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

filter(function, iterable)

filter() Parameters

The function takes two parameters:

• function - a function that runs for each item of an iterable
• iterable - a sequence that needs to be filtered like sets, lists, tuples, etc

filter() Return Value

The filter() function returns an iterator.

Example: Filter Vowels From List

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']
    if letter in vowels:
        return True 
    else:
        return False

# selects only vowel elements
filtered_vowels = filter(filter_vowels, letters)

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

# Output: ('a', 'e', 'i', 'o')

Here's how the above program works :

• each element of letters is passed to the filter_vowels() function
• if filter_vowels() returns True, filter() selects the element

Note: Here, the program returns the iterator, which we converted into a tuple using the vowels = tuple(fitered_vowels).

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]

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']