Add --cycle flag to :set

Chooses the next value from the provided list of values (string-wise
comparison). Technically, the 'option!' syntax for toggling bools is now
redundant, but a translation from 'option!' to '--cycle option false
true' is kept for backwards compatibility.

The '--cycle' flag could also be technically optionally, since the only
thing that depends on it is preserving the error message for specifying
multiple values. (But I think it's best to keep it explicit, as a
principle-of-least-surprise thing)

Note: The business logic of picking the next value and setting it was
moved out to a separate function to avoid tripping pylint's
too-many-branches detector.

Fixes #47

CHANGELOG.asciidoc

@@ -149,6 +149,7 @@ Changed
 - `ui -> window-title-format` now has a new `{backend} ` replacement
 - `:hint` has a new `--add-history` argument to add the URL to the history for
   yank/spawn targets.
+- `:set` now has a `--cycle` flag which lets you cycle through multiple options.
 
 Deprecated
 ~~~~~~~~~~

qutebrowser/config/config.py

@@ -772,13 +772,13 @@ class ConfigManager(QObject):
             raise cmdexc.CommandError("set: {} - {}".format(
                 e.__class__.__name__, e))
 
-    @cmdutils.register(name='set', instance='config')
+    @cmdutils.register(name='set', instance='config', star_args_optional=True)
     @cmdutils.argument('section_', completion=Completion.section)
     @cmdutils.argument('option', completion=Completion.option)
-    @cmdutils.argument('value', completion=Completion.value)
+    @cmdutils.argument('values', completion=Completion.value)
     @cmdutils.argument('win_id', win_id=True)
-    def set_command(self, win_id, section_=None, option=None, value=None,
+    def set_command(self, win_id, section_=None, option=None, *values,
-                    temp=False, print_=False):
+                    temp=False, print_=False, cycle=False):
         """Set an option.
 
         If the option name ends with '?', the value of the option is shown
@@ -793,9 +793,10 @@ class ConfigManager(QObject):
         Args:
             section_: The section where the option is in.
             option: The name of the option.
-            value: The value to set.
+            values: The value to set, or the values to cycle through.
             temp: Set value temporarily.
             print_: Print the value after setting.
+            cycle: Cycle through multiple provided values.
         """
         if section_ is not None and option is None:
             raise cmdexc.CommandError(
@@ -812,27 +813,48 @@ class ConfigManager(QObject):
             print_ = True
         else:
             with self._handle_config_error():
-                if option.endswith('!') and option != '!' and value is None:
+                if option.endswith('!') and option != '!' and not values:
+                    # Handle inversion as special cases of the cycle code path
                     option = option[:-1]
                     val = self.get(section_, option)
-                    layer = 'temp' if temp else 'conf'
                     if isinstance(val, bool):
-                        self.set(layer, section_, option, str(not val).lower())
+                        values = ['false', 'true']
                     else:
                         raise cmdexc.CommandError(
                             "set: Attempted inversion of non-boolean value.")
-                elif value is not None:
+                elif not values:
-                    layer = 'temp' if temp else 'conf'
-                    self.set(layer, section_, option, value)
-                else:
                     raise cmdexc.CommandError("set: The following arguments "
                                               "are required: value")
+                elif not cycle and len(values) > 1:
+                    raise cmdexc.CommandError("set: Too many values provided")
+
+                layer = 'temp' if temp else 'conf'
+                self._set_next(layer, section_, option, values)
 
         if print_:
             with self._handle_config_error():
                 val = self.get(section_, option, transformed=False)
             message.info("{} {} = {}".format(section_, option, val))
 
+    def _set_next(self, layer, section_, option, values):
+        """Set the next value out of a list of values."""
+        if len(values) == 1:
+            # If we have only one value, just set it directly (avoid
+            # breaking stuff like aliases or other pseudo-settings)
+            self.set(layer, section_, option, values[0])
+        else:
+            # Otherwise, use the next valid value from values, or the
+            # first if the current value does not appear in the list
+            assert len(values) > 1
+            val = self.get(section_, option, transformed=False)
+            try:
+                idx = values.index(str(val))
+                idx = (idx + 1) % len(values)
+                value = values[idx]
+            except ValueError:
+                value = values[0]
+            self.set(layer, section_, option, value)
+
     def set(self, layer, sectname, optname, value, validate=True):
         """Set an option.
 

tests/end2end/features/set.feature

@@ -15,6 +15,10 @@ Feature: Setting settings.
         When I run :set colors statusbar.bg
         Then the error "set: The following arguments are required: value" should be shown
 
+    Scenario: With too many values
+        When I run :set colors statusbar.bg green blue
+        Then the error "set: Too many values provided" should be shown
+
     Scenario: Invalid section
         When I run :set blah blub foo
         Then the error "set: Section 'blah' does not exist!" should be shown
@@ -32,6 +36,26 @@ Feature: Setting settings.
         When I run :set colors statusbar.bg!
         Then the error "set: Attempted inversion of non-boolean value." should be shown
 
+    Scenario: Cycling an option
+        When I run :set colors statusbar.bg magenta
+        And I run :set --cycle colors statusbar.bg green magenta blue yellow
+        Then colors -> statusbar.bg should be blue
+
+    Scenario: Cycling an option through the end of the list
+        When I run :set colors statusbar.bg yellow
+        And I run :set --cycle colors statusbar.bg green magenta blue yellow
+        Then colors -> statusbar.bg should be green
+
+    Scenario: Cycling an option that's not on the list
+        When I run :set colors statusbar.bg red
+        And I run :set --cycle colors statusbar.bg green magenta blue yellow
+        Then colors -> statusbar.bg should be green
+
+    Scenario: Cycling through a single option
+        When I run :set colors statusbar.bg red
+        And I run :set --cycle colors statusbar.bg red
+        Then colors -> statusbar.bg should be red
+
     Scenario: Getting an option
         When I run :set colors statusbar.bg magenta
         And I run :set colors statusbar.bg?