Make trailing comma logic more concise (#4202)

Signed-off-by: RedGuy12 <61329810+RedGuy12@users.noreply.github.com>

src/black/linegen.py

@@ -31,12 +31,12 @@
     BRACKETS,
     CLOSING_BRACKETS,
     OPENING_BRACKETS,
-    RARROW,
     STANDALONE_COMMENT,
     STATEMENT,
     WHITESPACE,
     Visitor,
     ensure_visible,
+    get_annotation_type,
     is_arith_like,
     is_async_stmt_or_funcdef,
     is_atom_with_invisible_parens,
@@ -1046,11 +1046,12 @@ def bracket_split_build_line(
         result.inside_brackets = True
         result.depth += 1
         if leaves:
-            # Ensure a trailing comma for imports and standalone function arguments, but
-            # be careful not to add one after any comments or within type annotations.
             no_commas = (
+                # Ensure a trailing comma for imports and standalone function arguments
                 original.is_def
+                # Don't add one after any comments or within type annotations
                 and opening_bracket.value == "("
+                # Don't add one if there's already one there
                 and not any(
                     leaf.type == token.COMMA
                     and (
@@ -1059,22 +1060,9 @@ def bracket_split_build_line(
                     )
                     for leaf in leaves
                 )
-                # In particular, don't add one within a parenthesized return annotation.
-                # Unfortunately the indicator we're in a return annotation (RARROW) may
-                # be defined directly in the parent node, the parent of the parent ...
-                # and so on depending on how complex the return annotation is.
-                # This isn't perfect and there's some false negatives but they are in
-                # contexts were a comma is actually fine.
-                and not any(
-                    node.prev_sibling.type == RARROW
-                    for node in (
-                        leaves[0].parent,
-                        getattr(leaves[0].parent, "parent", None),
-                    )
-                    if isinstance(node, Node) and isinstance(node.prev_sibling, Leaf)
-                )
-                # Except the false negatives above for PEP 604 unions where we
-                # can't add the comma.
+                # Don't add one inside parenthesized return annotations
+                and get_annotation_type(leaves[0]) != "return"
+                # Don't add one inside PEP 604 unions
                 and not (
                     leaves[0].parent
                     and leaves[0].parent.next_sibling

src/black/nodes.py

@@ -3,7 +3,18 @@
 """
 
 import sys
-from typing import Final, Generic, Iterator, List, Optional, Set, Tuple, TypeVar, Union
+from typing import (
+    Final,
+    Generic,
+    Iterator,
+    List,
+    Literal,
+    Optional,
+    Set,
+    Tuple,
+    TypeVar,
+    Union,
+)
 
 if sys.version_info >= (3, 10):
     from typing import TypeGuard
@@ -951,16 +962,21 @@ def is_number_token(nl: NL) -> TypeGuard[Leaf]:
     return nl.type == token.NUMBER
 
 
-def is_part_of_annotation(leaf: Leaf) -> bool:
-    """Returns whether this leaf is part of type annotations."""
+def get_annotation_type(leaf: Leaf) -> Literal["return", "param", None]:
+    """Returns the type of annotation this leaf is part of, if any."""
     ancestor = leaf.parent
     while ancestor is not None:
         if ancestor.prev_sibling and ancestor.prev_sibling.type == token.RARROW:
-            return True
+            return "return"
         if ancestor.parent and ancestor.parent.type == syms.tname:
-            return True
+            return "param"
         ancestor = ancestor.parent
-    return False
+    return None
+
+
+def is_part_of_annotation(leaf: Leaf) -> bool:
+    """Returns whether this leaf is part of a type annotation."""
+    return get_annotation_type(leaf) is not None
 
 
 def first_leaf(node: LN) -> Optional[Leaf]: