python的shelve库

1、shelve模块是一个简单的key，value将内存数据通过文件持久化的模块
2、shelve模块可以持久化任何pickle可支持的python数据格式
3、shelve就是pickle模块的一个封装
4、shelve模块是可以多次dump和load

https://docs.python.org/3.6/library/shelve.html

1、该模块底层使用dbm模块做存储，具体使用的是dbm.gnu还是dbm.ndbm，由运行时看哪个装了就用哪个，否则用dbm.dumb

2、不支持并发读写，但是支持并发读

#!/usr/bin/env python
#coding=utf-8
import shelve
import threading


class A(object):
  """一个用于测试的类 其中只打印出hello和bye
  """
  def __init__(self, name):
    self.name = name
    self.__actions = ['hello', 'bye']

  def _hello(self):
    print ('hello, %s' % self.name)

  def _bye(self):
    print ('bye, %s' % self.name)

  def act(self):
    for a in self.__actions:
      if callable(getattr(self, '_'+a)):
        getattr(self, '_'+a)()


FILENAME = 'shelve_test'
KEY = 'myobj'
DATA = A('qx')


def test1():
  """基本用法
  """
  with shelve.open(FILENAME) as d:
    d[KEY] = DATA
    data = d[KEY]
    print ('type(data)=%s' % type(data))
    del d[KEY]
    try:
      data = d[KEY]
    except KeyError:
      print ('key=%s not found' % KEY)
    d[KEY] = DATA

def test2():
  """当没有使用write_back=True时 shelve无法检测到可变对象的更新
  """
  with shelve.open(FILENAME) as d:
    d['xx'] = [0, 1, 2]
    print ("d['xx']=", d['xx'])  # [0, 1, 2]
    d['xx'].append(3)
    print ("d['xx']=", d['xx'])  # [0, 1, 2]

def test3():
  """继续test2 只能通过重新赋值来更新d['xx']
  """
  with shelve.open(FILENAME) as d:
    d['xx'] = [0, 1, 2]
    print ("d['xx']=", d['xx'])  # [0, 1, 2]
    data = d['xx']
    data.append(3)
    d['xx'] = data
    print ("d['xx']=", d['xx'])  # [0, 1, 2, 3]

def test4(flag=None):
  """不支持并发读写 这是由Unix文件系统的文件锁保证的 但是支持多线程并发读
  """
  if flag is None: flag = 'c'
  def _test():
    d = shelve.open(FILENAME, flag=flag)
    try:
      data = d[KEY]
    except Exception as exc:
      print ('exception:%s' % exc)
    else:
      data.act()
      print (d['xx'])
    finally:
      d.close()

  t1 = threading.Thread(target=_test)
  t2 = threading.Thread(target=_test)
  t1.start();t2.start()
  t1.join();t2.join()
  

def split(seq):
  """打印分隔线
  """
  print ('====test %d====' % seq)

if __name__ == '__main__':
  split(1);test1()  # key=myobj not found
  split(2);test2()
  split(3);test3()
  split(4);test4('r')  # 如果调用test4() 会报异常 Resource temporarily unavailable