# Buggy code
def _match_key(self, sequence):
    """Try to match a given keystring with any bound keychain.

    Args:
        sequence: The command string to find.

    Return:
        A tuple (matchtype, binding).
            matchtype: Match.definitive, Match.partial or Match.none.
            binding: - None with Match.partial/Match.none.
                     - The found binding with Match.definitive.
    """
    assert sequence
    assert not isinstance(sequence, str)
    result = QKeySequence.NoMatch

    for seq, cmd in self.bindings.items():
        assert not isinstance(seq, str), seq
        match = sequence.matches(seq)
        if match == QKeySequence.ExactMatch:
            return (match, cmd)
        elif match == QKeySequence.PartialMatch:
            result = QKeySequence.PartialMatch

    return (result, None)


# Traceback (ValueError)
16:39:44 DEBUG    init       app:_init_modules:468 Initializing websettings...
16:39:45 ERROR    misc       crashsignal:exception_hook:216 Uncaught exception
Traceback (most recent call last):
File "/bin/qutebrowser", line 11, in <module>
load_entry_point('qutebrowser==1.1.1', 'gui_scripts', 'qutebrowser')()
File "/usr/lib/python3.6/site-packages/qutebrowser/qutebrowser.py", line 188, in main
return app.run(args)
File "/usr/lib/python3.6/site-packages/qutebrowser/app.py", line 137, in run
init(args, crash_handler)
File "/usr/lib/python3.6/site-packages/qutebrowser/app.py", line 163, in init
_init_modules(args, crash_handler)
File "/usr/lib/python3.6/site-packages/qutebrowser/app.py", line 469, in _init_modules
websettings.init(args)
File "/usr/lib/python3.6/site-packages/qutebrowser/config/websettings.py", line 215, in init
webenginesettings.init(args)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 297, in init
websettings.init_mappings(MAPPINGS)
File "/usr/lib/python3.6/site-packages/qutebrowser/config/websettings.py", line 198, in init_mappings
mapping.set(value)
File "/usr/lib/python3.6/site-packages/qutebrowser/config/websettings.py", line 72, in set
self._set(value, settings=settings)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 154, in _set
filenames = [self._find_installed(code) for code in value]
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 154, in <listcomp>
filenames = [self._find_installed(code) for code in value]
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 143, in _find_installed
local_filename = spell.local_filename(code)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/spell.py", line 64, in local_filename
all_installed = local_files(code)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/spell.py", line 51, in local_files
for matching_dict in sorted(matching_dicts, key=version, reverse=True):
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/spell.py", line 36, in version
.format(filename))
ValueError: the given dictionary file name is malformed: /usr/share/qt/qtwebengine_dictionaries/en-US.bdic

# Fixed code
def _match_key(self, sequence):
    """Try to match a given keystring with any bound keychain.

    Args:
        sequence: The command string to find.

    Return:
        A tuple (matchtype, binding).
            matchtype: Match.definitive, Match.partial or Match.none.
            binding: - None with Match.partial/Match.none.
                     - The found binding with Match.definitive.
    """
    assert sequence
    assert not isinstance(sequence, str)
    result = QKeySequence.NoMatch

    for seq, cmd in self.bindings.items():
        assert not isinstance(seq, str), seq
        match = sequence.matches(seq)
        if match == QKeySequence.ExactMatch:
            return match, cmd
        elif match == QKeySequence.PartialMatch:
            result = QKeySequence.PartialMatch

    return result, None