Conditional Assignment of video reader?

I've seen many questions like this, but nothing that fit my problem quite right.

I'm running the Motion-Based Multiple Object Tracking example. In one of the nested functions, this is the assignment:

obj.reader = vision.VideoFileReader('atrium.avi');

I want to be able to select a different video and not have to open the function to do so. I want obj.reader to be assigned to that video only if it's not already assigned to a different video of my choice.

Is there an easy way of doing this, or should I just manually change the video in the function's code?

Thanks!