How do Python dictionaries work and what are the most important operations?

A dictionary is Python's hash-map: an unordered (insertion-ordered since Python 3.7) collection of key-value pairs. Keys must be hashable (strings, numbers, tuples of hashable items); values can be anything. Lookup, insertion, and deletion are O(1) average-case.

user = {
    'name': 'Alice',
    'age': 30,
    'active': True
}

# Access — raises KeyError if key missing
print(user['name'])              # Alice

# Safe access — returns default if key missing
print(user.get('email', 'N/A'))  # N/A

# Add / update
user['email'] = 'alice@example.com'
user.update({'age': 31, 'city': 'NYC'})

# Delete
del user['active']
role = user.pop('city', None)  # removes and returns; default avoids KeyError

# Iterating
for key, value in user.items():
    print(f'{key}: {value}')

# Keys and values as views
print(list(user.keys()))    # ['name', 'age', 'email']
print(list(user.values()))  # ['Alice', 31, 'alice@example.com']

Dict comprehension builds dictionaries from iterables in one line:

squares = {x: x**2 for x in range(1, 6)}  # {1:1, 2:4, 3:9, 4:16, 5:25}

Checking membership tests keys only: 'name' in user is O(1). To check values you must iterate, which is O(n). For counting occurrences, collections.Counter is a dict subclass that auto-initialises missing keys to zero, making frequency analysis much cleaner.

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