2025-06-25

defaultdict vs dict

from collections import defaultdict

d = defaultdict(list)
d['key'].append('value') # Append directly without init

d = {}
d['key'].append(1) # KeyError

d = {}
if 'key' not in d:
    d['key'] = []
d['key'].append('value')

The key must be hashable.

key = (1, 2, 3)
print(hash(key))

In other words, Python must be able to compute hash(key), and the key must be immutable(its value cannot change)

A tuple cannot contain unhashable types(like lists or dicts)

key = ([1, 2], 3)
print(hash(key)) # TypeError: unhashable type: 'list'

The hash() function returns an integer hash value for a given object.

why is hash() important?

In dictionaries or sets, Python uses hash() to decide where to:

This makes operations like insert and lookup super fast - average O(1) time.

print(hash("apple"))  # Example: 2991632850760436371
print(hash(42))       # Example: 42
print(hash((1, 2, 3)))  # Tuples are hashable

Behind the scenes in a dict

d = {"apple": 1}