Python filter()

# returns True if the argument passed is even

def check_even(number):
    if number % 2 == 0:
          return True  

    return False


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

# if an element passed to check_even() returns True, select it
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]

filter() Syntax

The syntax of filter() is:

filter(function, iterable)

filter() Arguments

The filter() function takes two arguments:

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

filter() Return Value

The filter() function returns an iterator.

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

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')

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.

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.

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.