Regarding the Fibonacci Sequence example in Python's function tutorial

Question

This is what they have:

def fib(n):
    a, b = 0, 1
    while a < n:
        print a,
        a, b = b, a+b

This is what I have:

def fib(n):
    a = 0
    b = 1
    while a < n:
        print a
        a = b
        b = b+a

The first returns the correct sequence when employed, whereas mine proceeds 0, 1, 2, 4, 8, 16, 32...

I am currently learning programming (no prior computer science education), and it's clear that the problem is how I've defined my variables. What is the difference between separating the variables by commas and separating variables by a new line (assuming that is the issue)?

Answer 1

This is a tuple assignment:

a, b = 0, 1

You can also think of it as

(a, b) = (0, 1)

A temporary tuple is created with the values 0, and 1 and then unpacked onto the variable a and b

This is also a tuple assignment

a, b = b, a+b

Again, you can think of it as

(a, b) = (b, a+b)

The temporary tuple is created from the values of b and a+b before either of them is updated. The assignment only happens after the temporary tuple is created.

By breaking it out to separate steps, you are changing the meaning of the code.

Lets see what happens here

a, b = 0, 1        # a=0 , b=1
a, b = b, a+b      # a=1 , b=1

Compare with

a = 0              # a=0
b = 1              # a=0 , b=1
a = b              # a=1 , b=1
b = b+a            # a=1 , b=2