pylightning: Exception if we have unfulfilled positional arguments

This caused me to backtrack quite a bit, so this should help debugging
in the future.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

contrib/pylightning/lightning/plugin.py

@@ -242,7 +242,7 @@ class Plugin(object):
 
         arguments = OrderedDict()
         for name, value in sig.parameters.items():
-            arguments[name] = inspect.Signature.empty
+            arguments[name] = inspect._empty
 
         # Fill in any injected parameters
         if 'plugin' in arguments:
@@ -257,7 +257,7 @@ class Plugin(object):
         else:
             pos = 0
             for k, v in arguments.items():
-                if v is not inspect.Signature.empty:
+                if v != inspect._empty:
                     continue
                 if pos < len(params):
                     # Apply positional args if we have them
@@ -267,6 +267,15 @@ class Plugin(object):
                     arguments[k] = sig.parameters[k].default
                 pos += 1
 
+        missing = [k for k, v in arguments.items() if v == inspect._empty]
+        if missing:
+            raise ValueError("Missing positional arguments ({given} given, "
+                             "expected {expected}): {missing}".format(
+                                 missing=", ".join(missing),
+                                 given=len(arguments) - len(missing),
+                                 expected=len(arguments)
+                             ))
+
         ba = sig.bind(**arguments)
         ba.apply_defaults()
         return func(*ba.args, **ba.kwargs)