Doc: add example on how to do some pre-processing from scan processing state

close #964

doc/user_corner/python_scripts.rst

@@ -199,3 +199,34 @@ On this example:
         # reset key for coherence
         scan._distance = None
     out_data = scan
+
+
+
+Filter scan having failled processes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Here is an example retrieving processing states of a dataset during is life time.
+We can imagine having this as pre-processing of the 'email notification for example' in order to receive a notification only when a scan has some 'failure' during processing.
+
+
+.. code-block:: python
+
+    from processview.core.manager import ProcessManager, DatasetState
+    scan = in_data
+
+    def get_processes_with_state(state_search, tomo_obj):
+        states = {}
+        for process in ProcessManager().get_processes():
+            state = ProcessManager().get_dataset_state(
+                dataset_id=tomo_obj.get_identifier(),
+                process=process,
+            )
+            if state is state_search:
+                states[process] = state
+        return states
+
+    failed_processes = get_processes_with_state(DatasetState.FAILED, tomo_obj=scan)
+    if len(failed_processes) > 0:
+        out_data = scan
+    else:
+        out_data = None