Python: Usage Of Variables And Their Difference ("a, B = 0, 1" Vs "a = 0", "b = 1")

I was looking at the Python Manual and found this snippet for a Fibonacci-Number generator: def fib(n): # write Fibonacci series up to n a, b = 0, 1 while b < n:

Solution 1:

Doing:

a, b = b, a+b

is equivalent to:

temp = a
a = b
b += temp

It lets you simultaneously do two calculations without the need of an intermediate/temporary variable.

The difference is that in your second piece of code, when you do the second line b = a+b, you have already modifed a in the previous line which is not the same as the first piece of code.

Examples

>>>a = 2>>>b = 3>>>a,b
2 3
>>>a,b = b,a>>>a,b
3 2

On the other hand, if you use the second approach shown in your question:

>>>a = 2>>>b = 3>>>a,b
2 3
>>>a = b>>>b = a>>>a,b
3 3

Solution 2:

In

    a, b = b, a+b

the right-hand expressions are evaluated first, and their results are assigned to a and b. This is similar to the following:

_new_a = b
    _new_b = a+b
    a = _new_a
    b = _new_b

On the other hand, in

a = b
    b = a+b

you are modifying a before adding it to b. This is equivalent to

    a, b = b, b+b

which explains where the powers of two are coming from.