python之shelve模块

Posted 2021-01-09 RobotsRising

1、shevle简介

　　利用 shelve 模块， 你可以将 Python 程序中的变量保存到二进制的 shelf 文件中。这样， 程序就可以从硬盘中恢复变量的数据。 shelve 模块让你在程序中添加“保存”和“打开” 功能。例如， 如果运行一个程序，并输入了一些配置设置，就可以将这些设置保存到一个 shelf 文件，然后让程序下一次运行时加载它们。

2、方法

2.1、写入数据

import shelve
import datetime
d = shelve.open(‘shelve_test‘)  
cats = ["Zophie", "Pooka", "Simon"]
d["cats"] = cats
d[‘date‘] = datetime.datetime.now()
d.close()

2.2、读取数据

import shelve
d = shelve.open("shelve_test") # 打开一个文件
print(d["cats"])
print(d["date"])

3、实例

import shelve
d = shelve.open("shelve_data") # open -- file may get suffix added by low-level library

key = "you"
data = 250

d[key] = 250 # store data at key (overwrites old data if using an existing key)
data = d[key] # retrieve a COPY of data at key (raise KeyError if no such key)
del d[key] # delete data stored at key (raises KeyError if no such key)
flag = key in d # true if the key exists
klist = list(d.keys()) # a list of all existing keys (slow!)

# as d was opened WITHOUT writeback=True, beware:
d[‘xx‘] = [0, 1, 2] # this works as expected, but...
d[‘xx‘].append(3) # *this doesn‘t!* -- d[‘xx‘] is STILL [0, 1, 2]!

# having opened d without writeback=True, you need to code carefully:
temp = d[‘xx‘] # extracts the copy
temp.append(5) # mutates the copy
d[‘xx‘] = temp # stores the copy right back, to persist it

# or, d=shelve.open(filename,writeback=True) would let you just code
# d[‘xx‘].append(5) and have it work as expected, BUT it would also
# consume more memory and make the d.close() operation slower.
d.close()