搁置,Python,更新字典

Question

我正在使用Python中的Shelve,我遇到了一个问题:

In [391]: x
Out[391]: {'broken': {'position': 25, 'page': 1, 'letter': 'a'}}

In [392]: x['broken'].update({'page':1,'position':25,'letter':'b'})

In [393]: x
Out[393]: {'broken': {'position': 25, 'page': 1, 'letter': 'a'}}

我不明白为什么不更新？有什么想法吗？

Answer 1

这在文档中有所涉及.基本上,关键字参数writebackto shelve.open负责:

如果可选writeback参数设置为True,访问的所有项目也缓存在内存中,并在写回sync()和 close(); 这可以使持久化字典中的可变条目变得更容易,但是,如果访问了许多条目,它可能会占用大量的内存用于缓存,并且它可以使关闭操作非常慢,因为所有访问的条目都被写回(没有办法确定哪些访问的条目是可变的,哪些是实际变异的.

同一页面的示例:

d = shelve.open(filename) # open -- file may get suffix added by low-level
                          # library
# 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