← Python

Nested Lists

A nested list is a list that contains other lists as its elements

What it does: Sets automatically remove duplicate values — each item appears only once. Use set() for an empty set. .add() inserts one element. {} creates an empty dict, not a set — you must use set(). Sets have no guaranteed order, so s[0] doesn't work — use in to check membership. .append() is for lists; sets use .add() to insert a single item. Sets don't support index-based pop — use .remove() or .discard() by value. in checks membership — 3 is in the set so it returns True. len() counts the number of unique items in the set.

Common mistakes: {} creates an empty dict, not a set. Use set() for an empty set. Sets use .add(), not .append() (that's for lists).

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Learning

Duo

What makes a set special?

Items are unique no duplicatesItems are ordered

Matches

❌ s = {1, 2, 3} print(s[0])

↓

Error: Sets have no index

Applying

Meanings

Term

Set

Meaning

An unordered collection of unique items in curly braces.

Practice

colors = {'red', 'blue', 'green'}

Testing

Gaps

s = ()

Check Answer

Spots

1# Create an empty set

2s = {}

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Quick Questions

1. What makes a set special?

A. Items are unique no duplicates ✓

B. Items are ordered

Sets automatically remove duplicates — great for deduplication tasks

2. How do you create an empty set?

A. {}

B. set() ✓

{} creates an empty dict, not a set — you must use set()

3. {1, 2, 2, 3} becomes?

A. {1, 2, 3} ✓

B. {1, 2, 2, 3}

Sets silently drop duplicate values — 2 appears only once

4. Can you access set items by index?

A. Yes like lists

B. No sets are unordered ✓

Sets have no guaranteed order, so s[0] doesn't work — use in to check membership

5. How do you add to a set?

A. .add() ✓

B. .append()

.append() is for lists; sets use .add() to insert a single item

Match the Pairs

s = {1, 2, 3} print(s[0]) → Error: Sets have no index

empty = {} → Error: {} creates dict, not set

s = {1, 2, 2, 3} → Valid: Becomes {1, 2, 3}

s = set() s.add(5) → Ok: Empty set then add 5

s = {3, 1, 2, 1} print(len(s)) → Output: 3

s = {1, 2, 3} print(2 in s) → Output: True

Key Term

Set

An unordered collection of unique items in curly braces.~~set()

Fill in the Blanks

1. s = set()

2. s = {1, 2, 3} s.add(4)

3. s = {1, 2, 3} s.discard(2)

4. print(3 in {1, 2, 3})

5. s = {1, 2, 2, 3} print(len(s))

Spot the Error

Question 1

A # Create an empty set

B s = {}

C s.add(1)

D print('Set:', s)

Line B — {} creates an empty dict, not a set. Use set() for an empty set.

Question 2

A s = {1, 2, 3}

B s.append(4)

C print('Set:', s)

D print('Done')

Line B — Sets use .add(), not .append() (that's for lists)

Question 3

A s = {1, 2, 3}

B first = s[0]

C print('First:', first)

D print('Done')

Line B — Sets are unordered: you can't access items by index

Question 4

A s = {1, 2, 3}

B s.remove(5)

C print('Set:', s)

D print('Done')

Line B — 5 is not in the set: remove() raises KeyError. Use discard() instead.

Question 5

A s = {[1, 2], [3, 4]}

B label = 'Coords'

C print('Set:', s)

D print('Done')

Line A — Lists can't be in sets (unhashable). Use tuples instead.

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