Dictionaries

Introduction

Dicts are hash maps — O(1) get/set/delete. Insertion-ordered since Python 3.7. The most-used data structure in Python.

Understanding the topic

Core concepts to understand:

• {'k': 'v'} or dict().
• d.get(k, default) avoids KeyError.
• d.setdefault, collections.defaultdict.
• Iterate: items(), keys(), values().

Syntax reference

Visual flow / code:

user = {"id": 1, "name": "Alice", "role": "admin"}

user["email"] = "a@x.com"   # add
user.pop("role")             # remove
name = user.get("name", "?")

# Iterate
for k, v in user.items():
    print(f"{k}: {v}")

# defaultdict — auto-init missing keys
from collections import defaultdict
counts = defaultdict(int)
for c in "hello":
    counts[c] += 1

# Dict merge (3.9+)
merged = {"a": 1} | {"b": 2}

Execution workflow

Real-world use

Dicts are the JSON of Python: every API response, config file, and DB row maps to/from a dict. Master get, setdefault, and defaultdict.

Best practices

• Use get with defaults to avoid KeyError.
• defaultdict for grouped/counted data.
• Use Counter for histograms.

Common mistakes

• Mutating a dict during iteration → RuntimeError.

Hands-on exercise

Interview preparation — practice these questions:

• Dict vs list lookup?
• What's defaultdict?
• Are dicts ordered?

Summary

In summary: O(1) hash map; insertion-ordered. get/setdefault/defaultdict are everyday tools.