Dictionaries & Hashing

Lore scroll · Dictionaries & Hashing

Keys and Values

bag = {"ash": 3, "ember": 5}
print(bag["ember"])  # 5
bag["clay"] = 1
print(len(bag))  # 3

A dict maps each key to a value. You look things up by key, not by position.

Lore scroll · Dictionaries & Hashing

The Hashing Rune

key = "ember"
slot = hash(key) % 4
print(slot == hash(key) % 4)  # True

A dict hashes the key to pick a bucket, then looks only there. No searching.

Lore scroll · Dictionaries & Hashing

One Key, One Value

d = {}
d["ash"] = 1
d["ash"] = 2
print(d, len(d))  # {'ash': 2} 1

The same key always hashes to the same bucket, so writing it again replaces the value.

Lore scroll · Dictionaries & Hashing

Keys Must Not Change

ok = {(1, 2): "tuple"}
print(ok[(1, 2)])  # tuple
bad = {[1, 2]: "list"}  # TypeError!

A list can change, so its hash could not be trusted. Keys must be str, int, tuple...

This code stops with TypeError, and that error is the point of the scroll.

Lore scroll · Dictionaries & Hashing

Asking Politely

bag = {"ash": 3}
print(bag.get("clay", 0))  # 0
print("ash" in bag)  # True

bag["clay"] would raise KeyError. get() hands back a default instead.