# Buggy code
def on_load_progress(self, tab, perc):
    """Adjust tab indicator on load progress."""
    try:
        idx = self._tab_index(tab)
    except TabDeletedError:
        # We can get signals for tabs we already deleted...
        return
    start = config.val.colors.tabs.indicator.start
    stop = config.val.colors.tabs.indicator.stop
    system = config.val.colors.tabs.indicator.system
    color = utils.interpolate_color(start, stop, perc, system)
    self.set_tab_indicator_color(idx, color)
    self._update_tab_title(idx)
    if idx == self.currentIndex():
        self._update_window_title()


# 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 on_load_progress(self, tab, perc):
    """Adjust tab indicator on load progress."""
    try:
        idx = self._tab_index(tab)
    except TabDeletedError:
        # We can get signals for tabs we already deleted...
        return
    start = config.val.colors.tabs.indicator.start
    stop = config.val.colors.tabs.indicator.stop
    system = config.val.colors.tabs.indicator.system
    color = utils.interpolate_color(start, stop, perc, system)
    self.widget.set_tab_indicator_color(idx, color)
    self.widget.update_tab_title(idx)
    if idx == self.widget.currentIndex():
        self._update_window_title()