Python Fibonacci Generator Sequence

Question

I am currently studying Python and have become a bit stuck on the Fibonacci Number Generator. After a few hours of research I haven't made much progress. I understand the output to a certain extent, I just don't feel comfortable moving to another topic until I know what is happening here for every iteration.

    def fibonacci_generator() :
        a , b = 0 , 1
        while True :
            yield a
            a , b = b , a + b

    fib = fibonacci_generator() 

    for i in fib :
        if i > 100 :
            break
        else :
            print( 'Generated:' , i )

After running this code, I see the following output.

    Generated: 0
    Generated: 1
    Generated: 1
    Generated: 2
    Generated: 3
    Generated: 5
    Generated: 8
    Generated: 13
    Generated: 21
    Generated: 34
    Generated: 55
    Generated: 89

According to several answers to similar questions here on Stack Overflow, I do understand that the first time the generator is called, a and be are initialized to 0 and 1 respectively, and then the value with the yield statement ( a = 0) is returned back to the caller.

To save a little time and make things perfectly clear, there are a few questions have.

1. In the first iteration, is the only occurrence the return of the initial value of a? Does it completely stop at yield?

2. In the second iteration, where does it get 1? If a, b = b, then a + b would be 1+1, so why isn't the second value returned as 2?

3. I'm also not sure how the same value of 1 gets returned twice.

To sum it all up, I would like to know what the specific operation is for every line of the output. I can't seem to get that part straight in my head. Let me reiterate that I have looked in quite a few different places but nothing I've seen has helped.

Answer 1

To give the background, fibonacci_generator is a generator function that returns a generator , which is in fact just an iterator object .

In the first iteration, the generator yields a, and it stops there until you call it again. After you call it again, it recalculates a and b, goes in the loop since it is True, and yields the recalculated a. It just keeps repeating that process.

For your second and third questions:

Whenever you have an assignment statement, the variables on the left don't get assigned until the right side of the assignment statement is finished.

Therefore, in

a, b = b, a + b

a is not equal to b when you are calculating a + b , but it is equal to what it was originally. In other words, a, b = b, a + b is equivalent to:

temp = a
a = b
b = temp + b

If you follow this logic, you will understand why that code is behaving the way it is.

Answer 2

Whenever you have an assignment statement, the variables on the left don't get assigned until the right side of the assignment statement is finished.