make sure keyword-only arguments have a default

Fixes #1872.

This prevents inspect.Parameter.empty from slipping through to the
command.

qutebrowser/commands/command.py

@@ -246,6 +246,10 @@ class Command:
                     continue
                 if self._inspect_special_param(param):
                     continue
+                if (param.kind == inspect.Parameter.KEYWORD_ONLY and
+                      param.default is inspect.Parameter.empty):
+                    raise TypeError("{}: handler has keyword only argument "
+                                    "without default!".format(self.name))
                 typ = self._get_type(param)
                 is_bool = typ is bool
                 kwargs = self._param_to_argparse_kwargs(param, is_bool)

tests/unit/commands/test_cmdutils.py

@@ -349,6 +349,24 @@ class TestRegister:
         with pytest.raises(IndexError):
             cmd.get_pos_arg_info(2)
 
+    def test_keyword_only_without_default(self):
+        # https://github.com/The-Compiler/qutebrowser/issues/1872
+        def fun(*, target):
+            """Blah."""
+            pass
+
+        with pytest.raises(TypeError):
+            fun = cmdutils.register()(fun)
+
+    def test_typed_keyword_only_without_default(self):
+        # https://github.com/The-Compiler/qutebrowser/issues/1872
+        def fun(*, target: int):
+            """Blah."""
+            pass
+
+        with pytest.raises(TypeError):
+            fun = cmdutils.register()(fun)
+
 
 class TestArgument: