Dictionaries

Dictionaries

What it means

A dictionary (dict) stores key-value pairs — {"name": "Alice", "age": 30} — and gives you fast, average O(1) lookup by key instead of by numeric position. Keys must be hashable (strings, numbers, tuples of hashable items); values can be anything. Since Python 3.7, dicts preserve insertion order.

Examples

person = {"name": "Alice", "age": 30}

person["city"] = "Boston"        # add a new key
age = person["age"]              # 30 -- KeyError if "age" doesn't exist
age = person.get("age", 0)       # 30 -- safe lookup, returns 0 if missing

for key, value in person.items():
    print(key, value)

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

Common mistake

Accessing a missing key with dict[key] instead of dict.get(key) — this raises KeyError and crashes the program instead of returning a safe default.

person = {"name": "Alice"}

# Crashes if "age" isn't set
age = person["age"]     # KeyError: 'age'

# Safe -- returns a default instead of crashing
age = person.get("age", "unknown")   # 'unknown'

Quick practice

1. What’s the difference between d[key] and d.get(key) when the key doesn’t exist?

   Answer

   d[key] raises a KeyError; d.get(key) returns None (or a provided default) instead.

2. Can a list be used as a dictionary key?

   Answer

   No — lists are mutable and therefore unhashable. Use a tuple instead if you need a compound, immutable key.

3. How do you iterate over both keys and values at once?

   Answer

   for key, value in my_dict.items():