Fix finding of mono events when mono_event_generator is used for real data.

ctapipe_io_magic/__init__.py

@@ -1447,7 +1447,11 @@ class MarsRun:
         mono_ids['M1'] = []
         mono_ids['M2'] = []
 
+        n_m1_events = len(self.event_data['M1']['stereo_event_number'])
+        n_m2_events = len(self.event_data['M2']['stereo_event_number'])
+
         if not self.is_mc:
+            if (n_m1_events != 0) and (n_m2_events != 0):
                 m1_data = self.event_data['M1']['stereo_event_number'][np.where(self.event_data['M1']['trigger_pattern'] == DATA_TRIGGER_PATTERN)]
                 m2_data = self.event_data['M2']['stereo_event_number'][np.where(self.event_data['M2']['trigger_pattern'] == DATA_TRIGGER_PATTERN)]
 
@@ -1474,6 +1478,12 @@ class MarsRun:
 
                 mono_ids['M1'] = m1_ids_mono.tolist()
                 mono_ids['M2'] = m2_ids_mono.tolist()
+            elif (n_m1_events != 0) and (n_m2_events == 0):
+                m1_ids_data = np.where(self.event_data['M1']['trigger_pattern'] == DATA_TRIGGER_PATTERN)[0]
+                mono_ids['M1'] = m1_ids_data.tolist()
+            elif (n_m1_events == 0) and (n_m2_events != 0):
+                m2_ids_data = np.where(self.event_data['M2']['trigger_pattern'] == DATA_TRIGGER_PATTERN)[0]
+                mono_ids['M2'] = m2_ids_data.tolist()
         else:
             # just find ids where event stereo number is 0 (which is given to mono events) and pattern is MC trigger
             m1_mono_mask = np.logical_and(self.event_data['M1']['trigger_pattern'] == MC_TRIGGER_PATTERN, self.event_data['M1']['stereo_event_number'] == 0)