From caa3fcc2de3cfb6162bee0bb6f68b10bc38e612f Mon Sep 17 00:00:00 2001
From: James Addison <jay@jp-hosting.net>
Date: Thu, 4 Feb 2021 17:30:05 +0000
Subject: [PATCH] Clarity: isolate and extract each responsibility from an
 overloaded variable

---
 src/black/__init__.py | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/src/black/__init__.py b/src/black/__init__.py
index e398ed0..44017b5 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -1481,6 +1481,7 @@ class Line:
     bracket_tracker: BracketTracker = field(default_factory=BracketTracker)
     inside_brackets: bool = False
     should_explode: bool = False
+    magic_trailing_comma: Optional[Leaf] = None
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
         """Add a new `leaf` to the end of the line.
@@ -1508,7 +1509,7 @@ class Line:
             self.bracket_tracker.mark(leaf)
             if self.mode.magic_trailing_comma:
                 if self.has_magic_trailing_comma(leaf):
-                    self.should_explode = True
+                    self.magic_trailing_comma = leaf
             elif self.has_magic_trailing_comma(leaf, ensure_removable=True):
                 self.remove_trailing_comma()
         if not self.append_comment(leaf):
@@ -1792,6 +1793,7 @@ class Line:
             depth=self.depth,
             inside_brackets=self.inside_brackets,
             should_explode=self.should_explode,
+            magic_trailing_comma=self.magic_trailing_comma,
         )
 
     def __str__(self) -> str:
@@ -2710,7 +2712,7 @@ def transform_line(
     transformers: List[Transformer]
     if (
         not line.contains_uncollapsable_type_comments()
-        and not line.should_explode
+        and not (line.should_explode or line.magic_trailing_comma)
         and (
             is_line_short_enough(line, line_length=mode.line_length, line_str=line_str)
             or line.contains_unsplittable_type_ignore()
@@ -4385,6 +4387,7 @@ class StringParenWrapper(CustomSplitMapMixin, BaseStringSplitter):
             depth=line.depth + 1,
             inside_brackets=True,
             should_explode=line.should_explode,
+            magic_trailing_comma=line.magic_trailing_comma,
         )
         string_leaf = Leaf(token.STRING, string_value)
         insert_str_child(string_leaf)
@@ -5946,7 +5949,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
     """
 
     omit: Set[LeafID] = set()
-    if not line.should_explode:
+    if not line.should_explode and not line.magic_trailing_comma:
         yield omit
 
     length = 4 * line.depth
@@ -5968,7 +5971,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
             elif leaf.type in CLOSING_BRACKETS:
                 prev = line.leaves[index - 1] if index > 0 else None
                 if (
-                    line.should_explode
+                    (line.should_explode or line.magic_trailing_comma)
                     and prev
                     and prev.type == token.COMMA
                     and not is_one_tuple_between(
@@ -5996,7 +5999,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 yield omit
 
             if (
-                line.should_explode
+                (line.should_explode or line.magic_trailing_comma)
                 and prev
                 and prev.type == token.COMMA
                 and not is_one_tuple_between(leaf.opening_bracket, leaf, line.leaves)
@@ -6629,7 +6632,7 @@ def can_omit_invisible_parens(
 
     penultimate = line.leaves[-2]
     last = line.leaves[-1]
-    if line.should_explode:
+    if line.should_explode or line.magic_trailing_comma:
         try:
             penultimate, last = last_two_except(line.leaves, omit=omit_on_explode)
         except LookupError:
@@ -6656,7 +6659,9 @@ def can_omit_invisible_parens(
             # unnecessary.
             return True
 
-        if line.should_explode and penultimate.type == token.COMMA:
+        if (
+            line.should_explode or line.magic_trailing_comma
+        ) and penultimate.type == token.COMMA:
             # The rightmost non-omitted bracket pair is the one we want to explode on.
             return True
 
-- 
2.39.2