I have just started learning python3 and I came across the following line of code :
a = 1
b = 2
a,b = b,a
print(a) #prints 2
print(b) #prints 1
How is this line a,b = b,a working? Is Python automatically creating some temp variables? And what are the unseen possibilities? I mean, can I do the same for 3 or more variables in one line?
disassembling with dis:
from dis import dis
def swap(a, b):
a, b = b, a
return a, b
dis(swap)
gives
7 0 LOAD_FAST 1 (b)
3 LOAD_FAST 0 (a)
6 ROT_TWO
7 STORE_FAST 0 (a)
10 STORE_FAST 1 (b)
8 13 LOAD_FAST 0 (a)
16 LOAD_FAST 1 (b)
19 BUILD_TUPLE 2
22 RETURN_VALUE
where ROT_TWO means
Swaps the two top-most stack items.
python does not need to create a temporary variable.
You can also do that with multiple variables but it gets tricky:
>>> a = 1
>>> b = 2
>>> c = 3
>>> a, b, c = c, a, b
>>> c
2
>>> a
3
>>> b
1
>>>
This is useful when you want to update some values. For example, if new value of y needs to be incremented by x, and x takes the value of y:
x, y = y, y + x
