How does the AI Vision "exists" attribute work?

[touretzkyds]

Looking at the AI Vision documentation here: https://api.vex.com/aim/home/python/AI_Vision.html#exists

I can't make sense of the "exists" attribute. In the section on ball detection, the example code does this:

# Move forward if a sports ball is detected
while True:
    ball = robot.vision.get_data(SPORTS_BALL)
    if ball[0].exists:
        robot.move_for(10, 0)
    wait(50, MSEC)

But if there is no sports ball detected, get_data will return an empty list [ ], so ball[0] will cause an error.

Also, looking at the source code, the "exists" attribute is only mentioned once in the code, in aim.py, where it is set to True.

[jpearman]

The documentation at https://api.vex.com/aim is primarily for the internal micropython implementation, many things are the same for websockets, but not everything. Internally we return a class based on tuple that handles out of bounds access, this was due to feedback from a different product that used to just return None for similar functionality, students were struggling with that. For websocket an empty list is returned if no objects are found, if the list has objects, exists is just set to True and really has no use.

[touretzkyds]

Okay. I did check the documentation on Websocket differences at https://api.vex.com/aim/home/websocket/websocket_api/api_differences.html

I guess you'll want to update this to explain that the "exists" examples won't work.