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

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Tue, 19 Nov 2002 06:35:37 -0800


Bugs item #640645, was opened at 2002-11-19 11:42
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Category: Python Library
Group: Python 2.2.2
Status: Open
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: 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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