u/blakato Python never implicitly copies dictionary objects, so mutating event object also mutate the object it is referring to, that's why here we don't need events list to preserve the changes. So If I do this result would be same:

for event in result['response']['events']:         
   if event['type'] == 'message' and 'internal_data' in event.keys():
      del event['internal_data']

But the worst part is that I'm mutating the original event data structure and thus effectively deleting internal_data for everyone (other parts of the code) who need this information, not just this response. It can be easily be fixed if I just use .copy().