Frozen set behavior in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a frozen set in Python?

A frozen set is an immutable version of a set. Once created, you cannot add or remove elements from it.

beginner

Can you add elements to a frozen set after it is created?

No, frozen sets are immutable, so you cannot add or remove elements after creation.

intermediate

How do frozen sets behave when used as dictionary keys?

Frozen sets can be used as dictionary keys because they are immutable and hashable, unlike regular sets.

beginner

What happens if you try to call add() on a frozen set?

You get an AttributeError because frozen sets do not support methods that modify the set, like add() or remove().

beginner

How can you create a frozen set from a list?

Use the frozenset() function and pass the list as an argument, for example: frozenset([1, 2, 3]).

Which of the following is true about frozen sets?

A They cannot be used as dictionary keys.

B They can be modified after creation.

C They support the add() method.

D They are immutable and hashable.

Frozen sets are immutable and hashable, so they can be used as dictionary keys. They do not support modification methods like add().

What error do you get if you try to add an element to a frozen set?

A TypeError

B AttributeError

C ValueError

D KeyError

Calling add() on a frozen set raises an AttributeError because frozen sets do not have this method.

How do you create a frozen set from a list named my_list?

A frozenset(my_list)

B set(my_list)

C list(frozenset)

D frozen(my_list)

Use frozenset(my_list) to create an immutable frozen set from the list.

Which method is NOT supported by frozen sets?

A union()

B intersection()

C add()

D difference()

Frozen sets do not support add() because they are immutable, but they support union(), intersection(), and difference().

Why can frozen sets be used as dictionary keys but regular sets cannot?

A Frozen sets are hashable and immutable.

B Frozen sets are mutable.

C Regular sets are hashable.

D Regular sets are immutable.

Only immutable and hashable objects can be dictionary keys. Frozen sets are immutable and hashable, unlike regular sets.

Explain what a frozen set is and how it differs from a regular set.

Think about what 'frozen' means in real life.

Describe a situation where using a frozen set would be better than a regular set.

Consider when you want to keep a collection fixed and safe.

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