为什么附加二进制泡菜不起作用? [英] Why doesn't appending binary pickles work?

问题描述

我知道这并不是应该使用pickle模块的方式，但是我认为这会起作用.我正在使用Python 3.1.2

I know this isn't exactly how the pickle module was intended to be used, but I would have thought this would work. I'm using Python 3.1.2

这是背景代码:

import pickle

FILEPATH='/tmp/tempfile'

class HistoryFile():
    """
    Persistent store of a history file  
    Each line should be a separate Python object
    Usually, pickle is used to make a file for each object,
        but here, I'm trying to use the append mode of writing a file to store a sequence
    """

    def validate(self, obj):
        """
        Returns whether or not obj is the right Pythonic object
        """
        return True

    def add(self, obj):
        if self.validate(obj):
            with open(FILEPATH, mode='ba') as f:    # appending, not writing
                f.write(pickle.dumps(obj))
        else:
            raise "Did not validate"

    def unpack(self):
        """
        Go through each line in the file and put each python object
        into a list, which is returned
        """
        lst = []
        with open(FILEPATH, mode='br') as f:
            # problem must be here, does it not step through the file?
            for l in f:
                lst.append(pickle.loads(l))
        return lst

现在，当我运行它时，它只会打印出传递给该类的第一个对象.

Now, when I run it, it only prints out the first object that is passed to the class.

if __name__ == '__main__':

    L = HistoryFile()
    L.add('a')
    L.add('dfsdfs')
    L.add(['dfdkfjdf', 'errree', 'cvcvcxvx'])

    print(L.unpack())       # only prints the first item, 'a'!

是因为看到了早期的EOF吗?也许追加仅适用于ASCII? (在那种情况下，为什么让我执行mode ='ba'?)有没有更简单的方法?

Is this because it's seeing an early EOF? Maybe appending is intended only for ascii? (in which case, why is it letting me do mode='ba'?) Is there a much simpler duh way to do this?

推荐答案

为什么您认为附加二进制泡菜会产生单个泡菜?借助酸洗，您可以一个接一个地放置(并取回)多个项目，因此很明显，它必须是自终止"序列化格式.忘掉台词，把它们找回来！例如:

Why would you think appending binary pickles would produce a single pickle?! Pickling lets you put (and get back) several items one after the other, so obviously it must be a "self-terminating" serialization format. Forget lines and just get them back! For example:

>>> import pickle
>>> import cStringIO
>>> s = cStringIO.StringIO()
>>> pickle.dump(23, s)
>>> pickle.dump(45, s)
>>> s.seek(0)
>>> pickle.load(s)
23
>>> pickle.load(s)
45
>>> pickle.load(s)
Traceback (most recent call last):
   ...
EOFError
>>> 

只需按一下EOFError即可告诉您完成脱酸的时间.

just catch the EOFError to tell you when you're done unpickling.

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