chore: Refactor delimiter_split() (#4257)

Signed-off-by: RedGuy12 <[email protected]>

src/black/linegen.py

@@ -1135,6 +1135,14 @@ def _get_last_non_comment_leaf(line: Line) -> Optional[int]:
     return None
 
 
+def _can_add_trailing_comma(leaf: Leaf, features: Collection[Feature]) -> bool:
+    if is_vararg(leaf, within={syms.typedargslist}):
+        return Feature.TRAILING_COMMA_IN_DEF in features
+    if is_vararg(leaf, within={syms.arglist, syms.argument}):
+        return Feature.TRAILING_COMMA_IN_CALL in features
+    return True
+
+
 def _safe_add_trailing_comma(safe: bool, delimiter_priority: int, line: Line) -> Line:
     if (
         safe
@@ -1156,20 +1164,21 @@ def delimiter_split(
     If the appropriate Features are given, the split will add trailing commas
     also in function signatures and calls that contain `*` and `**`.
     """
-    try:
-        last_leaf = line.leaves[-1]
-    except IndexError:
+    if len(line.leaves) == 0:
         raise CannotSplit("Line empty") from None
+    last_leaf = line.leaves[-1]
 
     bt = line.bracket_tracker
     try:
         delimiter_priority = bt.max_delimiter_priority(exclude={id(last_leaf)})
     except ValueError:
         raise CannotSplit("No delimiters found") from None
 
-    if delimiter_priority == DOT_PRIORITY:
-        if bt.delimiter_count_with_priority(delimiter_priority) == 1:
-            raise CannotSplit("Splitting a single attribute from its owner looks wrong")
+    if (
+        delimiter_priority == DOT_PRIORITY
+        and bt.delimiter_count_with_priority(delimiter_priority) == 1
+    ):
+        raise CannotSplit("Splitting a single attribute from its owner looks wrong")
 
     current_line = Line(
         mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
@@ -1198,15 +1207,8 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
             yield from append_to_line(comment_after)
 
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
-        if leaf.bracket_depth == lowest_depth:
-            if is_vararg(leaf, within={syms.typedargslist}):
-                trailing_comma_safe = (
-                    trailing_comma_safe and Feature.TRAILING_COMMA_IN_DEF in features
-                )
-            elif is_vararg(leaf, within={syms.arglist, syms.argument}):
-                trailing_comma_safe = (
-                    trailing_comma_safe and Feature.TRAILING_COMMA_IN_CALL in features
-                )
+        if trailing_comma_safe and leaf.bracket_depth == lowest_depth:
+            trailing_comma_safe = _can_add_trailing_comma(leaf, features)
 
         if last_leaf.type == STANDALONE_COMMENT and leaf_idx == last_non_comment_leaf:
             current_line = _safe_add_trailing_comma(
@@ -1220,6 +1222,7 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
             current_line = Line(
                 mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
             )
+
     if current_line:
         current_line = _safe_add_trailing_comma(
             trailing_comma_safe, delimiter_priority, current_line