Set membership testing in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the in keyword do when used with a set in Python?

It checks if a value is present in the set and returns True if it is, otherwise False.

beginner

How do you check if an element is NOT in a set?

Use the not in keyword. For example, if element not in my_set: checks if element is not a member of my_set.

intermediate

Why is set membership testing faster than list membership testing?

Sets use a hash table internally, so checking membership is on average very fast (constant time), while lists check each item one by one (linear time).

beginner

What will be the output of this code?

my_set = {1, 2, 3}
print(2 in my_set)
print(5 in my_set)

True
False

Because 2 is in the set but 5 is not.

beginner

Can you use the in keyword to check membership in other data types besides sets?

Yes! You can use in with lists, tuples, strings, dictionaries (checks keys), and more.

What does element in my_set return if element is not in my_set?

A Raises an error

B False

C None

D True

The expression returns False if the element is not found in the set.

Which data structure generally provides the fastest membership testing?

A Set

B Tuple

C List

D String

Sets use hash tables internally, making membership testing very fast.

What keyword checks if an item is NOT in a set?

A not in

B not

C out

D no in

The 'not in' keyword checks if an item is absent from a set.

What will print('a' in {'a', 'b', 'c'}) output?

A Error

B False

C True

D None

The letter 'a' is in the set, so it prints True.

Which of these is NOT a valid use of in for membership testing?

A Checking if a key is in a dictionary

B Checking if a value is in a list

C Checking if a character is in a string

D Checking if a number is in an integer

You cannot check membership inside an integer because it is not a collection.

Explain how to check if an item is in a set and why this is efficient.

Think about how sets store items differently than lists.

Describe the difference between using 'in' and 'not in' with sets.

One checks if something is there, the other checks if it is missing.