Section 11.3 Tuple Assignment

One of the unique syntactic features of Python is the ability to have a tuple on the left side of an assignment statement. This allows you to assign more than one variable at a time when the left side is a sequence.

In this example we have a two-element list (which is a sequence) and assign the first and second elements of the sequence to the variables x and y in a single statement.

This isn’t magic! Python roughly translates the tuple assignment syntax to the following:

It’s worth noting that Python does not translate the syntax literally. For example, if you try this with a dictionary, it won’t work as you might expect.

Stylistically, when we use a tuple on the left side of the assignment statement, we omit the parentheses, but the following is an equally valid syntax:

random_list = ['Harden', 'James']
(x, y) = random_list

A particularly clever application of tuple assignment allows us to swap the values of two variables in a single statement:

>>> a, b = b, a

Both sides of this statement are tuples, but Python interprets the left side to be a tuple of variables and the right side to be a tuple of expressions. All of the expressions on the right side are evaluated before any of the assignments. This means that the values of b and a on the right side are evaluated, then a and b on the left side take on their values.

The number of variables on the left and the number of values on the right must be the same:

>>> a, b = 1, 2, 3
ValueError: too many values to unpack

More generally, the right side can be any kind of sequence (string, list, or tuple). For example, to split an email address into a username and a domain, you could write:

>>> addr = '[email protected]'
>>> uname, domain = addr.split('@')

The return value from split() is a list with two elements; the first element is assigned to uname, the second to domain.

>>> print(uname)
monty
>>> print(domain)
python.org

email = [email protected]
uname, domain = email.split('@')