Add a @cmdutils.argument decorator

For now the only available keyword argument is 'flag' which customizes
the flag an argument will get.

See #637.

CONTRIBUTING.asciidoc

@@ -439,6 +439,18 @@ then automatically checked. Possible values:
     - A tuple of multiple types above: Any of these types are valid values,
       e.g. `('foo', 'bar')` or `(int, 'foo')`.
 
+You can customize how an argument is handled using the `@cmdutils.argument`
+decorator *after* `@cmdutils.register`. This can e.g. be used to customize the
+flag an argument should get:
+
+[source,python]
+----
+@cmdutils.register(...)
+@cmdutils.arg('bar', flag='c')
+def foo(bar):
+    ...
+----
+
 The name of an argument will always be the parameter name, with any trailing
 underscores stripped.
 

qutebrowser/browser/commands.py

@@ -575,8 +575,8 @@ class CommandDispatcher:
             widget.keyReleaseEvent(release_evt)
 
     @cmdutils.register(instance='command-dispatcher', hide=True,
-                       scope='window', count='count',
-                       flags={'horizontal': 'x'})
+                       scope='window', count='count')
+    @cmdutils.argument('horizontal', flag='x')
     def scroll_perc(self, perc: {'type': float}=None, horizontal=False,
                     count=None):
         """Scroll to a specific percentage of the page.

qutebrowser/commands/cmdutils.py

@@ -24,6 +24,8 @@ Module attributes:
     aliases: A list of all aliases, needed for doc generation.
 """
 
+import inspect
+
 from qutebrowser.utils import qtutils, log
 from qutebrowser.commands import command, cmdexc
 
@@ -164,3 +166,36 @@ class register:  # pylint: disable=invalid-name
             cmd_dict[name] = cmd
         aliases += names[1:]
         return func
+
+
+class argument:  # pylint: disable=invalid-name
+
+    """Decorator to customize an argument for @cmdutils.register.
+
+    This could also be a function, but as a class (with a "wrong" name) it's
+    much cleaner to implement.
+
+    Attributes:
+        _argname: The name of the argument to handle.
+        _kwargs: Keyword arguments, valid ArgInfo members
+    """
+
+    def __init__(self, argname, **kwargs):
+        self._argname = argname
+        self._kwargs = kwargs
+
+    def __call__(self, func):
+        if self._argname not in inspect.signature(func).parameters:
+            raise ValueError("{} has no argument {}!".format(func.__name__,
+                                                             self._argname))
+
+        # Fill up args which weren't passed
+        for arg in command.ArgInfo._fields:
+            if arg not in self._kwargs:
+                self._kwargs[arg] = None
+
+        if not hasattr(func, 'qute_args'):
+            func.qute_args = {}
+        func.qute_args[self._argname] = command.ArgInfo(**self._kwargs)
+
+        return func

qutebrowser/commands/command.py

@@ -35,6 +35,9 @@ def arg_name(name):
     return name.rstrip('_').replace('_', '-')
 
 
+ArgInfo = collections.namedtuple('ArgInfo', ['flag'])
+
+
 class Command:
 
     """Base skeleton for a command.
@@ -54,7 +57,6 @@ class Command:
         win_id_arg: The name of the win_id parameter, or None.
         flags_with_args: A list of flags which take an argument.
         no_cmd_split: If true, ';;' to split sub-commands is ignored.
-        _flags: A mapping of argument names to alternative flags
         _type_conv: A mapping of conversion functions for arguments.
         _needs_js: Whether the command needs javascript enabled
         _modes: The modes the command can be executed in.
@@ -73,7 +75,7 @@ class Command:
     def __init__(self, *, handler, name, instance=None, maxsplit=None,
                  hide=False, completion=None, modes=None, not_modes=None,
                  needs_js=False, debug=False, ignore_args=False,
-                 deprecated=False, no_cmd_split=False, flags=None,
+                 deprecated=False, no_cmd_split=False,
                  star_args_optional=False, scope='global', count=None,
                  win_id=None):
         # I really don't know how to solve this in a better way, I tried.
@@ -102,7 +104,6 @@ class Command:
         self._scope = scope
         self._needs_js = needs_js
         self._star_args_optional = star_args_optional
-        self._flags = flags or {}
         self.debug = debug
         self.ignore_args = ignore_args
         self.handler = handler
@@ -131,11 +132,6 @@ class Command:
             raise ValueError("Got {} completions, but only {} "
                              "arguments!".format(len(self.completion),
                                                  len(args)))
-        for argname in self._flags:
-            if argname not in args:
-                raise ValueError("Got argument {} in flags param, but no such "
-                                 "argument exists for {}".format(
-                                     argname, self.name))
 
     def _check_prerequisites(self, win_id):
         """Check if the command is permitted to run currently.
@@ -239,13 +235,21 @@ class Command:
         if not self.ignore_args:
             for param in signature.parameters.values():
                 annotation_info = self._parse_annotation(param)
+
+                try:
+                    arg_info = self.handler.qute_args[param.name]
+                except (KeyError, AttributeError):
+                    arg_info = ArgInfo(**{name: None
+                                          for name in ArgInfo._fields})
+
                 if param.name == 'self':
                     continue
                 if self._inspect_special_param(param):
                     continue
                 typ = self._get_type(param, annotation_info)
                 kwargs = self._param_to_argparse_kwargs(param, annotation_info)
-                args = self._param_to_argparse_args(param, annotation_info)
+                args = self._param_to_argparse_args(param, annotation_info,
+                                                    arg_info)
                 self._type_conv.update(self._get_typeconv(param, typ))
                 callsig = debug_utils.format_call(
                     self.parser.add_argument, args, kwargs,
@@ -294,19 +298,25 @@ class Command:
             kwargs['nargs'] = '?'
         return kwargs
 
-    def _param_to_argparse_args(self, param, annotation_info):
+    def _param_to_argparse_args(self, param, annotation_info, arg_info):
         """Get argparse positional arguments for a parameter.
 
         Args:
             param: The inspect.Parameter object to get the args for.
             annotation_info: An AnnotationInfo tuple for the parameter.
+            arg_info: An ArgInfo tuple for the parameter or None
 
         Return:
             A list of args.
         """
         args = []
         name = arg_name(param.name)
-        shortname = self._flags.get(param.name, name[0])
+
+        if arg_info.flag is not None:
+            shortname = arg_info.flag
+        else:
+            shortname = name[0]
+
         if len(shortname) != 1:
             raise ValueError("Flag '{}' of parameter {} (command {}) must be "
                              "exactly 1 char!".format(shortname, name,

tests/unit/commands/test_cmdutils.py

@@ -21,7 +21,7 @@
 
 import pytest
 
-from qutebrowser.commands import cmdutils, cmdexc, argparser
+from qutebrowser.commands import cmdutils, cmdexc, argparser, command
 
 
 class TestCheckOverflow:
@@ -208,7 +208,8 @@ class TestRegister:
         assert not parser.parse_args([]).arg
 
     def test_flag_argument(self):
-        @cmdutils.register(flags={'arg': 'b'})
+        @cmdutils.register()
+        @cmdutils.argument('arg', flag='b')
         def fun(arg=False):
             """Blah."""
             pass
@@ -218,9 +219,44 @@ class TestRegister:
         with pytest.raises(argparser.ArgumentParserError):
             parser.parse_args(['-a'])
 
-    def test_unknown_argument_in_flags(self):
-        with pytest.raises(ValueError):
-            @cmdutils.register(flags={'foobar': 'f'})
-            def fun():
+    def test_partial_arg(self):
+        """Test with only some arguments decorated with @cmdutils.argument."""
+        @cmdutils.register()
+        @cmdutils.argument('arg1', flag='b')
+        def fun(arg1=False, arg2=False):
+            """Blah."""
+            pass
+
+
+class TestArgument:
+
+    """Test the @cmdutils.argument decorator."""
+
+    # pylint: disable=unused-variable
+
+    def _arginfo(self, **kwargs):
+        """Helper method to get an ArgInfo tuple."""
+        for arg in command.ArgInfo._fields:
+            if arg not in kwargs:
+                kwargs[arg] = None
+        return command.ArgInfo(**kwargs)
+
+    def test_invalid_argument(self):
+        with pytest.raises(ValueError) as excinfo:
+            @cmdutils.argument('foo')
+            def fun(bar):
                 """Blah."""
                 pass
+        assert str(excinfo.value) == "fun has no argument foo!"
+
+    def test_storage(self):
+        @cmdutils.argument('foo', flag='x')
+        @cmdutils.argument('bar', flag='y')
+        def fun(foo, bar):
+            """Blah."""
+            pass
+        expected = {
+            'foo': self._arginfo(flag='x'),
+            'bar': self._arginfo(flag='y')
+        }
+        assert fun.qute_args == expected