← Python

Sets – Operations

Sets support mathematical operations: union |, intersection &, difference -, and subset tests

What it does: people[0] gets the first dict, then ['name'] gets its name value. d['nums'] returns the list, then .append() adds to it. Think of it as a table — each dict is a row with named columns. Lists hold multiple records; dicts give each record named fields. .add() is for sets; lists use .append() to add items. Dict values can be any Python object — strings, numbers, lists, even other dicts. d['tags'] is the list ['python', 'code'] — len() of that list is 2. The value is a list, so use an integer index: d['tags'][0], not d['tags']['a'].

Common mistakes: people is a list: must index first: people[0]['name']. Dicts use keys not indices. Use ['name'] not [0].

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Learning

Duo

What is a list of dictionaries?

A list where each item is a dictionaryA dictionary of lists

Matches

❌ people = [{'name':'Ali'}] print(people['name'])

↓

Error: people is a list, use [0]

Applying

Meanings

Term

List of dicts

Meaning

A list where each element is a dictionary like records.

Practice

people = [{'name': 'Alice'}, {'name': 'Bob'}]

Testing

Gaps

print(people[]['name'])

Check Answer

Spots

1people = [{'name': 'Alice'}]

2greeting = people['name']

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Quick Questions

1. What is a list of dictionaries?

A. A list where each item is a dictionary ✓

B. A dictionary of lists

Think of it as a table — each dict is a row with named columns

2. people = [{'name':'Al'}]\npeople[0]['name'] returns?

A. {'name':'Al'}

B. 'Al' ✓

people[0] gets the first dict, then ['name'] looks up the key in it

3. d = {'items': [1,2,3]}\nd['items'][0] returns?

A. 1 ✓

B. [1, 2, 3]

d['items'] gets the list, then [0] picks the first element of that list

4. Why combine lists and dicts?

A. For faster code

B. To store structured records ✓

Lists hold multiple records; dicts give each record named fields

5. How do you add a dict to a list of dicts?

A. .append({'key': 'val'}) ✓

B. .add({'key': 'val'})

.add() is for sets; lists use .append() to add items

Match the Pairs

people = [{'name':'Ali'}] print(people['name']) → Error: people is a list, use [0]

d = {'tags': ['a','b']} print(d['tags']['a']) → Error: Use integer index for list

people = [{'name':'Ali'},{'name':'Bo'}] people[0]['name'] → Valid: Returns 'Ali'

d = {'nums': [1,2,3]} d['nums'].append(4) → Ok: Append to list inside dict

p = [{'n':'Ali'},{'n':'Bo'}] for x in p: print(x['n']) → Output: Ali Bo

d = {'c':['r','g']} print(d['c'][1]) → Output: g

Key Term

List of dicts

A list where each element is a dictionary like records.~~list[i]['key']

Fill in the Blanks

1. people = [{'name': 'Alice'}] print(people[0]['name'])

2. d = {'items': [1, 2, 3]} print(d['items'][0])

3. people = [] people.append( {'name': 'Bob'} )

4. for person in people: print(person['name'])

5. student = {'scores': [90, 85]} student['scores'].append(95)

Spot the Error

Question 1

A people = [{'name': 'Alice'}]

B greeting = people['name']

C print('Hello,', greeting)

D print('Done')

Line B — people is a list: must index first: people[0]['name']

Question 2

A people = [{'name': 'Alice'}]

B greeting = people[0][0]

C print('Hello,', greeting)

D print('Done')

Line B — Dicts use keys not indices. Use ['name'] not [0].

Question 3

A d = {'items': [1, 2]}

B d['items'].add(3)

C print('Items:', d['items'])

D print('Done')

Line B — Lists use .append(), not .add() (that's for sets)

Question 4

A people = {'name': 'Alice'}, {'name': 'Bob'}

B first = people[0]['name']

C print('First:', first)

D print('Count:', len(people))

Line A — Missing square brackets: this creates a tuple, not a list

Question 5

A d = {'scores': [90, 85]}

B print(d.scores[0])

Line B — Use bracket notation d['scores'], not dot notation d.scores

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