How to Filter List Using List Comprehension in Python

You can filter a list in Python using list comprehension by adding a condition after the expression inside the brackets. The syntax is [expression for item in list if condition], which creates a new list with only the items that meet the condition.

Syntax

The basic syntax for filtering a list using list comprehension is:

• expression: the value to include in the new list (often the item itself)
• item: each element from the original list
• list: the original list you want to filter
• condition: a test that each item must pass to be included

This syntax creates a new list with only the items where the condition is True.

python

[expression for item in list if condition]

Example

This example shows how to filter a list of numbers to keep only the even ones using list comprehension.

python

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = [num for num in numbers if num % 2 == 0]
print(even_numbers)

Output

[2, 4, 6]

Common Pitfalls

Common mistakes when filtering lists with list comprehension include:

• Forgetting the if keyword before the condition.
• Using assignment = instead of comparison == in the condition.
• Modifying the original list inside the comprehension (which is not allowed).

Here is an example of a wrong and right way:

python

# Wrong: missing 'if'
numbers = [1, 2, 3, 4]
even = [num for num in numbers if num % 2 == 0]  # Correct syntax

# Right:
even = [num for num in numbers if num % 2 == 0]

Quick Reference

Remember these tips when filtering lists with list comprehension:

• Use if to filter items.
• The expression can transform items if needed.
• List comprehension creates a new list, original stays unchanged.

Key Takeaways

Use list comprehension with an if condition to filter lists in Python.

The syntax is [expression for item in list if condition] to create a new filtered list.

Always include the if keyword before the condition to avoid syntax errors.

List comprehension does not change the original list; it returns a new one.

You can also transform items while filtering by changing the expression part.