Don't explode a one-element collection ending with a comma.

README.md

@@ -178,9 +178,9 @@ example above).
 
 If a data structure literal (tuple, list, set, dict) or a line of "from"
 imports cannot fit in the allotted length, it's always split into one
-per line.  This minimizes diffs as well as enables readers of code to
-find which commit introduced a particular entry.  This also makes
-*Black* compatible with [isort](https://pypi.org/p/isort/).  Use
+element per line.  This minimizes diffs as well as enables readers of
+code to find which commit introduced a particular entry.  This also
+makes *Black* compatible with [isort](https://pypi.org/p/isort/).  Use
 `multi_line_output=3`, `include_trailing_comma=True`,
 `force_grid_wrap=0`, and `line_length=88` in your isort config.
 

black.py

@@ -2532,13 +2532,21 @@ def ensure_visible(leaf: Leaf) -> None:
 
 def should_explode(line: Line, opening_bracket: Leaf) -> bool:
     """Should `line` immediately be split with `delimiter_split()` after RHS?"""
-    return bool(
+    if not (
         opening_bracket.parent
         and opening_bracket.parent.type in {syms.atom, syms.import_from}
         and opening_bracket.value in "[{("
-        and line.bracket_tracker.delimiters
-        and line.bracket_tracker.max_delimiter_priority() == COMMA_PRIORITY
-    )
+    ):
+        return False
+
+    try:
+        last_leaf = line.leaves[-1]
+        exclude = {id(last_leaf)} if last_leaf.type == token.COMMA else set()
+        max_priority = line.bracket_tracker.max_delimiter_priority(exclude=exclude)
+    except (IndexError, ValueError):
+        return False
+
+    return max_priority == COMMA_PRIORITY
 
 
 def is_python36(node: Node) -> bool:

tests/composition.py

@@ -147,7 +147,18 @@ def tricky_asserts(self) -> None:
             "in one line because it's too long"
         )
 
-        # This is weird but true.
+        dis_c_instance_method = """\
+        %3d           0 LOAD_FAST                1 (x)
+                      2 LOAD_CONST               1 (1)
+                      4 COMPARE_OP               2 (==)
+                      6 LOAD_FAST                0 (self)
+                      8 STORE_ATTR               0 (x)
+                     10 LOAD_CONST               0 (None)
+                     12 RETURN_VALUE
+        """ % (
+            _C.__init__.__code__.co_firstlineno + 1,
+        )
+
         assert (
             expectedexpectedexpectedexpectedexpectedexpectedexpectedexpectedexpect
             == {