Lambda with filter() in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a lambda function in Python?

A lambda function is a small anonymous function defined with the keyword lambda. It can take any number of arguments but has only one expression, which is returned.

beginner

What does the filter() function do in Python?

filter() takes a function and a list (or any iterable) and returns a new iterator with only the items for which the function returns True.

beginner

How do you combine lambda and filter() to get even numbers from a list?

Use filter() with a lambda that checks if a number is even:
filter(lambda x: x % 2 == 0, numbers)

intermediate

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

filter() returns an iterator, which can be converted to a list using list().

intermediate

Why use lambda with filter() instead of a named function?

Using lambda keeps the code short and readable when the filtering logic is simple and used only once.

What does this code return?
list(filter(lambda x: x > 3, [1, 4, 2, 5]))

A [1, 2]

B [4, 5]

C [1, 4, 2, 5]

D [3, 4, 5]

The lambda keeps only numbers greater than 3, which are 4 and 5.

Which of these is a correct use of filter() with a lambda to get odd numbers?

A filter(lambda x: x % 2 == 1, numbers)

B filter(lambda x: x % 2 == 0, numbers)

C filter(lambda x: x > 2, numbers)

D filter(lambda x: x < 0, numbers)

The lambda checks if a number is odd by using x % 2 == 1.

What type of argument does filter() expect as its first parameter?

A A number

B A list

C A string

D A function that returns True or False

filter() needs a function that decides which items to keep.

What will list(filter(lambda x: x, [0, 1, '', 'hello'])) return?

A [1, 'hello']

B [0, 1]

C [0, '', 'hello']

D []

The lambda returns the item itself, so only truthy values (1 and 'hello') are kept.

Why might you convert the result of filter() to a list?

A Because <code>filter()</code> returns a dictionary

B Because <code>filter()</code> returns a string

C Because <code>filter()</code> returns an iterator, and lists are easier to use and print

D Because <code>filter()</code> returns a number

filter() returns an iterator, which you often convert to a list to see all items at once.

Explain how to use a lambda function with filter() to select items from a list.

Think about how lambda defines a small function and filter uses it to keep certain items.

Describe the difference between the output of filter() in Python 2 and Python 3.

Consider what you get directly from filter() and how you can use it.