数据读写
import shelve as sv
db = sv.open('db')
db['key1'] = {'1':1, '2':2, '3':3}
db.close()
db = sv.open('db')
print(db['key1'])
db['key1']['1'] = 10 # 不能改变单个条目的值
print(db['key1'])
db['key1'] = {'1':'a', '2':'b', '3':'c'} # 可以覆盖整个key的值
print(db['key1'])
db.close()
db = sv.open('db', writeback = True)
print(db['key1'])
db['key1']['1'] = 10 # 可以改变单个条目的值
print(db['key1'])
db['key2'] = {'a':100, 'b':200, 'c':300}
db.close()
db = sv.open('db')
print(db['key1'])
print(db['key2'])
db.close()
输入结果
{'1': 1, '2': 2, '3': 3}
{'1': 1, '2': 2, '3': 3}
{'1': 'a', '2': 'b', '3': 'c'}
{'1': 'a', '2': 'b', '3': 'c'}
{'1': 10, '2': 'b', '3': 'c'}
{'1': 10, '2': 'b', '3': 'c'}
{'a': 100, 'b': 200, 'c': 300}
总结：
- 从shelve的db文件中重新再访问一个key拿的是它的拷贝！
修改此拷贝后不做拷贝写回并不影响原来的key，
但你要是直接做的操作是赋值新的值到一个key里，那肯定就是指向原来的key，会被覆盖的。 - writeback=True，DB在close()的时候会将缓存中的每一个对象都写入到DB
open()参数说明
shelve.open(filename, flag='c', protocol=None, writeback=False)
The optional flag argument can be:
| Value | Meaning |
|---|---|
| 'r' | Open existing database for reading only (default) |
| 'w' | Open existing database for reading and writing |
| 'c' | Open database for reading and writing, creating it if it doesn’t exist |
| 'n' | Always create a new, empty database, open for reading and writing |
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