Weird behavior when accessing tag data via readBlocking

Ignition
1

I'm trying to use system.tag.readBlocking to access a dataset at a given tag path. However, my transform script is behaving oddly when I try to access the data. Specifically: Python treats my line_states variable like a Dataset object only in return statements. Otherwise, when I try to perform typical database operations on the line_states variable, I get errors treating it like a NoneType object.

def transform(self, value, quality, timestamp):
	item = {
  		"id": "",
  		"label": "New Item",
 		"iconConfig": {
    		"path": "",
    		"color": "",
    		"style": {}
  		},
  		"headerStyle": {
    		"classes": ""
  		},
  		"rowStyle": {
    		"classes": ""
  		}
	}
	
	items = []
	
	for line in value:
		item["id"] = line["value"]
		item["label"] = line["label"]
		
		line_states_qualified_value = system.tag.readBlocking(str(line["EquipmentPath"]) + "/config/stateDS")[0]
		line_states = line_states_qualified_value.value
		state_code_qualified_value = system.tag.readBlocking(str(line["EquipmentPath"]) + "/OEE_Events/state")[0]
		state_code = state_code_qualified_value.value
		state_data = []
		
		line_states_row_count = line_states.getRowCount()
		return line_states_row_count
		
		if line_states_row_count > 0 and state_code:
			for i in range(line_states_row_count):
				if int(line_states.getValueAt(i, "ReasonCode")) == int(state_code):
					line_states_column_count = line_states.getColumnCount()
					for j in range(line_states_column_count):
						states_data.append(line_states.getValueAt(i, j))
					item["iconConfig"]["path"] = line_states.getValueAt()
					item["iconConfig"]["color"] = ""
					item["iconConfig"]["style"] = ""
		
		items.append(item.copy())
	
	return items

For example, the code above returns 28 because of the return line_states_row_count line. However, if said line is removed, the preceding line line_states_row_count = line_states.getRowCount() generates the following error:

'NoneType' object has no attribute 'getRowCount'

2

Maybe because removing the early return causes you to iterate through the entire value sequence, and eventually you end up with a line that results in None after this:

		line_states_qualified_value = system.tag.readBlocking(str(line["EquipmentPath"]) + "/config/stateDS")[0]
		line_states = line_states_qualified_value.value

add some print statements to debug.

2 Likes
3

Also, for optimization, you should have 2 loops. One for building a list of all the tags you need to read, then doing a single system.tag.readBlockingstatement to read them in a single call. Then use those results as part of your iteration on the second loop with something like for i, line in enumerate(value): to pull the index and reference the 2 values using tagValues[i*2].value and tagValues[i*2+1].value for line_states and state_data respectively.

3 Likes
4

If that's the case, what change would you recommend adding to my code?

5

Add print statements to print the equipment path of the item, so you can Identify which one is returning a null value for its line states. Last one to be printed before the error is the one with the issue.

1 Like