Multiple value assignment

Question

a, b, n = 0, 1, 500
while a < n:
    print(a)
    a, b = b, a+b

and

a, b, n = 0, 1, 500
while a < n:
    print(a)
    a = b
    b = a+b

Both give different output. What is the difference between a, b = b, a+b and a = b; b = a+b?

score 1 · Answer 1 · answered May 29 '19 at 05:21

1

The below code is like swapping.

a, b = b, a+b

Its like doing this

temp = a
a = b
b = temp + b

answered May 29 '19 at 05:21

Praveenkumar

2,056
1
9
18

score 0 · Answer 2 · answered May 29 '19 at 05:21

0

a, b = b, a+b

is equivalent to

tmp = a
a = b
b = tmp+b

answered May 29 '19 at 05:21

Dmitry Kuzminov

6,180
6
18
40

score 0 · Answer 3 · answered May 29 '19 at 05:24

This is not comma separated values. You are doing tuple unpacking.

a, b, n = 0, 1, 500

Is the same as:

a, b, n = (0, 1, 500)

The reason they are different is the first line assigns b to a then adds a and b together. It’s essentially the same as:

a = b
b = a+b

score 0 · Accepted Answer · answered May 29 '19 at 05:39

a, b, n = 0, 1, 500
while a < n:
    print(a)
    a = b
    b = a+b

In above line of code - after print(a), code value in b variable will be assigned to a first and hence the value of a is updated and the updated value of a is being used in b = a+b

Lets say, a =0, b =1 . So after print(a), value of a will be 1 first and b will have 1+1 =2.

Whereas,

a, b, n = 0, 1, 500
while a < n:
    print(a)
    a, b = b, a+b

In the above code, after print(a), a and b values are simultaneously assigned. In this case, whatever value of a is printed in print(a) will be used in assigning value for a in further step.

Lets say, a = 0 and b = 1 , after print(a) which will print 0 first, value in a will be 1 and value in b will be 0+1 = 1 because b = a+b will use a =0 and not a =1.

Multiple value assignment

4 Answers4