python持久化存储之shelve

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。

示例1:

#/usr/bin/env python
# coding=utf-8
# code from  www.361way.com
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
    #   0: ascii串保存, 默认形式, 方便人读取
    #   1: 旧式兼容性较强2进制形式
    #   2: 支持新式类的2进制模式,Python2.3开始引入.
    # 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()
test_shelve()

示例2:

# code from  www.361way.com
import shelve
s = shelve.open('test_shelf.db')
try:
    s['key1'] = { 'int': 10, 'float':9.5, 'string':'Sample data' }
finally:
    s.close()
s = shelve.open('test_shelf.db', writeback=True)
try:
    print s['key1']
    s['key1']['new_value'] = 'this was not here before'
    print s['key1']
finally:
    s.close()
s = shelve.open('test_shelf.db')
try:
    print s['key1']
finally:
    s.close()

执行结果如下:

$ python shelve_writeback.py
{'int': 10, 'float': 9.5, 'string': 'Sample data'}
{'int': 10, 'new_value': 'this was not here before', 'float': 9.5, 'string': 'Sample data'}
{'int': 10, 'new_value': 'this was not here before', 'float': 9.5, 'string': 'Sample data'}

update值如下:

>>> import shelve
>>> d = shelve.open("test_shelf.db")
>>> x = d["key1"]
>>> x.update({'xyz': '11111'})
>>> print x
{'int': 10, 'new_value': 'this was not here before', 'xyz': '11111', 'float': 9.5, 'string': 'Sample data'}
>>> x.update({'int': 'int upate'})
>>> print x
{'string': 'Sample data', 'int': 'int upate', 'new_value': 'this was not here before', 'xyz': '11111', 'float': 9.5}
>>> 

示例3:

>>> import shelve
>>> d = shelve.open("shelve.db")
>>> len(d)
2
>>> d.keys()
['dfcfall', 'thsall']

参考页面如下:shelve – Persistent storage of arbitrary Python objects




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