Source: 📖 Effective Python item 17

defaultdict automatically sets a default value to missing keys

The defaultdict type can be imported from the collections module in the standard library. Upon defining the __init__ method, you must provide a valid callable or None as the first argument in the method. It will then be called automatically when you try to access a key that doesn't exist, creating an instance of that callable as the default value for the new (ex-missing) key.

from collections import defaultdict

my_dict = defaultdict(list)	# list callable passed as default value

my_dict['name'] = 'juan'	# adding key/value pair
dob = my_dict['dob']	# try to access missing key

print(my_dict['name'])
print(dob)

>>>
juan
[]	# list was called as the default value for key 'dob'

See also:

• Using get to check for existence of a key in a dict type