filter() function is used to extract elements from an iterable (like a list, tuple or set) that satisfy a given condition. It works by applying a function to each element and keeping only those for which function returns True.

Example: This example shows how to keep only the words starting with the letter 'a' from a list of fruits.

def starts_a(w):
    return w.startswith("a")

li = ["apple", "banana", "avocado", "cherry", "apricot"]
res = filter(starts_a, li)
print(list(res))

['apple', 'avocado', 'apricot']

Explanation: function starts_a checks if a word begins with 'a' and filter() applies this function to each fruit and returns only matching ones.

Syntax

filter(function, iterable)

Parameters:

• function: tests each element and if return, True Keep the element, if False Discard the element
• iterable: Any iterable (list, tuple, set, etc.).

Return Value: A filter object (an iterator), which can be converted into a list, tuple, set, etc.

Examples

Example 1: This code defines a regular function to check if a number is even and then uses filter() to extract all even numbers from a list.

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

a = [1, 2, 3, 4, 5, 6]
b = filter(even, a)
print(list(b)) # Convert filter object to a list

[2, 4, 6]

Explanation:

• even function checks if a number is divisible by 2.
• filter() applies this function to each item in a.
• Only even numbers are included in output.

Example 2: Below example uses a lambda function with filter() to select numbers divisible by 3.

a = [1, 2, 3, 4, 5, 6]
b = filter(lambda x: x % 3 == 0, a)
print(list(b))  

[3, 6]

Example 3: Here, lambda function is used with filter() to keep only words that have more than 5 letters from a list of fruits.

a = ["apple", "banana", "cherry", "kiwi", "grape"]
b = filter(lambda w: len(w) > 5, a)
print(list(b))

['banana', 'cherry']

Example 4: This code uses filter() with None as the function to remove all falsy values (like empty strings, None and 0) from a list.

L = ["apple", "", None, "banana", 0, "cherry"]
A = filter(None, L)
print(list(A))

['apple', 'banana', 'cherry']

Explanation: filter(None, L) removes all falsy values (empty string, None and 0) and keeps only truthy ones.