How swapping variables is implemented in Python?

Question

How does the below code work in Python:

a = input()
b = input()
a, b = b, a  # STATEMENT 1
print(a, b)

Does the statement 1 create a third variable in Python heap memory space to swap the two numbers or does it use some algorithm to do the swap?

Answer 1

ruohola did a good job in providing the translated bytecode of the python code.

I am duplicating here for reference:

Python code:

a = input()
b = input()
a, b = b, a  # STATEMENT 1
print(a, b)

Bytecode:

 2           0 LOAD_NAME                0 (input)
             2 CALL_FUNCTION            0
             4 STORE_NAME               1 (a)

 3           6 LOAD_NAME                0 (input)
             8 CALL_FUNCTION            0
            10 STORE_NAME               2 (b)

 4          12 LOAD_NAME                2 (b)
            14 LOAD_NAME                1 (a)
            16 ROT_TWO                  # swapping done here
            18 STORE_NAME               1 (a)
            20 STORE_NAME               2 (b)
            22 LOAD_CONST               0 (None)
            24 RETURN_VALUE

ROT_TWO operation swaps the top 2 values of the python stack. So what do we actually have so far:

Python swaps the 2 values by calling a swap (ROT_TWO) subroutine.

If this is how far you want to go and it answers your question it is fine. However for those who want to go deeper and see how this swap (ROT_TWO) subroutine works, here is the official CPython implementation:

#define TOP()             (stack_pointer[-1])
#define SECOND()          (stack_pointer[-2])
#define SET_TOP(v)        (stack_pointer[-1] = (v))
#define SET_SECOND(v)     (stack_pointer[-2] = (v))
/*..*/
case TARGET(ROT_TWO): {
   PyObject *top = TOP();
   PyObject *second = SECOND();
   SET_TOP(second);
   SET_SECOND(top);
   FAST_DISPATCH();
}

Or in other words the implementation of ROT_TWO actually performs the following steps (a,b are the top 2 values of the stack):

x1 = a
x2 = b
a = x2
b = x1

So the implementation uses auxiliary temporary locations (x1, x2) and in fact it uses 2 auxilliary memory locations instead of the minimum 1 auxiliary location to swap two values that a more memory-efficient implementation would do:

x = a
a = b
b = x

Under the current computing model, swapping two values can be done only in so many different ways and does not happen magically:

1. Using auxilliary temporary storage
2. Employing a series of XOR (or similar arithmetic) operations

So to sum it up, Python under the hood indeed uses auxilliary temporary locations in order to swap the two values.

Answer 2

It's a simple bytecode operation which doesn't need any intermediate variables to do the swap. See this demo:

import dis

code = '''
a = input()
b = input()
a, b = b, a
'''

dis.dis(code)

Output:

 2           0 LOAD_NAME                0 (input)
             2 CALL_FUNCTION            0
             4 STORE_NAME               1 (a)

 3           6 LOAD_NAME                0 (input)
             8 CALL_FUNCTION            0
            10 STORE_NAME               2 (b)

 4          12 LOAD_NAME                2 (b)
            14 LOAD_NAME                1 (a)
            16 ROT_TWO
            18 STORE_NAME               1 (a)
            20 STORE_NAME               2 (b)
            22 LOAD_CONST               0 (None)
            24 RETURN_VALUE

Note: Like bytecode as a whole, this is of course just an implementation detail of CPython.