读取腌制数据时出现EOFError

Question

I'm trying to unpickle chunks of a webpage stored in the Google App Engine memcache. First I get the chunks and store them as a dictionary with the key

def get_by_key_name(key_name):
    result = memcache.get_multi(['%s.%s' % (key_name, i) for i in xrange(32)])
    serialized = ''
    for k, v in sorted(result.items()):
        if v is not None:
            serialized = serialized.join(v)
        else:
            return None

    return pickle.loads(serialized) #Line that fails

For some reason it raises EOFError. The code that originally pickled the data is:

serialized = pickle.dumps(content, 2)
values = {}
for i in xrange(0, len(serialized), chunksize):
  values['%s.%s' % (key_name, i//CHUNKSIZE) ] = serialized[i:i+chunksize]

Anybody have any idea why? By the way, CHUNKSIZE is 950000 bytes. I tried to load reddit's front page onto the memcache, so I don't think it is exceeding this limit.

Answer 1

You want to concatenate the string, not join.

serialized += v

Join will add a copy of the original string between each character of the new string

>>> 'hello'.join('there')
'thellohhelloehellorhelloe'

I'm kinda impressed you didn't run out of memory!

Answer 2

You are joining your string incorrectly:

serialized = ''
for k, v in sorted(result.items()):
    if v is not None:
        serialized = serialized.join(v)

This uses selialized as built so far as the joining string, with the new string treated as individual characters:

>>> serialized = ''
>>> for v in ('foo', 'bar', 'baz'):
...     serialized = serialized.join(v)
... 
>>> serialized
'bbfooafoorabfooafoorz'

where 'foo'.join('bar') produced 'bfooafoor' , which then was used to join the characters of baz .

Build a list , then return that:

if None in result.viewvalues():
    # one or more keys came back empty, abort
    return
serialized = ''.join([v for k, v in sorted(result.items())])