shelve

一、 shelve

• shelve提供python的持久化操作。
• 持久化：把數據寫到硬碟上
• shelve操作與字典非常類似

import shelve

d = shelve.open("tom")
d["name"] = "Tom"
print(d["name"])  # Tom
d.close()

文件關閉後無法讀取其內容

import shelve

d = shelve.open("tom")
d["name"] = "Tom"
d.close()

print(d["name"])  # ValueError: invalid operation on closed shelf

 遍歷：

import shelve

d = shelve.open("namelist", writeback=True)

d["name"] = "Tom"
d["age"] = 18
d.close()


d = shelve.open("namelist")

for k in d:  # 遍歷所有的key
    print(k)  # name age

for k in d.keys():  # 遍歷所有key
    print(k)  # name age

for k, v in d.items():  # 遍歷所有的鍵-值對
    print(k, v)  # name Tom   age 18
d.close()

二、 writeback

• 把修改的內容自動回寫到文件中

1. 修改內容

shelve.open()預設writeback=False，所以，在預設情況下修改字典的數據，修改內容不會寫入到文件中：

import shelve

d = shelve.open("namedict")
d["one"] = {"name": "Tom", "age": 18, "hobby": "football"}
d.close()

import shelve

d = shelve.open("namedict")
d["one"]["name"] = "Linda"
d.close()


d = shelve.open("namedict")
print(d["one"])  # {'name': 'Tom', 'age': 18, 'hobby': 'football'}
d.close()

 當writeback=True時，修改的數據保存在文件中，再次讀取為修改後的結果

import shelve

d = shelve.open("namedict", writeback=True)
d["one"]["name"] = "Linda"
d.close()


d = shelve.open("namedict")
print(d["one"])  # {'name': 'Linda', 'age': 18, 'hobby': 'football'}
d.close()

 2. 刪除內容

    在writeback=True條件下，刪除內容，文件中內容也會被刪除

import shelve

d = shelve.open("namedict", writeback=True)
del d["one"]
d.close()


d = shelve.open("namedict")
print(d["one"])  # KeyError: b'one' 報錯
d.close()