# Buggy code
def append_event(self, ev):
    """Create a new KeySequence object with the given QKeyEvent added."""
    key = ev.key()
    modifiers = ev.modifiers()

    _assert_plain_key(key)
    _assert_plain_modifier(modifiers)

    if key == 0x0:
        raise KeyParseError(None, "Got nil key!")

    # We always remove Qt.GroupSwitchModifier because QKeySequence has no
    # way to mention that in a binding anyways...
    modifiers &= ~Qt.GroupSwitchModifier

    # We change Qt.Key_Backtab to Key_Tab here because nobody would
    # configure "Shift-Backtab" in their config.
    if modifiers & Qt.ShiftModifier and key == Qt.Key_Backtab:
        key = Qt.Key_Tab

    # We don't care about a shift modifier with symbols (Shift-: should
    # match a : binding even though we typed it with a shift on an
    # US-keyboard)
    #
    # However, we *do* care about Shift being involved if we got an
    # upper-case letter, as Shift-A should match a Shift-A binding, but not
    # an "a" binding.
    #
    # In addition, Shift also *is* relevant when other modifiers are
    # involved. Shift-Ctrl-X should not be equivalent to Ctrl-X.
    if (
        modifiers == Qt.ShiftModifier
        and _is_printable(ev.key())
        and not ev.text().isupper()
    ):
        modifiers = Qt.KeyboardModifiers()

    keys = list(self._iter_keys())
    keys.append(key | int(modifiers))

    return self.__class__(*keys)


# 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 append_event(self, ev):
    """Create a new KeySequence object with the given QKeyEvent added."""
    key = ev.key()
    modifiers = ev.modifiers()

    _assert_plain_key(key)
    _assert_plain_modifier(modifiers)

    if key == 0x0:
        raise KeyParseError(None, "Got nil key!")

    # We always remove Qt.GroupSwitchModifier because QKeySequence has no
    # way to mention that in a binding anyways...
    modifiers &= ~Qt.GroupSwitchModifier

    # We change Qt.Key_Backtab to Key_Tab here because nobody would
    # configure "Shift-Backtab" in their config.
    if modifiers & Qt.ShiftModifier and key == Qt.Key_Backtab:
        key = Qt.Key_Tab

    # We don't care about a shift modifier with symbols (Shift-: should
    # match a : binding even though we typed it with a shift on an
    # US-keyboard)
    #
    # However, we *do* care about Shift being involved if we got an
    # upper-case letter, as Shift-A should match a Shift-A binding, but not
    # an "a" binding.
    #
    # In addition, Shift also *is* relevant when other modifiers are
    # involved. Shift-Ctrl-X should not be equivalent to Ctrl-X.
    if (
        modifiers == Qt.ShiftModifier
        and _is_printable(ev.key())
        and not ev.text().isupper()
    ):
        modifiers = Qt.KeyboardModifiers()

    # On macOS, swap Ctrl and Meta back
    # WORKAROUND for https://bugreports.qt.io/browse/QTBUG-51293
    if utils.is_mac:
        if modifiers & Qt.ControlModifier and modifiers & Qt.MetaModifier:
            pass
        elif modifiers & Qt.ControlModifier:
            modifiers &= ~Qt.ControlModifier
            modifiers |= Qt.MetaModifier
        elif modifiers & Qt.MetaModifier:
            modifiers &= ~Qt.MetaModifier
            modifiers |= Qt.ControlModifier

    keys = list(self._iter_keys())
    keys.append(key | int(modifiers))

    return self.__class__(*keys)