Why does (a,b)=(b,a) work when a=b+a; b=a doesn't?

Question

I've tried to generate the fibonacci in python but I've noticed that if I do it with swapping it gives me a different value than if I was to do it with simple assignment

def fib_num(max):
    a = 0
    b = 1
    for i in range(max):
         # a,b = b+a,a  this way it is right

         # but if I will implement it like below with simple assigment,
         # I am not going to get the same result Why???
         a  = b+a  
         b = a

         yield a

Because in the second example you've already changed the value of `a` *before* you then assign that new value to `b`. — jonrsharpe, Jun 12 '19 at 15:48

score 2 · Answer 1 · answered Jun 12 '19 at 15:52

Multiple assignment is implicitly creating a temp variable for you. Your code is assigning a new value to a, and when you use it again it'll have the new value, not the old one:

a = b + a  
b = a

The correct, equivalent solution would be:

temp = a
a = b + a  
b = temp

Why does (a,b)=(b,a) work when a=b+a; b=a doesn't?

1 Answers1