How do Python dictionaries work and when do you use them?

A dictionary stores entries in a compact array and uses a hash index to find a slot. Key equality is checked with == after hashing, so keys need consistent __hash__ and __eq__ implementations. Average operations are O(1), but a poor hash or heavy collisions can degrade performance.

counts = {}
for word in ["a", "b", "a"]:
    counts[word] = counts.get(word, 0) + 1

counts.setdefault("c", 0)

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

Idioms worth knowing

• Use d.get(key, default) instead of try/except when a missing key is normal
• Use collections.defaultdict when every access should create a default
• Use dict comprehension for transforms: {k: v for k, v in pairs}
• Use d | other or {a, b} to merge, with the right-hand side winning

Because keys must be hashable, mutable objects such as lists cannot be keys. Insertion order is guaranteed by the language since 3.7, but equality ignores order: two dicts with the same pairs are equal regardless of when the keys were inserted.