Avoid magic-trailing-comma in single-element subscripts (#2942)

Closes #2918.

CHANGES.md

@@ -15,6 +15,7 @@
 <!-- Changes that affect Black's preview style -->
 
 - Code cell separators `#%%` are now standardised to `# %%` (#2919)
+- Avoid magic-trailing-comma in single-element subscripts (#2942)
 
 ### _Blackd_
 

src/black/linegen.py

@@ -8,7 +8,12 @@
 from black.nodes import WHITESPACE, RARROW, STATEMENT, STANDALONE_COMMENT
 from black.nodes import ASSIGNMENTS, OPENING_BRACKETS, CLOSING_BRACKETS
 from black.nodes import Visitor, syms, is_arith_like, ensure_visible
-from black.nodes import is_docstring, is_empty_tuple, is_one_tuple, is_one_tuple_between
+from black.nodes import (
+    is_docstring,
+    is_empty_tuple,
+    is_one_tuple,
+    is_one_sequence_between,
+)
 from black.nodes import is_name_token, is_lpar_token, is_rpar_token
 from black.nodes import is_walrus_assignment, is_yield, is_vararg, is_multiline_string
 from black.nodes import is_stub_suite, is_stub_body, is_atom_with_invisible_parens
@@ -973,7 +978,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                     prev
                     and prev.type == token.COMMA
                     and leaf.opening_bracket is not None
-                    and not is_one_tuple_between(
+                    and not is_one_sequence_between(
                         leaf.opening_bracket, leaf, line.leaves
                     )
                 ):
@@ -1001,7 +1006,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 prev
                 and prev.type == token.COMMA
                 and leaf.opening_bracket is not None
-                and not is_one_tuple_between(leaf.opening_bracket, leaf, line.leaves)
+                and not is_one_sequence_between(leaf.opening_bracket, leaf, line.leaves)
             ):
                 # Never omit bracket pairs with trailing commas.
                 # We need to explode on those.

src/black/lines.py

@@ -17,12 +17,12 @@
 from blib2to3.pgen2 import token
 
 from black.brackets import BracketTracker, DOT_PRIORITY
-from black.mode import Mode
+from black.mode import Mode, Preview
 from black.nodes import STANDALONE_COMMENT, TEST_DESCENDANTS
 from black.nodes import BRACKETS, OPENING_BRACKETS, CLOSING_BRACKETS
 from black.nodes import syms, whitespace, replace_child, child_towards
 from black.nodes import is_multiline_string, is_import, is_type_comment
-from black.nodes import is_one_tuple_between
+from black.nodes import is_one_sequence_between
 
 # types
 T = TypeVar("T")
@@ -254,6 +254,7 @@ def has_magic_trailing_comma(
         """Return True if we have a magic trailing comma, that is when:
         - there's a trailing comma here
         - it's not a one-tuple
+        - it's not a single-element subscript
         Additionally, if ensure_removable:
         - it's not from square bracket indexing
         """
@@ -268,6 +269,20 @@ def has_magic_trailing_comma(
             return True
 
         if closing.type == token.RSQB:
+            if (
+                Preview.one_element_subscript in self.mode
+                and closing.parent
+                and closing.parent.type == syms.trailer
+                and closing.opening_bracket
+                and is_one_sequence_between(
+                    closing.opening_bracket,
+                    closing,
+                    self.leaves,
+                    brackets=(token.LSQB, token.RSQB),
+                )
+            ):
+                return False
+
             if not ensure_removable:
                 return True
             comma = self.leaves[-1]
@@ -276,7 +291,7 @@ def has_magic_trailing_comma(
         if self.is_import:
             return True
 
-        if closing.opening_bracket is not None and not is_one_tuple_between(
+        if closing.opening_bracket is not None and not is_one_sequence_between(
             closing.opening_bracket, closing, self.leaves
         ):
             return True

src/black/mode.py

@@ -127,6 +127,7 @@ class Preview(Enum):
     """Individual preview style features."""
 
     string_processing = auto()
+    one_element_subscript = auto()
 
 
 class Deprecated(UserWarning):
@@ -162,9 +163,7 @@ def __contains__(self, feature: Preview) -> bool:
         """
         if feature is Preview.string_processing:
             return self.preview or self.experimental_string_processing
-        # TODO: Remove type ignore comment once preview contains more features
+        return self.preview
-        #  than just ESP
-        return self.preview  # type: ignore
 
     def get_cache_key(self) -> str:
         if self.target_versions:

src/black/nodes.py

@@ -9,6 +9,7 @@
     List,
     Optional,
     Set,
+    Tuple,
     TypeVar,
     Union,
 )
@@ -559,9 +560,14 @@ def is_one_tuple(node: LN) -> bool:
     )
 
 
-def is_one_tuple_between(opening: Leaf, closing: Leaf, leaves: List[Leaf]) -> bool:
+def is_one_sequence_between(
-    """Return True if content between `opening` and `closing` looks like a one-tuple."""
+    opening: Leaf,
-    if opening.type != token.LPAR and closing.type != token.RPAR:
+    closing: Leaf,
+    leaves: List[Leaf],
+    brackets: Tuple[int, int] = (token.LPAR, token.RPAR),
+) -> bool:
+    """Return True if content between `opening` and `closing` is a one-sequence."""
+    if (opening.type, closing.type) != brackets:
         return False
 
     depth = closing.bracket_depth + 1

tests/data/one_element_subscript.py

@@ -0,0 +1,36 @@
+# We should not treat the trailing comma
+# in a single-element subscript.
+a: tuple[int,]
+b = tuple[int,]
+
+# The magic comma still applies to multi-element subscripts.
+c: tuple[int, int,]
+d = tuple[int, int,]
+
+# Magic commas still work as expected for non-subscripts.
+small_list = [1,]
+list_of_types = [tuple[int,],]
+
+# output
+# We should not treat the trailing comma
+# in a single-element subscript.
+a: tuple[int,]
+b = tuple[int,]
+
+# The magic comma still applies to multi-element subscripts.
+c: tuple[
+    int,
+    int,
+]
+d = tuple[
+    int,
+    int,
+]
+
+# Magic commas still work as expected for non-subscripts.
+small_list = [
+    1,
+]
+list_of_types = [
+    tuple[int,],
+]

tests/test_format.py

@@ -80,6 +80,7 @@
     "long_strings__edge_case",
     "long_strings__regression",
     "percent_precedence",
+    "one_element_subscript",
 ]
 
 SOURCES: List[str] = [