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举例简单讲解Python中的数据存储模块shelve的用法

WBOY
WBOYOriginal
2016-06-10 15:05:401310browse

shelve类似于一个key-value数据库,可以很方便的用来保存Python的内存对象,其内部使用pickle来序列化数据,简单来说,使用者可以将一个列表、字典、或者用户自定义的类实例保存到shelve中,下次需要用的时候直接取出来,就是一个Python内存对象,不需要像传统数据库一样,先取出数据,然后用这些数据重新构造一遍所需要的对象。下面是简单示例:

import shelve


def test_shelve():
  # open 返回一个Shelf类的实例
  #
  # 参数flag的取值范围:
  # 'r':只读打开
  # 'w':读写访问
  # 'c':读写访问,如果不存在则创建
  # 'n':读写访问,总是创建新的、空的数据库文件
  #
  # protocol:与pickle库一致
  # writeback:为True时,当数据发生变化会回写,不过会导致内存开销比较大
  d = shelve.open('shelve.db', flag='c', protocol=2, writeback=False)
  assert isinstance(d, shelve.Shelf)

  # 在数据库中插入一条记录
  d['abc'] = {'name': ['a', 'b']}
  d.sync()

  print d['abc']

  # writeback是False,因此对value进行修改是不起作用的
  d['abc']['x'] = 'x'
  print d['abc'] # 还是打印 {'name': ['a', 'b']}

  # 当然,直接替换key的value还是起作用的
  d['abc'] = 'xxx'
  print d['abc']

  # 还原abc的内容,为下面的测试代码做准备
  d['abc'] = {'name': ['a', 'b']}
  d.close()

  # writeback 为 True 时,对字段内容的修改会writeback到数据库中。
  d = shelve.open('shelve.db', writeback=True)

  # 上面我们已经保存了abc的内容为{'name': ['a', 'b']},打印一下看看对不对
  print d['abc']

  # 修改abc的value的部分内容
  d['abc']['xx'] = 'xxx'
  print d['abc']
  d.close()

  # 重新打开数据库,看看abc的内容是否正确writeback
  d = shelve.open('shelve.db')
  print d['abc']
  d.close()

这个有一个潜在的小问题,如下:

>>> import shelve 
>>> s = shelve.open('test.dat') 
>>> s['x'] = ['a', 'b', 'c'] 
>>> s['x'].append('d') 
>>> s['x'] 
['a', 'b', 'c'] 

存储的d到哪里去了呢?其实很简单,d没有写回,你把['a', 'b', 'c']存到了x,当你再次读取s['x']的时候,s['x']只是一个拷贝,而你没有将拷贝写回,所以当你再次读取s['x']的时候,它又从源中读取了一个拷贝,所以,你新修改的内容并不会出现在拷贝中,解决的办法就是,第一个是利用一个缓存的变量,如下所示

>>> temp = s['x'] 
>>> temp.append('d') 
>>> s['x'] = temp 
>>> s['x'] 
['a', 'b', 'c', 'd'] 

在python2.4以后有了另外的方法,就是把open方法的writeback参数的值赋为True,这样的话,你open后所有的内容都将在cache中,当你close的时候,将全部一次性写到硬盘里面。如果数据量不是很大的时候,建议这么做。

下面是一个基于shelve的简单数据库的代码

#database.py 
import sys, shelve 
 
def store_person(db): 
  """ 
  Query user for data and store it in the shelf object 
  """ 
  pid = raw_input('Enter unique ID number: ') 
  person = {} 
  person['name'] = raw_input('Enter name: ') 
  person['age'] = raw_input('Enter age: ') 
  person['phone'] = raw_input('Enter phone number: ') 
  db[pid] = person 
 
def lookup_person(db): 
  """ 
  Query user for ID and desired field, and fetch the corresponding data from 
  the shelf object 
  """ 
  pid = raw_input('Enter ID number: ') 
  field = raw_input('What would you like to know? (name, age, phone) ') 
  field = field.strip().lower() 
  print field.capitalize() + ':', \ 
    db[pid][field] 
 
def print_help(): 
  print 'The available commons are: ' 
  print 'store :Stores information about a person' 
  print 'lookup :Looks up a person from ID number' 
  print 'quit  :Save changes and exit' 
  print '?   :Print this message' 
 
def enter_command(): 
  cmd = raw_input('Enter command (? for help): ') 
  cmd = cmd.strip().lower() 
  return cmd 
 
def main(): 
  database = shelve.open('database.dat') 
  try:  
    while True: 
      cmd = enter_command() 
      if cmd == 'store': 
        store_person(database) 
      elif cmd == 'lookup': 
        lookup_person(database) 
      elif cmd == '?': 
        print_help() 
      elif cmd == 'quit': 
        return  
  finally: 
    database.close() 
if __name__ == '__main__': main() 

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