Python's defaultdict

Defaultdict is a subclass of Dict, accepting a factory function as a parameter, and when accessing a key that does not exist in Defaultdict, the return value of the factory function is used as the default value.

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

Test code

Complete code: https://github.com/blackmatrix7/python-learning/blob/master/other/defaultdict.py

__author__='Blackmatrix'defFactory_func ():return 'default_factory'test_defaultdict=defaultdict (Factory_func)if __name__=='__main__':    " "When a key is present, it returns the value corresponding to the key, which is no different from the normal dict." "test_defaultdict['a'] ='233333'    Print(test_defaultdict['a'])    " "The return value of the factory function is automatically used as the default value when accessing a nonexistent key" "    Print(test_defaultdict['b'])    Print(test_defaultdict['C'])

Run results

233333default_factorydefault_factory

If Defaultdict does not pass in the factory function when it is created, it throws keyerror when it accesses a nonexistent key.

 from Import  = defaultdict ()print(test_defaultdict2['x')  #  keyerror: ' x '

Python's defaultdict