举例简单讲解Python中的数据存储模块shelve的用法
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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