Python高级用法------字典无需提前定义key

from collections import defaultdict
import json

def tree():
    return defaultdict(tree)

categories = tree()

a = categories['Programming Languages']['Python']
b = categories['Programming Languages']['java']
a['Standard Library']['sys']='sys'
a['Standard Library']['os']='os'
b['Standard Library']['os']='jos'

print(json.dumps(categories))

输出：
{"Programming Languages": {"Python": {"Standard Library": {"sys": "sys", "os": "os"}}, "java": {"Standard Library": {"os": "jos"}}}}