Python List Comprehension: Efficient List Creation and Filtering

Learn about list comprehension in Python, a concise method for creating new lists from existing ones. Explore how to use list comprehension to filter and generate lists based on specific conditions, such as selecting items with certain characters. Understand the advantages of list comprehension over traditional for loops with practical examples.

Python - List Comprehension

List Comprehension

List comprehension offers a concise way to create new lists based on existing lists.

Suppose you have a list of fruits and you want to create a new list containing only the fruits with the letter "a" in their names.

Without using list comprehension, you would write a for loop with a conditional test inside:

Example


fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
  
for x in fruits:
  if "a" in x:
    newlist.append(x)
  
print(newlist)

Output


["apple", "banana", "mango"]

With list comprehension, you can achieve the same result in a single line of code:

Example


fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
  
 print(newlist)

Output


["apple", "banana", "mango"]

The Syntax

The syntax for list comprehension is:

Example


newlist = [expression for item in iterable if condition == True]

The return value is a new list, leaving the original list unchanged.

Condition

The condition acts as a filter that only includes items that evaluate to True.

Example

Only include items that are not "apple":


newlist = [x for x in fruits if x != "apple"]

Output


["banana", "cherry", "kiwi", "mango"]

The condition if x != "apple" returns True for all items except "apple".

The condition is optional and can be omitted:

Example

With no condition:


newlist = [x for x in fruits]

Iterable

The iterable can be any iterable object, such as a list, tuple, or set.

Example

You can use the range() function to create an iterable:


newlist = [x for x in range(10)]

Output


[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Same example, but with a condition:

Example

Include only numbers less than 5:

Example


newlist = [x for x in range(10) if x < 5]

Output


[0, 1, 2, 3, 4]

Expression

The expression is the current item in the iteration, but it can also be manipulated before being added to the new list.

Example

Convert all items to uppercase:


newlist = [x.upper() for x in fruits]

Output


["APPLE", "BANANA", "CHERRY", "KIWI", "MANGO"]

You can set the outcome to any value you like:

Example

Set all items to 'hello':


newlist = ['hello' for x in fruits

Output


["hello", "hello", "hello", "hello", "hello"]

The expression can also contain conditions to manipulate the outcome:

Example

Return "orange" instead of "banana":


newlist = [x if x != "banana" else "orange" for x in fruits]

Output


["apple", "orange", "cherry", "kiwi", "mango"]

The expression above means: "Return the item if it is not 'banana', otherwise return 'orange'".