Python: 初识shelve module
看一段开源的Python工具源码，发现采用了shelve这个module来存储数据，而且接口设计的非常好用。这里简单介绍一下。
这是python自带的一个非常实用的module，用来进行数据存储非常方便。
简单来说，
通过在当前或者指定路径建立一个文件来存储字典结构的数据：
字典的key是用字符串，value可以使任何数据对象。它与dbm不同的是，字典的值可以是
任意的数据对象。
下面我们暂从Python的官方Manuals中的例子来说明其用法：
import shelve
d = shelve.open(filename) # open -- file may get suffix added by low-level library
d[key] = data # 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 = d.has_key(key) # true if the key exists
klist = d.keys() # a list of all existing keys (slow!)
# as d was opened WITHOUT writeback=True, beware:
d['xx'] = range(4) # this works as expected, but...
d['xx'].append(5) # *this doesn't!* -- d['xx'] is STILL range(4)!
# 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() # close it