{"id":389,"date":"2020-10-09T01:57:14","date_gmt":"2020-10-09T01:57:14","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=389"},"modified":"2025-03-31T02:58:10","modified_gmt":"2025-03-31T02:58:10","slug":"python-list-comprehensions","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-basics\/python-list-comprehensions\/","title":{"rendered":"Python List Comprehensions"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about Python List comprehensions that allow you to create a new list from an existing one.<\/p>\n\n\n\n

Introduction to Python list comprehensions #<\/a><\/h2>\n\n\n\n

In programming, you often need to transform elements of a list<\/a> and return a new list. <\/p>\n\n\n\n

For example, suppose that you have a list of five numbers like this:<\/p>\n\n\n

numbers = [1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>, 5<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
And you want to get a list of squares based on this numbers<\/code> list<\/p>\n\n\n\n
numbers = [1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>, 5<\/span>]\n\nsquares = []\nfor<\/span> number in<\/span> numbers:\n    squares.append(number**2<\/span>)\n\nprint(squares)\n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[1<\/span>, 4<\/span>, 9<\/span>, 16<\/span>, 25<\/span>]<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the for<\/code> loop iterates over the elements of the numbers<\/code> list,  squares each number and adds the result to the squares list.<\/p>\n\n\n\n
Note that a square number is the product of the number multiplied by itself. For example, square number 2 is 2*2 = 4, square number of 3 is 3*3  = 9, and so on.<\/p>\n\n\n\n
numbers = [1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>, 5<\/span>]\n\nsquares = list(map(lambda<\/span> number: number**2<\/span>, numbers))\n\nprint(squares)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[1<\/span>, 4<\/span>, 9<\/span>, 16<\/span>, 25<\/span>]<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
Both the for<\/code> loop and map()<\/code> function can help you create a new list based on an existing one. But the code isn’t really concise and beautiful. <\/p>\n\n\n\n
To help you create a list based on the transformation of elements of an existing list, Python provides a feature called list comprehensions.<\/p>\n\n\n\n
The following shows how to use list comprehension to make a list of squares from the numbers<\/code> list:<\/p>\n\n\n
numbers = [1<\/span>, 2<\/span>, 3<\/span>, 4<\/span>, 5<\/span>]\nsquares = [number**2<\/span> for<\/span> number in<\/span> numbers]\n\nprint(squares)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[1<\/span>, 4<\/span>, 9<\/span>, 16<\/span>, 25<\/span>]<\/code><\/span>Code language:<\/span> JSON \/ JSON with Comments<\/span> (<\/span>json<\/span>)<\/span><\/small><\/pre>\n\n\n
And here’s the list comprehension part:<\/p>\n\n\n
squares = [number**2<\/span> for<\/span> number in<\/span> numbers]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
A list comprehension consists of the following parts:<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Since the map()<\/code> function returns an iterator<\/a>, you need to use the list()<\/code> function to convert the iterator to a list.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
To make the code more concise, you can use the built-in map()<\/a><\/code> function with a lambda expression:<\/p>\n\n\n
Try it<\/a><\/p>\n\n\n\n
The straightforward way is to use a for loop<\/a>:<\/p>\n\n\n