Python Dictionaries

Easy

A Python dictionary is a mutable collection of key-value pairs with O(1) average lookup time.

Why It Matters

Dictionaries are essential for fast lookups, counting, grouping, and caching. Most coding interviews test dictionary manipulation.

How It Works

Created with {} or dict(). Keys must be immutable (str, int, tuple of immutables). Values can be anything. Access: d[key] raises KeyError if missing; d.get(key, default) returns default instead. d.setdefault(key, val) returns existing value or sets and returns val. d.update(other_dict) merges. Iteration: d.keys(), d.values(), d.items(). Since Python 3.7, dictionaries maintain insertion order.

Example

scores = {'Alice': 85, 'Bob': 92} scores['Charlie'] = 78 scores.get('Dave', 0) # returns 0, no KeyError

Code Example

# Counting pattern
from collections import Counter
words = ['apple', 'banana', 'apple', 'cherry', 'banana', 'apple']
count = Counter(words)  # Counter({'apple': 3, 'banana': 2, 'cherry': 1})

# Manual counting
count = {}
for w in words:
    count[w] = count.get(w, 0) + 1

Common Mistakes

Using d[key] on a potentially missing key without get() or a try/except. Using mutable types (lists, dicts) as dictionary keys — they're unhashable and raise TypeError.