Raise explicit error when impl called with pos args

Raise an explicit TypeError to alert the user that they are calling
a hookimpl incorrectly with positional args; keyword args are allowed
only. Add a test too.

Fixes #53

Author: Tyler Goodlet

pluggy/__init__.py

@@ -664,7 +664,9 @@ def _add_hookimpl(self, hookimpl):
     def __repr__(self):
         return "<_HookCaller %r>" % (self.name,)
 
-    def __call__(self, **kwargs):
+    def __call__(self, *args, **kwargs):
+        if args:
+            raise TypeError("hook calling supports only keyword arguments")
         assert not self.is_historic()
         if self.argnames:
             notincall = set(self.argnames) - set(['__multicall__']) - set(

testing/test_hookrelay.py

@@ -57,7 +57,11 @@ def hello(self, arg):
             "api hook 1"
 
     pm.add_hookspecs(Api)
-    pytest.raises(TypeError, lambda: pm.hook.hello(3))
+    with pytest.raises(TypeError) as exc:
+        pm.hook.hello(3)
+
+    comprehensible = "hook calling supports only keyword arguments"
+    assert comprehensible in str(exc.value)
 
 
 def test_firstresult_definition(pm):