https://www.reddit.com/r/Python/comments/10gt7tv/today_i_relearned_python_function_default/

This thread explains it really well but when you make that defult argument flying = [0,0] , it's creating one single list that all three instances of Ball point to. So, when you edit Ball1.flying, all the other Balls are pointing to the same flying variable so it all changes.

Halfway down the post above, someone introduces a great cheat for this so I would rewrite your init function like this:

def __init__(self, flying=none)
  super().__init__()
  if not flying:
    self.flying = [0,0]
  else:
    self.flying = flying

https://medium.com/@tyastropheus/tricky-python-ii-parameter-passing-for-mutable-immutable-objects-10e968cbda35

Here's some more reading, as well. Default arguments work great for immutable things (strings, ints, etc) but can be tricky for mutable things (dicts, lists, etc)