Crash on assert and parenthesized % format

[JelleZijlstra]

$ cat author.py 
class xxxxxxxxxxxxxxxxxxxxx(xxxx.xxxxxxxxxxxxx):
    def xxxxxxx_xxxxxx(xxxx):
        assert xxxxxxx_xxxx in [
            x.xxxxx.xxxxxx.xxxxx.xxxxxx,
            x.xxxxx.xxxxxx.xxxxx.xxxx,
        ], ("xxxxxxxxxxx xxxxxxx xxxx (xxxxxx xxxx) %x xxx xxxxx" % xxxxxxx_xxxx)
$ black -v author.py 
Using configuration from /Users/jelle/py/black/pyproject.toml.
Traceback (most recent call last):
  File "/Users/jelle/py/black/src/black/__init__.py", line 648, in reformat_one
    if changed is not Changed.CACHED and format_file_in_place(
  File "/Users/jelle/py/black/src/black/__init__.py", line 791, in format_file_in_place
    dst_contents = format_file_contents(src_contents, fast=fast, mode=mode)
  File "/Users/jelle/py/black/src/black/__init__.py", line 912, in format_file_contents
    dst_contents = format_str(src_contents, mode=mode)
  File "/Users/jelle/py/black/src/black/__init__.py", line 977, in format_str
    for line in transform_line(
  File "/Users/jelle/py/black/src/black/__init__.py", line 2731, in transform_line
    result.extend(
  File "/Users/jelle/py/black/src/black/__init__.py", line 2725, in transform_line
    for transformed_line in transform(line, features):
  File "/Users/jelle/py/black/src/black/__init__.py", line 2834, in __call__
    for line_result in self.do_transform(line, string_idx):
  File "/Users/jelle/py/black/src/black/__init__.py", line 3315, in do_transform
    append_leaves(new_line, line, LL[: string_idx - 1])
  File "/Users/jelle/py/black/src/black/__init__.py", line 4601, in append_leaves
    new_line.append(new_leaf)
  File "/Users/jelle/py/black/src/black/__init__.py", line 1425, in append
    self.bracket_tracker.mark(leaf)
  File "/Users/jelle/py/black/src/black/__init__.py", line 1291, in mark
    opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
KeyError: (-1, 10)
error: cannot format author.py: (-1, 10)
Oh no! 💥 💔 💥
1 file failed to reformat.

[hugovk]

A third git bisect pointing to 544ea9c from PR #1132 "Improve String Handling".

[JelleZijlstra]

#1605 has another example of what's probably the same underlying bug. (I saw what looked like the same crash on several other files in my codebase too, but didn't try to minify the failures.)

[hugovk]

This file now formats without errors with master.

$ black --version
black, version 19.10b1.dev154+gcd3a93a
$ black --check author.py
All done! ✨ 🍰 ✨
1 file would be left unchanged.

[bbugyi200]

I was not able to replicate this on master with or without the --experimental-string-processing option. With that said, I included a test case for it in #1681 just in case.

I was, however, able to replicate #1605. This was fixed in #1681 and a corresponding test case was added to black's regression tests.