Make sure we never compare against an uninit'ed backend

qutebrowser/config/config.py

@@ -393,7 +393,7 @@ class Config(QObject):
 
     def _set_value(self, opt, value):
         """Set the given option to the given value."""
-        if objects.backend is not None:
+        if not isinstance(objects.backend, objects.NoBackend):
             if objects.backend not in opt.backends:
                 raise configexc.BackendError(objects.backend)
 

qutebrowser/misc/objects.py

@@ -22,5 +22,14 @@
 # NOTE: We need to be careful with imports here, as this is imported from
 # earlyinit.
 
+
+class NoBackend:
+
+    """Special object when there's no backend set so we notice that."""
+
+    def __eq__(self, other):
+        raise AssertionError("No backend set!")
+
+
 # A usertypes.Backend member
-backend = None
+backend = NoBackend()

tests/unit/misc/test_objects.py

@@ -0,0 +1,33 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2017 Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
+
+import pytest
+
+from qutebrowser.misc import objects
+from qutebrowser.utils import usertypes
+
+
+@pytest.mark.parametrize('func', [
+    lambda: objects.NoBackend() == usertypes.Backend.QtWebEngine,
+    lambda: objects.NoBackend() != usertypes.Backend.QtWebEngine,
+    lambda: objects.NoBackend() in [usertypes.Backend.QtWebEngine],
+])
+def test_no_backend(func):
+    with pytest.raises(AssertionError, match='No backend set!'):
+        func()