学习Django，但是与变量混淆

I am going through the Django Tutorial and i am on step 3 of creating polls app. There is a variable called "question_id" and i cant understand where exactly this is defined or where its coming from. I will post the files bellow. My only guess is that this variable is somehow created by Django internally when class Question is defined in Models.py, but i am not sure. Its not defined in "Question" class. I am going through the Django Tutorial and i am