Created on 2002-11-19 11:42 by marcoberi, last changed 2004-01-12 12:30 by arigo. This issue is now closed.
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| msg13394 - (view) | Author: Marco Beri (marcoberi) | Date: 2002-11-19 11:42 | |
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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| msg13395 - (view) | Author: Michael Hudson (mwh)  |
Date: 2002-11-19 12:00 | |
Logged In: YES
user_id=6656
Umm:
>>> a=([{}])
>>> cPickle.dumps(a)
'(lp1\n(da.'
you're only printing the first pickle...
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| msg13396 - (view) | Author: Marco Beri (marcoberi) | Date: 2002-11-19 12:07 | |
Logged In: YES user_id=588604 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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| msg13397 - (view) | Author: Michael Hudson (mwh) |
Date: 2002-11-19 12:56 | |
Logged In: YES user_id=6656 You're right, sorry. |
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| msg13398 - (view) | Author: Michael Hudson (mwh) |
Date: 2002-11-19 13:03 | |
Logged In: YES
user_id=6656
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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| msg13399 - (view) | Author: Marco Beri (marcoberi) | Date: 2002-11-19 13:56 | |
Logged In: YES user_id=588604 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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| msg13400 - (view) | Author: Michael Hudson (mwh) |
Date: 2002-11-19 14:35 | |
Logged In: YES user_id=6656 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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| msg13401 - (view) | Author: Tim Peters (tim_one) * |
Date: 2002-11-19 16:19 | |
Logged In: YES user_id=31435 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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| msg13402 - (view) | Author: Armin Rigo (arigo) * |
Date: 2004-01-12 12:30 | |
Logged In: YES user_id=4771 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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| History | |||
|---|---|---|---|
| Date | User | Action | Args |
| 2002-11-19 11:42:57 | marcoberi | create | |