filter() function in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the filter() function do in Python?

The filter() function takes a function and a sequence (like a list) and returns a new sequence with only the items for which the function returns True.

beginner

How do you use filter() with a lambda function to keep only even numbers from a list?

You write:
filter(lambda x: x % 2 == 0, my_list)
This keeps only numbers divisible by 2 (even numbers).

intermediate

What type of object does filter() return in Python 3?

It returns a filter object, which is an iterator. You can convert it to a list using list() to see the filtered items.

intermediate

What happens if the function passed to filter() is None?

If the function is None, filter() removes all items that are false in a Boolean context (like False, 0, '', None).

beginner

Can filter() be used with any iterable, or only lists?

filter() works with any iterable, such as lists, tuples, sets, or even strings.

What does filter() return in Python 3?

A A dictionary

B A list

C A tuple

D A filter object (an iterator)

filter() returns a filter object, which is an iterator. You can convert it to a list if needed.

What happens if you pass None as the function to filter()?

A It returns the original iterable unchanged

B It raises an error

C It filters out all false values from the iterable

D It returns an empty list

Passing None makes filter() remove all items that are false in a Boolean context.

Which of these is a correct way to use filter() to keep only positive numbers from a list nums?

A filter(lambda x: x > 0, nums)

B filter(lambda x: x < 0, nums)

C filter(lambda x: x == 0, nums)

D filter(lambda x: x != 0, nums)

The function lambda x: x > 0 keeps only numbers greater than zero.

How can you see the filtered results from filter()?

A Print the filter object directly

B Convert the filter object to a list using <code>list()</code>

C Use <code>str()</code> on the filter object

D Use <code>dict()</code> on the filter object

The filter object is an iterator, so converting it to a list shows the filtered items.

Can filter() be used on a string?

A Yes, because strings are iterable

B No, strings are not iterable

C Only if converted to a list first

D Only with a special function

Strings are iterable, so filter() can be used directly on them.

Explain how the filter() function works and give an example using a lambda function to filter even numbers.

Think about how you pick only certain items from a list based on a test.

What happens when you pass None as the function to filter()? Why might this be useful?

Consider what values are treated as false in Python.