class defaultdict(dict):
    """
    defaultdict(default_factory[, ...]) --> dict with default factory
    
    The default factory is called without arguments to produce
    a new value when a key is not present, in __getitem__ only.
    A defaultdict compares equal to a dict with the same items.
    All remaining arguments are treated the same as if they were
    passed to the dict constructor, including keyword arguments.
    """

__author__ = ‘blackmatrix‘


def factory_func():
    return ‘default_factory‘

test_defaultdict = defaultdict(factory_func)

if __name__ == ‘__main__‘:
    ‘‘‘
    当key存在时，返回的是key对应的value
    这个时候和普通的dict没有什么区别
    ‘‘‘
    test_defaultdict[‘a‘] = ‘233333‘
    print(test_defaultdict[‘a‘])
    ‘‘‘
    当访问不存在的key时，自动将工厂函数的返回值作为默认值
    ‘‘‘
    print(test_defaultdict[‘b‘])
    print(test_defaultdict[‘c‘])

233333
default_factory
default_factory

from collections import defaultdict

test_defaultdict2 = defaultdict()

print(test_defaultdict2[‘x‘])

# KeyError: ‘x‘