[Tutor] Swapping values

[Steven D'Aprano]

On Tue, Jan 05, 2016 at 02:37:27AM +0200, yehudak . wrote:
> In Python we can swap values between two variable a and b this way:
> 
> a = 3; b = 7
> print(a, b)   # =====> 3  7
> 
> a, b = b, a  # swapping!
> print(a, b)   # =====> 7  3
> 
> How does this work?

The right-hand side of the assignment is evaluated fully before the 
assignment happens. Think about how you might do the same:

# right-hand side: ... = b, a
- write down b
- write down a

# left-hand side: a, b = ...
- take the first number written down (was b) and assign it to a
- take the second number written down (was a) and assign it to b

except of course Python doesn't literally "write things down".

To be technical, Python uses a stack. We can look at the byte code 
generated:

py> import dis
py> code = compile("a, b = b, a", "", "single")
py> dis.dis(code)
  1           0 LOAD_NAME                0 (b)
              3 LOAD_NAME                1 (a)
              6 ROT_TWO
              7 STORE_NAME               1 (a)
             10 STORE_NAME               0 (b)
             13 LOAD_CONST               0 (None)
             16 RETURN_VALUE


The first two commands LOAD_NAME pushes the values of b and a onto the 
internal stack. ROT_TWO swaps them around, then STORE_NAME gets called 
twice to assign them to a and b.


> If I split the 'magic' line into:
> a = b; b = a
> without a temp variable I get:
> print(a, b)   # =====> 7  7

In this case, what you are doing is:

- let a equal the value that b has right now;
- let b equal the value that a has right now 
  (which is of course, the same as b)


so the second part (b = a) doesn't actually do anything. If you just 
write

    a = b

of course now a and b are the same.



-- 
Steve