2.1. Collections Defaultdict

defaultdict is a subclass of the built-in dict class in Python. It overrides one method and adds one writable instance variable. The defaultdict class is used when you want to create a dictionary with default values for keys that have not been set yet.

The default value is specified as a parameter when creating the defaultdict. If a key is not found in the dictionary, instead of a KeyError being thrown, a new entry is created. The type of this new entry is specified by the argument of defaultdict.

Here's an example:

>>> from collections import defaultdict
>>>
>>> d = defaultdict(int)
>>> d['a'] += 1
>>> d['b'] += 2
>>> d['c']  # returns 0, since it was not set yet
0

In this example, we create a defaultdict with a default value of 0. We then set the values of keys 'a' and 'b'. When we try to access the value of key 'c', which was not set yet, it returns the default value of 0.

2.1.1. SetUp

>>> from collections import defaultdict

2.1.2. Example

>>> data = defaultdict(str)
>>>
>>>
>>> data
defaultdict(<class 'str'>, {})
>>>
>>> data['firstname'] = 'Mark'
>>> data['firstname']
'Mark'
>>>
>>> data['lastname']
''
>>>
>>> data
defaultdict(<class 'str'>, {'firstname': 'Mark', 'lastname': ''})

2.1.3. Empty Values

>>> data = defaultdict(lambda: None)
>>>
>>> data['latin_name']

>>> from types import NoneType
>>>
>>> data = defaultdict(NoneType)
>>>
>>> data['latin_name']
>>> data
defaultdict(<class 'NoneType'>, {'latin_name': None})

2.1.4. Dict to Defaultdict

>>> PLATYPUS = {'english_name': 'Platypus'}
>>>
>>> data = defaultdict(NoneType, **PLATYPUS)
>>>
>>> data['english_name']
'Platypus'
>>>
>>> data['latin_name']