Python Tip: map() and filter()

Cleaner and more efficient sequence operations

Learn how to use map() and filter() for cleaner and more efficient sequence operations.

Author

Shep Bryan IV

Python Tip: `map()` and `filter()`

Want to apply a function to every item in a sequence or filter elements? Use map() and filter() for cleaner and more efficient code.

Using `map()`

The map() function applies a function to all items in a list. It returns a map object, which is an iterator that can be converted into a list.

def myfunc(x):
    return x**2 + 1

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

new_numbers = map(myfunc, numbers)

print(list(new_numbers))  # [2, 5, 10, 17, 26]

[2, 5, 10, 17, 26]

Using `filter()`

The filter() function filters elements from an iterable based on a condition. It returns an iterator with elements that satisfy the condition.

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

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

even_numbers = filter(is_even, numbers)

print(list(even_numbers))  # [2, 4, 6, 8, 10]

[2, 4, 6, 8, 10]

Why This is Better Than Loops

Both map() and filter() use lazy evaluation, meaning they only process elements when called downstream.

By contrast, a loop will process and store all elements immediately.

def myfunc(x):
    return x**2 + 1

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

# This stores all results in memory immediately
list1 = [myfunc(x) for x in numbers]

# This only computes when needed
list2 = map(myfunc, numbers)

print(list(list2))

[2, 5, 10, 17, 26]

Wrap-Up

Now you can use map() and filter() to modify and filter sequences with ease.

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Shep Bryan IV