Python Variable Swapping

I read this stackoverflow post on python tuple swapping and decided to take some notes.

Swapping Variables With An Intermediary

When you want to swap the values between two variables, there are a few ways to do it. The simplest is to introduce an intermediary variable.

x = 'foo'
y = 'bar'

# Intermediary
z = x

x = y
y = z

assert x == 'bar'
assert y == 'foo'

XOR

A common “trick” is to use XOR bit operations. This only works for languages that support bit operations. Such as C.

x = 0b1111
y = 0b1010

x = x ^ y
y = x ^ y
x = x ^ y

This only works for binary data.

Python Tuple Swapping

In python, we have nicer sugars to solve this. Enter implicit tuple packing and unpacking.

x = 'foo'
y = 'bar'

x, y = y, x
assert x == 'bar'
assert y == 'foo'

The expression y, x is an implicit tuple, created by the presence of the comma.

And we use tuple unpacking to splat a list of values to a several variables.

x, y = y, x

# Syntactic sugar for implicit tuple
x, y = (y, x)

# Syntactic sugar for unpacking
x = (y, x)[0]
y = (y, x)[1]

x, y, *z = y, x
x = (y, x)[0]
y = (y, x)[1]
z = (y, x)[2:]

Understanding what is going on behind the sugar can go quite a ways in freeing yourself from memorizing the rules. We now know what the sugar is doing and can take advantage of its expressiveness.