step: set event source for bound methods

It is possible to call the step function not via the @step decorator but
directly. In case getattr() is used to retrieve an object's method it
will be a bound method object (instead of a function object when used
via the decorator). The signature of a bound method does not contain self
as a parameter, so the event source will always be None.

Currently only the GraphStrategy calls the step function directly. Fix
the event source in these cases by using func.__self__ for bound methods.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

labgrid/step.py

@@ -207,7 +207,7 @@ def decorator(func):
         def wrapper(*_args, **_kwargs):
             bound = signature.bind_partial(*_args, **_kwargs)
             bound.apply_defaults()
-            source = bound.arguments.get('self')
+            source = func.__self__ if inspect.ismethod(func) else bound.arguments.get('self')
             step = steps.get_new(title, tag, source)
             # optionally pass the step object
             if 'step' in signature.parameters: