[ python-Bugs-640645 ] cPickle.dumps change after for

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Bugs item #640645, was opened at 2002-11-19 11:42
Message generated for change (Comment added) made by arigo
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Category: Python Library
Group: Not a Bug
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: Marco Beri (marcoberi)
Assigned to: Nobody/Anonymous (nobody)
Summary: cPickle.dumps change after for

Initial Comment:
try this program:

import cPickle
a=([{}])
print cPickle.dumps(a)
for x in a:
  pass
cPickle.dumps(a)

It prints:
(lp1
(da.

Why on earth it does dumps return different string?
And after just a for with a pass in it...


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>Comment By: Armin Rigo (arigo)
Date: 2004-01-12 12:30

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Someone else ran into the problem of unexpectedly varying
cPickle strings:

http://sourceforge.net/tracker/?func=detail&atid=429622&aid=875331&group_id=41036

Maybe this behavior should be documented.

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Comment By: Tim Peters (tim_one)
Date: 2002-11-19 16:19

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Closed as Not-A-Bug.  The internals of pickle strings aren't 
guaranteed, just that "they work" when unpickled again, and 
these do.  If you want a hash code for a dict, don't dare use 
pickle for this either, even if it appears "to work":  it doesn't.  
The order in which dict keys are enumerated isn't defined 
either, and can and does vary across releases, and even 
across program runs.

So a reliable hash code for a dict needs to be independent of 
key iteration order, and no version of pickle spends time 
trying to force that issue (waste of time -- it isn't needed for 
pickling or unpickling).

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Comment By: Michael Hudson (mwh)
Date: 2002-11-19 14:35

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Well, that's asking for something you're not promised.  It
takes some imagination to categorize this as a bug, and I'm
not going to put any of my time into fixing it.

It seems using pickle in place of cPickle *may* work, fwiw.

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Comment By: Marco Beri (marcoberi)
Date: 2002-11-19 13:56

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I would like to use hash(cPickle.dumps(a)) to have a unique 
hash also for dict that aren't hashable.


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Comment By: Michael Hudson (mwh)
Date: 2002-11-19 13:03

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OK, I think I know what's happening.

Look at this:

>>> a = {}       
>>> cPickle.dumps(a)
'(dp1\n.'
>>> cPickle.dumps({})
'(d.'

the only difference is in the number of references to the
object getting pickled.  I don't know why that matters, but
it seems to.

Why does this matter to you?

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Comment By: Michael Hudson (mwh)
Date: 2002-11-19 12:56

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You're right, sorry.

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Comment By: Marco Beri (marcoberi)
Date: 2002-11-19 12:07

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I forgot to print the second dumps but the bug remains:
Before the for:
(lp1
(da.
After the for:
(lp1
(dp2
a.


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Comment By: Michael Hudson (mwh)
Date: 2002-11-19 12:00

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Umm:

>>> a=([{}])
>>> cPickle.dumps(a)
'(lp1\n(da.'

you're only printing the first pickle...

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