Python filter() Function

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The filter() function is a built-in Python function used to filter elements from an iterable (like a list or tuple) based on a specific condition or criteria. It processes each item in the iterable and returns only those that satisfy the condition.

filter() Syntax

The syntax of filter( ) is:

filter(function, iterable)

filter() Parameters

  • function: A function that tests if each item of the iterable meets a certain condition. The function should return True or False for each element.
  • iterable: The iterable (e.g., list, tuple, set) whose elements you want to filter.

filter() Return Value

The filter() function returns an iterator containing elements for which the function returns True.


Consider you have a list of numbers and you want to retain only the even numbers:

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

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

In this example, the is_even function checks if a number is even. We use this function as the test function for filter(), which then processes the numbers list and retains only the even numbers.

It’s also common to use lambda functions with filter() for shorter, throwaway functions:

numbers = [1, 2, 3, 4, 5]
even_numbers = filter(lambda x: x % 2 == 0, numbers)
print(list(even_numbers))  # Outputs: [2, 4]

In essence, the filter() function provides a convenient way to selectively process elements from an iterable based on a specified condition.

Filtering Out Empty Strings

Consider you have a list of strings, and you want to remove any empty strings:

strings = ["apple", "", "banana", "cherry", ""]
result = filter(lambda s: s != "", strings)
print(list(result))  # Outputs: ["apple", "banana", "cherry"]

Removing Outliers in Data

For a list of data points, you can filter out values that lie outside a specific range:

data = [10, 45, 60, 85, 150, 200]
filtered_data = filter(lambda x: 50 <= x <= 150, data)
print(list(filtered_data))  # Outputs: [60, 85, 150]

filter( ) with Lambda Functions

When using filter(), we often need a function to determine whether each item in an iterable should be included in the output. For simple filtering criteria, it can be overkill to define a separate function using def. Instead, we can use a lambda function directly within filter().

Example: Filtering Strings by Length

Imagine you have a list of words, and you only want to keep words with more than 3 characters:

words = ["apple", "pie", "and", "sky", "hi"]
long_words = filter(lambda word: len(word) > 3, words)
print(list(long_words))  # Outputs: ['apple']

Why Use Lambda with filter( ) ?

  1. Conciseness: Lambda functions provide a concise way to define simple functions. This makes the code shorter and often more readable.
  2. Throwaway Use: Lambda functions are anonymous, meaning they don’t have a name. This is beneficial for “throwaway” or one-time use functions, which are only needed temporarily and won’t be reused elsewhere in the code.
  3. Functional Style: Combining filter() with lambda functions promotes a functional programming style, emphasizing operations on data collections without side effects.

Using lambda functions with filter() provides a streamlined way to filter items from iterables. The combination allows for concise, clear, and efficient filtering of data without the need for verbose function definitions, especially for simple filtering conditions.


The filter() function is a powerful and versatile tool in Python that aids in processing sequences. Whether you’re cleaning data, transforming sequences, or performing various tasks that require conditional processing of iterables, filter() is an indispensable tool.

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