Python Unpacking Tuple

Summary: in this tutorial, you’ll learn how to unpack tuples in Python.

Reviewing Python tuples

Python defines a tuple using commas (,), not parentheses (). For example, the following defines a tuple that has two elements:

1,2 

Python uses the parentheses to make the tuple clearer:

(1, 2)

Python also uses the parentheses to create an empty tuple:

()

In addition, you can use the tuple() constructor like this:

tuple()

To define a tuple with only one element, you still need to use a comma. The following example illustrates how to define a tuple with one element:

1,

It’s equivalent to the following:

(1, )

Note that the following is an integer, not a tuple:

(1)

Unpacking a tuple

Unpacking a tuple means splitting the tuple’s elements into individual variables. For example:

x, y = (1, 2)

The left side:

x, y

is a tuple of two variables x and y.

The right side is also a tuple of two integers 1 and 2.

The expression assigns the elements of the tuple on the right side (1, 2) to each variable on the left side (x, y) based on the relative position of each element.

In the above example, x will take 1 and y will take 2.

See another example:

x, y ,z = 10, 20, 30

The right side is a tuple of three integers 10, 20, and 30. You can quickly check its type as follows:

numbers = 10, 20, 30
print(type(numbers))
Code language: PHP (php)

Output:

<class 'tuple'>
Code language: HTML, XML (xml)

In the above example, the x, y, and z variables will take the values 10, 20, and 30 respectively.

Using unpacking tuple to swap values of two variables

Traditionally, to swap the values of two variables, you would use a temporary variable like this:

x = 10
y = 20

print(f'x={x}, y={y}')

tmp = x
x = y
y = tmp

print(f'x={x}, y={y}')
Code language: PHP (php)

Output:

x=10, y=20
x=20, y=10    

In Python, you can use the unpacking tuple syntax to achieve the same result:

x = 10
y = 20

print(f'x={x}, y={y}')

x, y = y, x

print(f'x={x}, y={y}')
Code language: PHP (php)

Output:

x=10, y=20
x=20, y=10   

The following expression swaps the values of two variables x and y.

x, y = y, x

In this expression, Python evaluates the right-hand side first and then assign variable from the left-hand side to the values from the right-hand side.

ValueError: too many values to unpack

The following example unpacks the elements of a tuple into variables. However, it’ll result in an error:

x, y = 10, 20, 30

Error:

ValueError: too many values to unpack (expected 2)
Code language: HTTP (http)

The reason for this error is that the right hand side returns three values while the left hand side only has two variables.

To fix this, you can add a _ variable:

x, y, _ = 10, 20, 30

The _ variable is a regular variable in Python. By convention, it’s called a dummy variable.

Typically, you use the dummy variable in unpacking when you don’t care and use its value after that.

Extended unpacking using the * operator

Sometimes, you don’t want to unpack every single item in a tuple. For example, you may want to unpack the first and second elements. In this case, you can use the * operator. For example:

r, g, *other = (192, 210, 100, 0.5)

Output:

192
210
[100, 0.5]
Code language: JSON / JSON with Comments (json)

In this example, Python assigns 192 to r, 210 to g. Also, Python packs the remaining elements 100 and 0.5 into a list and assigns it to the other variable.

Notice that you can only use the * operator once on the left-hand side of an unpacking assignment.

The following example results in a error:

x, y, *z, *t = (10, 20, 30, '10:30')
Code language: JavaScript (javascript)

Error:

SyntaxError: two starred expressions in assignment
Code language: JavaScript (javascript)

Using the * operator on the right hand side

Python allows you to use the * operator on the right hand side. Suppose that you have two tuples:

odd_numbers = (1, 3, 5)
even_numbers = (2, 4, 6)

The following example uses the * operator to unpack those tuples and merge them into a single tuple:

numbers = (*odd_numbers, *even_numbers)
print(numbers)
Code language: PHP (php)

Output:

(1, 3, 5, 2, 4, 6)

Summary

• Python uses the commas (,) to define a tuple, not parentheses.
• Unpacking tuples means assigning individual elements of a tuple to multiple variables.
• Use the * operator to assign remaining elements of an unpacking assignment into a list and assign it to a variable.