Filtering in Comprehensions

Filtering in Comprehensions

In this tutorial, we'll include only the values that pass a condition.

You can add an if condition to a comprehension to keep only some values.

Normal loop:

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

for number in numbers:
    if number % 2 == 0:
        evens.append(number)

Comprehension:

numbers = [1, 2, 3, 4, 5]
evens = [number for number in numbers if number % 2 == 0]

Output:

[2, 4]

The Shape

[expression for item in collection if condition]

The condition decides whether the item is included.

Transform and Filter

You can transform values and filter them at the same time:

words = ["apple", "cat", "banana"]
long_words = [word.upper() for word in words if len(word) > 3]

Output:

["APPLE", "BANANA"]

Keep It Readable

Filtering comprehensions are useful, but do not make them too clever. If you need multiple conditions and several calculations, use a normal loop.

Filtering Without Changing

Sometimes the expression is just the original item:

names = ["Ada", "", "Grace"]
real_names = [name for name in names if name != ""]

This keeps only the non-empty strings. You are not transforming the names; you are only filtering them.

Practice

Start with a list of scores. Build a new list containing only scores greater than or equal to 50.