defaultdict是dict的一个子类，接受一个工厂函数作为参数，当访问defaultdict中不存在的key时，会将工厂函数的返回值作为默认的value。
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.
"""
测试代码
完整代码: https://github.com/blackmatrix7/python-learning/blob/master/other/defaultdict.py
__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'] = ''
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'])
运行结果
233333
default_factory
default_factory
如果defaultdict在创建的时候没有传入工厂函数，则在访问不存在的key时，会抛出KeyError
from collections import defaultdict test_defaultdict2 = defaultdict() print(test_defaultdict2['x']) # KeyError: 'x'