Fix --experiemental-string-processing crash when matching parens not found (#2283)

Fixes #2271

CHANGES.md

@@ -10,6 +10,7 @@
   `.gitignore` rules like `git` does) (#2225)
 - Restored compatibility with Click 8.0 on Python 3.6 when LANG=C used (#2227)
 - Add extra uvloop install + import support if in python env (#2258)
+- Fix --experimental-string-processing crash when matching parens are not found (#2283)
 
 ### _Blackd_
 

src/black/trans.py

@@ -25,7 +25,7 @@
 from black.mode import Feature
 from black.nodes import syms, replace_child, parent_type
 from black.nodes import is_empty_par, is_empty_lpar, is_empty_rpar
-from black.nodes import CLOSING_BRACKETS, STANDALONE_COMMENT
+from black.nodes import OPENING_BRACKETS, CLOSING_BRACKETS, STANDALONE_COMMENT
 from black.lines import Line, append_leaves
 from black.brackets import BracketMatchError
 from black.comments import contains_pragma_comment
@@ -1398,6 +1398,11 @@ class StringParenWrapper(CustomSplitMapMixin, BaseStringSplitter):
     def do_splitter_match(self, line: Line) -> TMatchResult:
         LL = line.leaves
 
+        if line.leaves[-1].type in OPENING_BRACKETS:
+            return TErr(
+                "Cannot wrap parens around a line that ends in an opening bracket."
+            )
+
         string_idx = (
             self._return_match(LL)
             or self._else_match(LL)
@@ -1665,9 +1670,10 @@ def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
                 right_leaves.pop()
 
             if old_parens_exist:
-                assert (
+                assert right_leaves and right_leaves[-1].type == token.RPAR, (
-                    right_leaves and right_leaves[-1].type == token.RPAR
+                    "Apparently, old parentheses do NOT exist?!"
-                ), "Apparently, old parentheses do NOT exist?!"
+                    f" (left_leaves={left_leaves}, right_leaves={right_leaves})"
+                )
                 old_rpar_leaf = right_leaves.pop()
 
             append_leaves(string_line, line, right_leaves)

tests/data/long_strings__regression.py

@@ -396,6 +396,16 @@ def xxxxxxx_xxxxxx(xxxx):
     " it has now"
 )
 
+
+def _legacy_listen_examples():
+    text += (
+        "    \"listen for the '%(event_name)s' event\"\n"
+        "\n    # ... (event logic logic logic) ...\n"
+        % {
+            "since": since,
+        }
+    )
+
 # output
 
 
@@ -886,3 +896,13 @@ def xxxxxxx_xxxxxx(xxxx):
     " it goes over 88 characters which"
     " it has now"
 )
+
+
+def _legacy_listen_examples():
+    text += (
+        "    \"listen for the '%(event_name)s' event\"\n"
+        "\n    # ... (event logic logic logic) ...\n"
+        % {
+            "since": since,
+        }
+    )