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.

Practice

1. What is a key characteristic of a frozenset in Python?

easy

A. It is a type of list.

B. It allows duplicate elements.

C. It can be modified by adding or removing elements.

D. It is immutable and cannot be changed after creation.

Solution

1. Step 1: Understand what a frozenset is

   A frozenset is a set that cannot be changed after it is created, meaning it is immutable.

2. Step 2: Compare options with frozenset properties

   'It is immutable and cannot be changed after creation.' correctly states immutability. Claims that it allows duplicate elements or can be modified are false because frozensets do not allow duplicates and cannot be modified. 'It is a type of list.' is incorrect because frozensets are not lists.

3. Final Answer:

   It is immutable and cannot be changed after creation. -> Option D

4. Quick Check:

   Frozen set = immutable set [OK]

Hint: Remember: frozenset means frozen, so no changes allowed [OK]

Common Mistakes:

• Thinking frozensets can be changed like normal sets
• Confusing frozenset with list or tuple
• Assuming duplicates are allowed

2. Which of the following is the correct way to create a frozenset from a list [1, 2, 3]?

easy

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

B. fs = frozen_set([1, 2, 3])

C. fs = frozenset{1, 2, 3}

D. fs = frozenset(1, 2, 3)

Solution

1. Step 1: Recall the syntax for creating a frozenset

   The correct syntax uses the function frozenset() with an iterable inside parentheses.

2. Step 2: Evaluate each option

   fs = frozenset([1, 2, 3]) uses frozenset with a list inside parentheses, which is correct. fs = frozen_set([1, 2, 3]) uses a wrong function name. fs = frozenset{1, 2, 3} uses curly braces which is invalid syntax for function calls. fs = frozenset(1, 2, 3) passes multiple arguments instead of one iterable.

3. Final Answer:

   fs = frozenset([1, 2, 3]) -> Option A

4. Quick Check:

   frozenset(iterable) = correct syntax [OK]

Hint: Use frozenset() with one iterable argument inside parentheses [OK]

Common Mistakes:

• Using wrong function name like frozen_set
• Using curly braces instead of parentheses
• Passing multiple arguments instead of one iterable

3. What will be the output of this code?

fs = frozenset([1, 2, 2, 3])
print(len(fs))

medium

A. 4

B. Error

C. 3

D. 2

Solution

1. Step 1: Understand frozenset removes duplicates

   The list has elements [1, 2, 2, 3]. When converted to frozenset, duplicates are removed, so it becomes {1, 2, 3}.

2. Step 2: Calculate length of frozenset

   The frozenset has 3 unique elements, so len(fs) returns 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   frozenset removes duplicates, length = 3 [OK]

Hint: Count unique elements only, duplicates are removed [OK]

Common Mistakes:

• Counting duplicates as separate elements
• Expecting an error due to duplicates
• Confusing frozenset with list length

4. What is wrong with this code?

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

medium

A. frozenset object has no attribute 'add'

B. It prints {1, 2, 3, 4}

C. It prints {1, 2, 3}

D. SyntaxError

Solution

1. Step 1: Understand frozenset immutability

   frozenset objects cannot be changed after creation, so they do not have methods like add().

2. Step 2: Identify the error when calling add()

   Calling fs.add(4) raises an AttributeError because 'frozenset' has no 'add' method.

3. Final Answer:

   frozenset object has no attribute 'add' -> Option A

4. Quick Check:

   frozenset is immutable, no add() method [OK]

Hint: frozenset has no add or remove methods [OK]

Common Mistakes:

• Trying to add or remove elements from frozenset
• Expecting frozenset to behave like set
• Confusing AttributeError with SyntaxError

5. Given two frozensets:

fs1 = frozenset([1, 2, 3])
fs2 = frozenset([3, 4, 5])

Which expression correctly finds the common elements between fs1 and fs2?

hard

A. fs1 + fs2

B. fs1 & fs2

C. fs1 | fs2

D. fs1 - fs2

Solution

1. Step 1: Recall set operations on frozensets

   frozensets support set operations like intersection (&), union (|), difference (-).

2. Step 2: Identify operation for common elements

   The intersection operator & returns elements common to both sets. So fs1 & fs2 gives {3}.

3. Final Answer:

   fs1 & fs2 -> Option B

4. Quick Check:

   Intersection (&) = common elements [OK]

Hint: Use & operator to find common elements in frozensets [OK]

Common Mistakes:

• Using + which is invalid for sets
• Confusing union (|) with intersection
• Using difference (-) instead of intersection