nonobie — Real dev skills, explained like a friend would

Warming up the neural circuits...

What you'll learn

By the end of this chapter you will:

Model real data with dicts and navigate nested structures safely
Access defensively: get(), setdefault(), and the KeyError fix patterns
Merge and transform dicts with |, comprehensions, and views
Apply set operations — union, intersection, difference — to real dedup problems

The why

Lookup by name beats search by position, every time. If you have a list of users and want to find Alice's age, you'd loop through the whole list. With a dict, you write users["Alice"]["age"] — instant lookup.

Dictionaries are Python's hash map — the most important data structure for modeling real-world data. Sets are hash-based collections for unique items and fast membership testing. Master both, and you can model any relationship.

Core concepts: Dictionaries at a glance

Creating & Accessing DictsOpen section

Let's learn

python

# Creating dicts
empty = {}
user = {"name": "Alice", "age": 30, "active": True}
from_list = dict([("a", 1), ("b", 2)])
 
# Accessing
print(user["name"])      # 'Alice'
# user["email"]          # KeyError!
print(user.get("email"))  # None (safe)
print(user.get("email", "N/A"))  # 'N/A' (default)

Operation	Code	Result
Create empty	`{}`	Empty dict
Create with values	`{"a": 1}`	Dict with one entry
Access	`d["key"]`	Value or KeyError
Safe access	`d.get("key")`	Value or None
With default	`d.get("key", default)`	Value or default

Modifying DictsOpen section

Let's learn

python

user = {"name": "Alice", "age": 30}
 
# Adding/Updating
user["email"] = "alice@example.com"  # add
user["age"] = 31                     #

Iterating DictsOpen section

Let's learn

python

user = {"name": "Alice", "age": 30, "active": True}
 
# Keys
for key in user.keys():
    print(key)
 
# Values

Merging DictsOpen section

Let's learn

python

defaults = {"color": "blue", "size": "medium"}
overrides = {"color": "red", "price": 29.99}
 
# | operator (Python 3.9+) — returns new dict
merged = defaults |

Sets: Unique CollectionsOpen section

Let's learn

python

# Creating sets
unique = {1, 2, 3, 3, 4}  # {1, 2, 3, 4}
from_list = set([1, 2, 2, 3])  # {1, 2, 3}
 
# Membership (O(1))
print

Mental model: Dicts as phone books

Think of dicts as phone books:

Key = name (unique, immutable)
Value = phone number (any type)
Lookup = instant (hash-based, O(1))
No order guarantee (insertion order preserved in Python 3.7+, but don't rely on it for sorting)

Common mistakes

Mistake	Why it's wrong	Fix
`d["key"]` when key might not exist	`KeyError`	`d.get("key")` or `d.get("key", default)`
`d = {}` then `d["key"]`	Works (empty dict)	Fine, but `set()` is for empty sets
`set = {1, 2, 3}` then `set.add(4)`	`set` shadows built-in	Use `my_set = {1, 2, 3}`
Iterating and modifying dict size	`RuntimeError: dictionary changed size during iteration`	Collect changes, apply after loop
Using mutable values as dict keys	`TypeError: unhashable type: 'list'`	Use tuples instead of lists as keys

Practice: Prove your Mastery

beginner Challenge

3 min

Dict Builder

Create a dict for a user with name, age, and email. Add a "phone" key, then remove "email".

Input— given data

data

Start with {"name": "Alice", "age": 30, "email": "alice@example.com"}

Expected Output— your code should produce this

expected

{"name": "Alice", "age": 30, "phone": "555-1234"}

Write your solutionCtrl+Enter to run

Ctrl+Enter

beginner Challenge

3 min

Safe Access

Use get() to safely access a missing key without raising KeyError.

Input— given data

data

user = {"name": "Alice"}

Expected Output— your code should produce this

expected

"N/A" for missing "email" key

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

Set Dedup

Remove duplicates from a list using a set.

Input— given data

data

names = ["Alice", "Bob", "Alice", "Charlie", "Bob"]

Expected Output— your code should produce this

expected

["Alice", "Bob", "Charlie"]

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

Dict Merge

Merge two dicts using the | operator (Python 3.9+).

Input— given data

data

defaults = {"color": "blue"}, overrides = {"color": "red", "size": "large"}

Expected Output— your code should produce this

expected

{"color": "red", "size": "large"}

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

Set Operations

Find users who are in both "active" and "premium" sets.

Input— given data

data

active = {"Alice", "Bob", "Charlie"}, premium = {"Bob", "Charlie", "Dave"}

Expected Output— your code should produce this

expected

{"Bob", "Charlie"}

Write your solutionCtrl+Enter to run

Ctrl+Enter

Interview Questions

Beginner:

"What's the difference between d["key"] and d.get("key")?"

d["key"] raises KeyError if the key doesn't exist. d.get("key") returns None (or a default value you specify). Use get() when the key might be missing.
"How do you remove duplicates from a list?"

Convert to a set: list(set(my_list)). Sets automatically remove duplicates. Note: this doesn't preserve order. For order preservation, use list(dict.fromkeys(my_list)).

Senior:

"What's the time complexity of dict lookup vs list lookup?"

Dict lookup is O(1) average — it uses a . List lookup with in is O(n) — it scans every . This is why dicts are preferred for large datasets when you need fast lookups by key.

Summary

Dictionaries store key-value pairs. Use {} to create, d[key] to access, d.get(key) for safe access.
Use setdefault() to get-or-create. Use | to merge (Python 3.9+).
Sets store unique items. Use {1, 2, 3} to create. in is O(1).
Set operations: | union, & intersection, - difference, ^ symmetric difference.
Keys must be immutable (strings, numbers, tuples). Lists cannot be keys.

Quick recall

text

I have a dict. I want to…
├── Access safely → d.get("key") or d.get("key", default)
├── Add/update → d["key"] = value
├── Remove → pop(key) or del d[key]
├── Merge → d1 | d2 (3.9+)
├── Iterate → for k, v in d.items():
└── Check exists → "key" in d
 
I have a set. I want to…
├── Dedup → set(list)
├── Intersection → a & b
├── Union → a | b
├── Difference → a - b
└── Check membership → x in s

Self-Check Quiz

Q1 / 3

MCQ

What does d.get('key') return if the key doesn't exist?

Method	Returns	Mutates	Use when
`d[key] = val`	—	✅ Yes	Add/update entry
`pop(key)`	The value	✅ Yes	Remove and return
`del d[key]`	None	✅ Yes	Remove (no return)
`setdefault(key, default)`	Value	✅ Yes	Get or create

Method	Returns	Use when
`keys()`	View of keys	Need all keys
`values()`	View of values	Need all values
`items()`	View of (key, value)	Need pairs
`for key in d:`	Keys	Direct iteration

Method	Mutates	Use when
`d1 \| d2`	❌ No	Merge into new dict
`d1.update(d2)`	✅ Yes	Update in place
`{d1, d2}`	❌ No	Merge (pre-3.9)

Operation	Code	Result
Union	`a \| b`	All items from both
Intersection	`a & b`	Items in both
Difference	`a - b`	Items in a, not in b
Symmetric diff	`a ^ b`	Items in either, not both

Chapter 2 — Dictionaries & Sets

The why

Core concepts: Dictionaries at a glance

Common mistakes

Practice: Prove your Mastery

Dict Builder

Safe Access

Set Dedup

Dict Merge

Set Operations

Interview Questions

Summary

Quick recall

Self-Check Quiz