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

# default_factory=None, defaultdict has the same performance as dict
import collections

def default_factory():
    return 'default value'

d = collections.defaultdict()

d['blue'] = 20 # defaultdict(None, {'blue': 20})

# print(d['red']) # key doesn't exist, raise KeyError
            

# default_factory=func_missing, if key does not exist, insert key with default value
import collections

def func_missing():
    return 0

d = collections.defaultdict(default_factory = func_missing)

d['blue'] = 20 # defaultdict(<function func_missing at 0x108af04a0>, {'blue': 20})

print(d['red']) # key doesn't exist, insert key with default value
# defaultdict(<function func_missing at 0x108af04a0>, {'blue': 20, 'red': 0})
            

# list
from collections import defaultdict

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d_list = defaultdict(list) # each value is a list
for k, v in s:
    # access key which does not exist
    # insert key with an empty list
    # append value to list
    d_list[k].append(v)

print(d_list) # defaultdict(list, {'yellow': [1, 3], 'blue': [2, 4], 'red': [1]})
            

# dict
from collections import defaultdict

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d_dict = defaultdict(dict) # each value is a list
for k, v in s:
    # access key which does not exist
    # insert key with an empty dict
    # add key/value to dict
    d_dict[k][v] = 0
    
print(d_dict) # defaultdict(, {'yellow': {1: 0, 3: 0}, 'blue': {2: 0, 4: 0}, 'red': {1: 0}})
            

# list
list(d_list.keys()) # ['yellow', 'blue', 'red']
list(d_list.values()) # [[1, 3], [2, 4], [1]]
list(d_list.items()) # [('yellow', [1, 3]), ('blue', [2, 4]), ('red', [1])]
            

# dict
list(d_dict.keys()) # ['yellow', 'blue', 'red']
list(d_dict.values()) # [{1: 0, 3: 0}, {2: 0, 4: 0}, {1: 0}]
list(d_dict.items()) # [('yellow', {1: 0, 3: 0}), ('blue', {2: 0, 4: 0}), ('red', {1: 0})]
            

# list
# convert to dict
dict(d_list) # {'yellow': [1, 3], 'blue': [2, 4], 'red': [1]}
            

# dict
# convert to dict
dict(d_dict) # {'yellow': {1: 0, 3: 0}, 'blue': {2: 0, 4: 0}, 'red': {1: 0}}
            

Python 3 Standard Library

PyMOTW-3