Assigning multiple variables in one line

Question

I am trying to make the Fibonacci sequence I don't get why this:

def fibonacci(n):
    f1 = 0
    f2 = 1
    i = 1

    while i < n:
        print(f2)
        f1 = f2
        f2 = f1 + f2

        i += 1
    return f3

returns 1, 2, 4, 8, while this:

def fibonacci(n):

    f1 = 0
    f2 = 1
    i = 1

    while i < n:
        print(f2)
        f1, f2 = f2, f1 + f2
        i += 1
    return f3

returns the Fibonacci sequence.

Answer 1

In the latter example, the right hand side is evaluated first :

f1, f2 = f2, f1 + f2

So the value of f1 used in the calculation of f2 is the "old" value.

In your code, when you do:

f1 = f2
f2 = f1 + f2

the value of f1 has already changed when you go to evaluate the new value for f2 .

Answer 2

In the first example, the value of f1 from the previous iteration is discarded before f2 is updated.

f1, f2 = f2, f1 + f2

can be seen as shorthand for

tmp = f1
f1 = f2
f2 = tmp + f2

if that helps it make more sense. The latter is what you'd have to do in many other languages to get the desired effect.

Answer 3

The variables f1 and f2 simultaneously get the new values f2 and f1 + f2, the expression

f1, f2 = f2, f1 + f2.

Demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place . f1 + f2 use the f1 old value we don't use the f1 new value. The right-hand side expressions are evaluated from the left to the right.