Frozen set behavior in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of frozen set union operation

What is the output of this Python code?

Python

a = frozenset([1, 2, 3])
b = frozenset([3, 4, 5])
result = a.union(b)
print(result)

A frozenset({1, 2, 3, 4, 5})

B {1, 2, 3, 4, 5}

C frozenset([1, 2, 3, 4, 5])

D TypeError

💡 Hint

Remember that union returns a set or frozen set depending on the caller.

The union method of a frozen set returns a new frozen set containing all unique elements from both sets.

❓ Predict Output

intermediate

Frozen set mutability test

What happens when you try to add an element to a frozen set?

Python

fs = frozenset([1, 2, 3])
fs.add(4)

A frozenset({1, 2, 3, 4})

B TypeError

C AttributeError

D No output, element added silently

💡 Hint

Frozen sets do not support methods that modify them.

Frozen sets are immutable and do not have an add method, so calling add raises AttributeError.

❓ Predict Output

advanced

Frozen set as dictionary key behavior

What is the output of this code?

Python

fs1 = frozenset([1, 2])
fs2 = frozenset([2, 1])
d = {fs1: 'value1'}
print(d[fs2])

A None

B KeyError

C TypeError

D value1

💡 Hint

Frozen sets with the same elements are equal and hash the same.

Frozen sets are hashable and two frozen sets with the same elements are equal, so fs2 can be used to access the dictionary key fs1.

❓ Predict Output

advanced

Frozen set intersection with mutable set

What is the output of this code?

Python

fs = frozenset([1, 2, 3])
s = {2, 3, 4}
result = fs.intersection(s)
print(result)

A frozenset({2, 3})

B {2, 3}

C TypeError

D frozenset({4})

💡 Hint

The intersection method returns the same type as the caller.

The intersection of a frozen set and a set returns a frozen set containing elements common to both.

🧠 Conceptual

expert

Why are frozen sets hashable but sets are not?

Which statement best explains why frozen sets can be used as dictionary keys but sets cannot?

A Sets are mutable but frozen sets are implemented in C, making them hashable.

B Frozen sets are immutable, so their hash value does not change, while sets are mutable and unhashable.

C Frozen sets store elements in sorted order, sets do not.

D Frozen sets use less memory than sets, making them hashable.

💡 Hint

Think about what makes an object usable as a dictionary key.

Only immutable objects with a fixed hash value can be dictionary keys. Frozen sets are immutable, so their hash is stable. Sets can change, so they are unhashable.