Set creation in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a set in Python?

A set is a collection of unique items. It does not keep order and does not allow duplicates.

beginner

How do you create an empty set in Python?

Use set(). Using {} creates an empty dictionary, not a set.

beginner

Which of these creates a set with elements 1, 2, and 3?

1) {1, 2, 3}
2) set([1, 2, 3])
3) {1, 2, 2, 3}
4) set()

Options 1, 2, and 3 all create a set with elements 1, 2, and 3. Option 3 has a duplicate 2, but sets remove duplicates automatically.

intermediate

Can sets contain mutable items like lists?

No. Sets can only contain immutable (unchangeable) items like numbers, strings, or tuples.

beginner

What happens if you try to create a set with duplicate elements?

Duplicates are automatically removed. The set will only keep one copy of each unique element.

Which syntax creates an empty set in Python?

A {}

B set([])

C []

D set()

Only set() creates an empty set. {} creates an empty dictionary.

What will be the output of {1, 2, 2, 3}?

A {1, 2, 3}

B {1, 2, 2, 3}

C [1, 2, 3]

D (1, 2, 3)

Sets remove duplicates automatically, so only unique elements remain.

Which of these can be an element of a set?

A (1, 2)

B {1: 'a'}

C [1, 2]

D {1, 2}

Tuples are immutable and can be set elements. Lists and dictionaries are mutable and cannot.

What type of collection is a set?

A Unordered and allows duplicates

B Unordered and no duplicates

C Ordered and allows duplicates

D Ordered and no duplicates

Sets are unordered collections that do not allow duplicate elements.

How do you create a set from a list [1, 2, 3]?

A [1, 2, 3].set()

B {[1, 2, 3]}

C set([1, 2, 3])

D set{1, 2, 3}

Use set() with the list as argument to create a set from it.

Explain how to create a set in Python and what makes sets different from lists.

Think about curly braces and the set() function.

Describe what types of elements can be added to a set and why some types are not allowed.

Consider what it means for an object to be unchangeable.

Practice

1. Which of the following is the correct way to create a set with the elements 1, 2, and 3 in Python?

easy

A. (1, 2, 3)

B. [1, 2, 3]

C. {1, 2, 3}

D. set[1, 2, 3]

Solution

1. Step 1: Understand set creation syntax

   Sets are created using curly braces with comma-separated values or the set() function.

2. Step 2: Identify correct syntax for set with elements

   {1, 2, 3} correctly creates a set with elements 1, 2, and 3.

3. Final Answer:

   {1, 2, 3} -> Option C

4. Quick Check:

   Curly braces with values = set [OK]

Hint: Use curly braces with commas for sets, not brackets [OK]

Common Mistakes:

• Using square brackets creates a list, not a set
• Using parentheses creates a tuple, not a set
• Using set[] is invalid syntax

2. What is the correct way to create an empty set in Python?

easy

A. set()

B. {}

C. []

D. empty_set()

Solution

1. Step 1: Recognize empty set syntax

   Using {} creates an empty dictionary, not a set.

2. Step 2: Use the set() function for empty sets

   set() correctly creates an empty set.

3. Final Answer:

   set() -> Option A

4. Quick Check:

   Empty set = set() function [OK]

Hint: Empty sets need set(), not curly braces [OK]

Common Mistakes:

• Using {} creates an empty dictionary, not a set
• Using [] creates an empty list
• Using undefined functions like empty_set()

3. What will be the output of the following code?

my_set = {1, 2, 2, 3, 4, 4}
print(my_set)

medium

A. {1, 2, 2, 3, 4, 4}

B. [1, 2, 3, 4]

C. (1, 2, 3, 4)

D. {1, 2, 3, 4}

Solution

1. Step 1: Understand set uniqueness property

   Sets automatically remove duplicate elements, so repeated values appear only once.

2. Step 2: Apply uniqueness to given elements

   Duplicates 2 and 4 are removed, leaving {1, 2, 3, 4}.

3. Final Answer:

   {1, 2, 3, 4} -> Option D

4. Quick Check:

   Sets remove duplicates = {1, 2, 3, 4} [OK]

Hint: Sets keep unique items only, duplicates vanish [OK]

Common Mistakes:

• Expecting duplicates to remain in the set
• Confusing set output with list or tuple syntax
• Thinking sets preserve order

4. Identify the error in the following code snippet:

my_set = set{1, 2, 3}
print(my_set)

medium

A. SyntaxError due to incorrect set creation syntax

B. Prints {1, 2, 3} correctly

C. TypeError because set() expects a list

D. NameError because set is not defined

Solution

1. Step 1: Check set creation syntax

   The correct way to create a set using the function is set() with parentheses, not curly braces.

2. Step 2: Identify syntax error

   set{1, 2, 3} is invalid syntax and causes a SyntaxError.

3. Final Answer:

   SyntaxError due to incorrect set creation syntax -> Option A

4. Quick Check:

   Use set() with parentheses, not braces [OK]

Hint: Use parentheses with set(), not curly braces [OK]

Common Mistakes:

• Using curly braces after set instead of parentheses
• Confusing set() with dictionary syntax
• Assuming set is undefined

5. Given the list nums = [1, 2, 2, 3, 4, 4, 5], which code snippet correctly creates a set of unique elements from this list?

hard

A. unique_nums = {nums}

B. unique_nums = set(nums)

C. unique_nums = list(set(nums))

D. unique_nums = {1, 2, 3, 4, 5}

Solution

1. Step 1: Understand how to convert list to set

   The set() function can take an iterable like a list and return a set of unique elements.

2. Step 2: Analyze each option

   unique_nums = {nums} creates a set with the entire list as one element (invalid). unique_nums = set(nums) correctly converts the list to a set. unique_nums = list(set(nums)) converts to set then back to list, which is not a set. unique_nums = {1, 2, 3, 4, 5} manually writes the set but is not dynamic.

3. Final Answer:

   unique_nums = set(nums) -> Option B

4. Quick Check:

   Use set() on list to get unique elements [OK]

Hint: Use set() on list to get unique items fast [OK]

Common Mistakes:

• Trying to put list inside curly braces directly
• Converting set back to list when set is needed
• Hardcoding values instead of using the list variable