Don't make default config a mutable global.

Before, configdata.DATA only existed once - that means when something
manipulated it, instantiating a new ConfigManager actually gave us the
*modified* rather than the default data.

There's still a (now readonly) configdata.DATA for performance reasons -
before, the settings completion model called data() many times, which caused
initializing of it taking a few (instead of nearly 0) seconds.

See https://github.com/hackebrot/qutebrowser/pull/16#discussion-diff-27770433

qutebrowser/config/config.py

@@ -281,7 +281,7 @@ class ConfigManager(QObject):
     def __init__(self, configdir, fname, relaxed=False, parent=None):
         super().__init__(parent)
         self._initialized = False
-        self.sections = configdata.DATA
+        self.sections = configdata.data()
         self._interpolation = configparser.ExtendedInterpolation()
         self._proxies = {}
         for sectname in self.sections.keys():

qutebrowser/config/sections.py

@@ -29,6 +29,7 @@ class Section:
     """Base class for KeyValue/ValueList sections.
 
     Attributes:
+        _readonly: Whether this section is read-only.
         values: An OrderedDict with key as index and value as value.
                 key: string
                 value: SettingValue
@@ -38,6 +39,7 @@ class Section:
     def __init__(self):
         self.values = None
         self.descriptions = {}
+        self._readonly = False
 
     def __getitem__(self, key):
         """Get the value for key.
@@ -99,13 +101,15 @@ class KeyValue(Section):
     set of keys.
     """
 
-    def __init__(self, *defaults):
+    def __init__(self, *defaults, readonly=False):
         """Constructor.
 
         Args:
             *defaults: A (key, value, description) list of defaults.
+            readonly: Whether this config is readonly.
         """
         super().__init__()
+        self._readonly = readonly
         if not defaults:
             return
         self.values = collections.OrderedDict()
@@ -115,6 +119,8 @@ class KeyValue(Section):
             self.descriptions[k] = desc
 
     def setv(self, layer, key, value, interpolated):
+        if self._readonly:
+            raise ValueError("Trying to modify a read-only config!")
         self.values[key].setv(layer, value, interpolated)
 
     def dump_userconfig(self):
@@ -143,17 +149,20 @@ class ValueList(Section):
         keytype: The type to use for the key (only used for validating)
         valtype: The type to use for the value.
         _ordered_value_cache: A ChainMap-like OrderedDict of all values.
+        _readonly: Whether this section is read-only.
     """
 
-    def __init__(self, keytype, valtype, *defaults):
+    def __init__(self, keytype, valtype, *defaults, readonly=False):
         """Wrap types over default values. Take care when overriding this.
 
         Args:
             keytype: The type instance to be used for keys.
             valtype: The type instance to be used for values.
             *defaults: A (key, value) list of default values.
+            readonly: Whether this config is readonly.
         """
         super().__init__()
+        self._readonly = readonly
         self._ordered_value_cache = None
         self.keytype = keytype
         self.valtype = valtype
@@ -182,6 +191,8 @@ class ValueList(Section):
         return self._ordered_value_cache
 
     def setv(self, layer, key, value, interpolated):
+        if self._readonly:
+            raise ValueError("Trying to modify a read-only config!")
         self.keytype.validate(key)
         if key in self.layers[layer]:
             self.layers[layer][key].setv(layer, value, interpolated)