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"}}}}