Python filter()

In this tutorial, we will learn about the Python filter() function with the help of examples.

The filter() function extracts elements from an iterable (list, tuple etc.) for which a function returns True.

Example

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# returns True if number is even
def check_even(number):
    if number % 2 == 0:
          return True  

    return False

# Extract elements from the numbers list for which check_even() returns True
even_numbers_iterator = filter(check_even, numbers)

# converting to list
even_numbers = list(even_numbers_iterator)

print(even_numbers)

# Output: [2, 4, 6, 8, 10]

1. filter() Syntax

Its syntax is:

filter(function, iterable)

2. filter() Arguments

The filter() function takes two arguments:

• function – a function
• iterable – an iterable like sets, lists, tuples etc.

3. filter() Return Value

The filter() function returns an iterator.

Note: You can easily convert iterators to sequences like lists, tuples, strings etc.

4. Example 1: Working of filter()

letters = ['a', 'b', 'd', 'e', 'i', 'j', 'o']

# a function that returns True if letter is vowel
def filter_vowels(letter):
    vowels = ['a', 'e', 'i', 'o', 'u']
    return True if letter in vowels else False

filtered_vowels = filter(filter_vowels, letters)

# converting to tuple
vowels = tuple(filtered_vowels)
print(vowels)

Output

('a', 'e', 'i', 'o', 'u')

Here, the filter() function extracts only the vowel letters from the letters list. Here’s how this code works:

• Each element of the letters list is passed to the filter_vowels() function.
• If filter_vowels() returns True, that element is extracted otherwise it’s filtered out.

Note: It’s also possible to filter lists using a loop, however, using the filter() function is much more cleaner.

5. Example 2: Using Lambda Function Inside filter()

numbers = [1, 2, 3, 4, 5, 6, 7]

# the lambda function returns True for even numbers 
even_numbers_iterator = filter(lambda x: (x%2 == 0), numbers)

# converting to list
even_numbers = list(even_numbers_iterator)

print(even_numbers)

Output

[2, 4, 6]

Here, we have directly passed a lambda function inside filter().

Our lambda function returns True for even numbers. Hence, the filter() function returns an iterator containing even numbers only.

6. Example 3: Using None as a Function Inside filter()

# random list
random_list = [1, 'a', 0, False, True, '0']

filtered_iterator = filter(None, random_list)

#converting to list
filtered_list = list(filtered_iterator)

print(filtered_list)

Output

[1, 'a', True, '0']

When None is used as the first argument to the filter() function, all elements that are truthy values (gives True if converted to boolean) are extracted.