma.parent and comma.parent.type == syms.listmaker)
+            if comma.parent is None:
+                return False
+            if Preview.skip_magic_trailing_comma_in_subscript in self.mode:
+                return (
+                    comma.parent.type != syms.subscriptlist
+                    or closing.opening_bracket is None
+                    or not is_one_sequence_between(
+                        closing.opening_bracket,
+                        closing,
+                        self.leaves,
+                        brackets=(token.LSQB, token.RSQB),
+                    )
+                )
+            return comma.parent.type == syms.listmaker
 
         if self.is_import:
             return True

src/black/mode.py

@@ -156,6 +156,7 @@ class Preview(Enum):
     remove_block_trailing_newline = auto()
     remove_redundant_parens = auto()
     string_processing = auto()
+    skip_magic_trailing_comma_in_subscript = auto()
 
 
 class Deprecated(UserWarning):

tests/data/preview/skip_magic_trailing_comma.py

@@ -0,0 +1,34 @@
+# We should not remove the trailing comma in a single-element subscript.
+a: tuple[int,]
+b = tuple[int,]
+
+# But commas in multiple element subscripts should be removed.
+c: tuple[int, int,]
+d = tuple[int, int,]
+
+# Remove commas for non-subscripts.
+small_list = [1,]
+list_of_types = [tuple[int,],]
+small_set = {1,}
+set_of_types = {tuple[int,],}
+
+# Except single element tuples
+small_tuple = (1,)
+
+# output
+# We should not remove the trailing comma in a single-element subscript.
+a: tuple[int,]
+b = tuple[int,]
+
+# But commas in multiple element subscripts should be removed.
+c: tuple[int, int]
+d = tuple[int, int]
+
+# Remove commas for non-subscripts.
+small_list = [1]
+list_of_types = [tuple[int,]]
+small_set = {1}
+set_of_types = {tuple[int,]}
+
+# Except single element tuples
+small_tuple = (1,)

tests/test_format.py

@@ -36,7 +36,12 @@ def test_simple_format(filename: str) -> None:
 
 @pytest.mark.parametrize("filename", all_data_cases("preview"))
 def test_preview_format(filename: str) -> None:
-    check_file("preview", filename, black.Mode(preview=True))
+    magic_trailing_comma = filename != "skip_magic_trailing_comma"
+    check_file(
+        "preview",
+        filename,
+        black.Mode(preview=True, magic_trailing_comma=magic_trailing_comma),
+    )
 
 
 @pytest.mark.parametrize("filename", all_data_cases("preview_39"))