What is a dictionary (dict) and how does it work?

Dictionary (dict) is a mutable collection of key-value pairs implemented using a hash table.

Key characteristics:

• Access, insertion, and deletion are performed in O(1) on average.
• Keys must be hashable (strings, numbers, tuples).
• Since Python 3.7, dictionaries preserve insertion order.

user = {
    "name": "Anna",
    "age": 25,
    "city": "Moscow"
}

Main methods:

user["name"]              # Access by key (KeyError if missing)
user.get("email", "—")    # Safe access with default value

user.keys()               # All keys
user.values()             # All values
user.items()              # Key-value pairs

user.pop("city")          # Remove and return value
user.update({"age": 26})  # Update values

Creating a dictionary:

# Literal
d1 = {"a": 1, "b": 2}

# From a list of tuples
d2 = dict([("a", 1), ("b", 2)])

# Using dict comprehension
d3 = {x: x ** 2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}