How to Transform List Elements with Python map() Function

Summary: in this tutorial, you’ll learn how to use the Python map() function with lists.

Introduction to the Python map() function #

When working with a list (or a tuple), you often need to transform the elements of the list and return a new list that contains the transformed element.

Suppose, you want to double every number in the following bonuses list:

bonuses = [100, 200, 300]Copy

To do it, you can use a for loop to iterate over the elements, double each of them, and add it to a new list like this:

bonuses = [100, 200, 300]

new_bonuses = []

for bonus in bonuses:
    new_bonuses.append(bonus*2)

print(new_bonuses)Copy

Try it

Output:

[200, 400, 600]Copy

Python provides a nicer way to do this kind of task by using the map() built-in function.

The map() function iterates over all elements in a list (or a tuple), applies a function to each, and returns a new iterator of the new elements.

The following shows the basic syntax of the map() function:

iterator = map(fn, list)Copy

In this syntax, fn is the name of the function that will call on each element of the list.

In fact, you can pass any iterable to the map() function, not just a list or tuple.

Back to the previous example, to use the map() function, you define a function that doubles a bonus first and then use the map() function as follows:

def double(bonus):
    return bonus * 2


bonuses = [100, 200, 300]

iterator = map(double, bonuses)Copy

Or you make this code more concise by using a lambda expression like this:

bonuses = [100, 200, 300]
iterator = map(lambda bonus: bonus*2, bonuses)Copy

Once you have an iterator, you can iterate over the new elements using a for loop.

Or you can convert an iterator to a list by using the the list() function:

bonuses = [100, 200, 300]
iterator = map(lambda bonus: bonus*2, bonuses)
print(list(iterator))Copy

Using the Python map() function with a list of strings #

The following example uses the map() function to return a new list where each element is transformed into the proper case:

names = ['david', 'peter', 'jenifer']
new_names = map(lambda name: name.capitalize(), names)
print(list(new_names))Copy

Try it

Output:

['David', 'Peter', 'Jenifer']Copy

Using the Python map() function with a list of tuples #

Suppose that you have the following shopping cart represented as a list of tuples:

carts = [['SmartPhone', 400],
         ['Tablet', 450],
         ['Laptop', 700]]Copy

And you need to calculate the tax amount for each product with a 10% tax. In addition, you need to add the tax amount to the third element of each item in the list.

The return list should be something like this:

[['SmartPhone', 400, 40.0],
['Tablet', 450, 45.0],
['Laptop', 700, 70.0]]Copy

In order to do so, you can use the map() function to create a new element of the list and add the new tax amount to each like this:

carts = [['SmartPhone', 400],
         ['Tablet', 450],
         ['Laptop', 700]]

TAX = 0.1
carts = map(lambda item: [item[0], item[1], item[1] * TAX], carts)

print(list(carts))Copy

Try it

Output:

[['SmartPhone', 400, 40.0], ['Tablet', 450, 45.0], ['Laptop', 700, 70.0]]Copy

Summary #

• Use the Python map() function to call a function on every item of a list and returns an iterator.

Quiz #