What is the := operator? I know about = operator (assignment). Also, I know that there are += and -= syntax which also helps to simplify the incrementing/decrementing operations. But I have never seen := before.
It is a so-called "walrus operator". You have never seen it before because it appeared only in Python version 3.8. And it is needed to perform assignment expressions. They allow to assign a value to the variable and return this value immediately.