Add fallback argument to ConfigManager.get.

This is needed for interpolation since this change in Python 3.4:

https://hg.python.org/cpython/rev/267422f7c927

This broke qutebrowser in Debian experimental when updating python from
3.4.3-8 to 3.4.3-9 as they pulled from hg.

Fixes #968.

qutebrowser/config/config.py

@@ -42,6 +42,9 @@ from qutebrowser.utils import (message, objreg, utils, standarddir, log,
 from qutebrowser.utils.usertypes import Completion
 
 
+UNSET = object()
+
+
 class change_filter:  # pylint: disable=invalid-name
 
     """Decorator to filter calls based on a config section/option matching.
@@ -621,9 +624,13 @@ class ConfigManager(QObject):
         return existed
 
     @functools.lru_cache()
-    def get(self, sectname, optname, raw=False, transformed=True):
+    def get(self, sectname, optname, raw=False, transformed=True,
+            fallback=UNSET):
         """Get the value from a section/option.
 
+        We don't support the vars argument from configparser.get as it's not
+        hashable.
+
         Args:
             sectname: The section to get the option from.
             optname: The option name
@@ -636,13 +643,18 @@ class ConfigManager(QObject):
         if not self._initialized:
             raise Exception("get got called before initialization was "
                             "complete!")
+
         try:
             sect = self.sections[sectname]
         except KeyError:
+            if fallback is not UNSET:
+                return fallback
             raise configexc.NoSectionError(sectname)
         try:
             val = sect[optname]
         except KeyError:
+            if fallback is not UNSET:
+                return fallback
             raise configexc.NoOptionError(optname, sectname)
         if raw:
             return val.value()

tests/unit/config/test_config.py

@@ -66,6 +66,9 @@ class TestConfigParser:
 
     def test_transformed_option_old(self):
         """Test a transformed option with the old name."""
+        # WORKAROUND
+        # Instance of 'object' has no 'name' member (no-member)
+        # pylint: disable=no-member
         self.cp.read_dict({'colors': {'tab.fg.odd': 'pink'}})
         self.cfg._from_cp(self.cp)
         actual = self.cfg.get('colors', 'tabs.fg.odd').name()
@@ -74,6 +77,9 @@ class TestConfigParser:
 
     def test_transformed_option_new(self):
         """Test a transformed section with the new name."""
+        # WORKAROUND
+        # Instance of 'object' has no 'name' member (no-member)
+        # pylint: disable=no-member
         self.cp.read_dict({'colors': {'tabs.fg.odd': 'pink'}})
         self.cfg._from_cp(self.cp)
         actual = self.cfg.get('colors', 'tabs.fg.odd').name()
@@ -154,6 +160,27 @@ class TestConfigParser:
         with pytest.raises(configexc.NoOptionError):
             self.cfg.get('general', 'bar')  # pylint: disable=bad-config-call
 
+    def test_fallback(self):
+        """Test getting an option with fallback.
+
+        This is done during interpolation in later Python 3.4 versions.
+
+        See https://github.com/The-Compiler/qutebrowser/issues/968
+        """
+        # pylint: disable=bad-config-call
+        assert self.cfg.get('general', 'blabla', fallback='blub') == 'blub'
+
+    def test_sectionproxy(self):
+        """Test getting an option via the section proxy."""
+        self.cp.read_dict({'general': {'ignore-case': 'false'}})
+        self.cfg._from_cp(self.cp)
+        assert not self.cfg['general'].get('ignore-case')
+
+    def test_sectionproxy_keyerror(self):
+        """Test getting an inexistent option via the section proxy."""
+        with pytest.raises(configexc.NoOptionError):
+            self.cfg['general'].get('blahblahblub')
+
 
 class TestKeyConfigParser: