Python filter() Function

The filter() function in Python lets you filter elements from an iterable (like a list or tuple) based on a condition (a function that returns True or False).

This is useful when you want to keep only certain items from a collection.

Syntax

filter(function, iterable)

Parameters:

• function: A function that returns True or False for each item.
• iterable: The collection you want to filter (e.g. list, tuple, set).

Returns:

• An iterator with elements for which the function returned True.

Example 1: Filter Even Numbers

def is_even(n):
    return n % 2 == 0

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = filter(is_even, numbers)
print(list(even_numbers))

[2, 4, 6]

Example 2: Using Lambda with filter()

numbers = [10, 25, 30, 45, 50]
result = filter(lambda x: x > 30, numbers)
print(list(result))

[45, 50]

Use Case: Remove Empty Strings

names = ["Alice", "", "Bob", "", "Charlie"]
filtered_names = filter(None, names)
print(list(filtered_names))

['Alice', 'Bob', 'Charlie']

When None is passed as the function, filter() removes all "falsy" values (like '', 0, None).

How is filter() Different from List Comprehension?

• filter() uses a function to decide which items to keep.
• List comprehension uses inline syntax.

# Using filter()
print(list(filter(lambda x: x > 0, [-2, 0, 3, 5])))  # [3, 5]

# Using list comprehension
print([x for x in [-2, 0, 3, 5] if x > 0])           # [3, 5]

Common Mistakes

• Forgetting to convert the result to a list: list(filter(...))
• Not providing a valid function that returns True/False
• Passing a non-iterable as the second argument

Interview Tip

filter() is often used in functional programming questions, especially when you need to reduce or process data streams efficiently without writing loops.

Summary

• filter() helps you pick elements that match a condition.
• It returns an iterator, which you often convert to a list.
• Use with named functions or lambdas.

Practice Problem

Write a program that takes a list of numbers and filters out the negative ones.

nums = [-5, 3, -1, 0, 4, -2]
positive_nums = list(filter(lambda x: x >= 0, nums))
print(positive_nums)

Expected Output:

[3, 0, 4]