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Sets – Operations

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

Part of: Python Coding — Module 41

Understanding Sets – Operations in Python

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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This module's key terms are also in a printable crossword (Modules 41–50) and word search (Modules 41–63).

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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. To store structured records ✓
B. For faster code
Lists hold multiple records; dicts give each record named fields
5. How do you add a dict to a list of dicts?
A. .add({'key': 'val'})
B. .append({'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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