How do I swap two variables in Python?

a, b = 1, 2
a, b = b, a
print(a, b)      # 2 1

#Why it works

The right-hand side is evaluated completely before any assignment happens. Python builds the tuple (b, a) — that is, (2, 1) — and only then unpacks it into the names on the left. The old value of a is still intact when b is read.

This is the same mechanism behind:

first, second, third = [1, 2, 3]
head, *tail = [1, 2, 3, 4]         # head=1, tail=[2,3,4]
first, *middle, last = [1,2,3,4]   # middle=[2,3]

#Swapping list elements

items[i], items[j] = items[j], items[i]

This is the swap inside every sorting algorithm, and it is why Python implementations of bubble sort read so much more clearly than C ones.

#The one place it does not apply

If the two sides share an index that is itself being reassigned, evaluation order can surprise you:

i = 0
items = [1, 2, 3]
i, items[i] = 1, 99      # items[1] = 99, not items[0]

Targets are assigned left to right, so i becomes 1 before items[i] is resolved. Rare, but worth knowing it exists.