defaultdict

• Returns a new dictionary-like object
• overrides one method and adds one writable instance variable
• the remaining functionality is the same as for the dict class
• the type of the value for each key can be specified

Initialization

from collections import defaultdict

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(list) # each value is a list
for k, v in s:
    d[k].append(v)
    
print(sorted(d.items())) #[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(dict) # each value is a dict
for k, v in s:
    d[k][v] = 0
    
print(sorted(d.items())) # [('blue', {2: 0, 4: 0}), ('red', {1: 0}), ('yellow', {1: 0, 3: 0})]

Dict Methods

k = d.keys()
print(list(k)) # ['yellow', 'blue', 'red']

v = d.values()
print(list(v)) # [{1: 0, 3: 0}, {2: 0, 4: 0}, {1: 0}]

i = d.items()
print(list(i)) # [('yellow', {1: 0, 3: 0}), ('blue', {2: 0, 4: 0}), ('red', {1: 0})]

Built-in Functions

print(len(d)) # 3

Reference

• Python STL Library
• Python 3 Module of Week