python - Truncating and rewriting a pickle file raises KeyError '\x00'

Question

I am trying to truncate pickle file after I dumped my data, but it does not seem to work like .txt file.

dic1 = {'a': 1, 'b': 2}
dic2 = {'c': 3, 'd': 4}
f = open("data.pk", "wb+")
pickle.dump(dic1, f)
f.truncate(0)
pickle.dump(dic2, f)
f.seek(0)
print pickle.load(f)
f.close()

This code raises KeyError: '\x00'. It seems like truncate(0) does not truncate file, but adds some characters like '\x00'.

Can I delete the content of the pickle file without closing and reopening it ?

标签： pythonfilepicklepython-2.xtruncate

Answer 1

file.truncate更改文件大小，但是，根据文档

当前文件位置没有改变。

这可以通过tell在截断后调用文件的方法来证明。

>>> d1 = dict(a=1, b=2)
>>> d2 = dict(c=3, d=4) 

>>> with open('file.bin', 'wb+') as f:
...     pickle.dump(d1, f)
...     f.truncate(0)
...     print 'File pointer position:', f.tell()
...     pickle.dump(d2, f)
...     f.seek(0)
...     pickle.load(f)
... 
File pointer position: 30
Traceback (most recent call last):
  File "<stdin>", line 7, in <module>
  File "/usr/lib64/python2.7/pickle.py", line 1384, in load
    return Unpickler(file).load()
  File "/usr/lib64/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
KeyError: '\x00'

将文件指针重置到文件的开头会产生所需的行为^*

>>> with open('file.bin', 'wb+') as f:
...     pickle.dump(d1, f)
...     f.truncate(0)
...     f.seek(0)
...     pickle.dump(d2, f)
...     f.seek(0)
...     pickle.load(f)
... 
{'c': 3, 'd': 4}

^*至少，它会在我的 Linux 机器上产生所需的行为。对此问题的评论中的讨论表明这可能不适用于所有平台。