Question

I apologize since this might be a common question, but I think I am looking for a quite specific answer that wouldn’t be found in other topics. Basically, I am quite confused about the flow of adding numbers. Here are two similar codes that compute numbers differently. Is there any simple explanation for this?

>>> a = 0
>>> b = 1
>>> while b <1000:
    print b
        a, b = b, a+b


1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987

>>> a =0
>>> b=1
>>> while b<1000:
       print b
       a = b
       b = a+b


1,2,4,8,16,32,64,128,256,512

Answer 1

The difference lies in what the values are WHEN swapped

a, b = b, a+b

sets a to b and sets a to a+b but the swaps are done relatively at the same time so it’s not in order, ie the change in b doesn’t respect that a was changed first.

In the second example

a = b

b = a+b

the values are changed and the 2nd statement respects the change of the first