The Python statement gList1[i1][1] += gList2[i2][1] modifies not only gList1 (a volatile storage object) but dBasis[163] (a record within shelve object dBasis).  Both gList and dBasis[163] are printed before and after execution of the statement by cFract2.combine.  They are obviously not the same record, but the statement modifies gList1[2][1] and dBasis[163][2][1] simultaniously.

Any chance you could reduce this to a simpler test case?  It may be a while before anyone gets around to analyzing your complete example.

> They are obviously not the same record

Have you tried printing gList1 and dBasis[163], their repr and their ids?

>	dBasis = shelve.open('cFract2.tmp','n')
>	i,testVal,dBasis,oldTime = 0,1,{},int(time.clock())	# Initialize

As I understand, you don't use shelve object at all (except creating an empty base).  Such a lot of code has no relation to the bugs in Python.  Ask on the Python mailing list or on forums and may be someone help you.

Indeed.

This is easy to reply to.  You better believe I've tried printing gList1 and dBasis[163].  I've also scrapped a predecessor function named combo (which is included in the attachment) and written its successor, viz., "combine".  I bent over backwards to make sure that the two functions did NOTHING the same way.  Guess what.  Both combo and combine screw up exactly the same record that they aren't supposed to know about in exactly the same way.  So I'm pretty sure Python remembers that it originally got one ITEM on gList from dBasis[161].  So I think I know that the problem is to determine WHY Python remembers that.  Of course what oneo thinks one knows is always subject to revision--especially what one thinks one knows about the truth.

--Terry

-----Original Message-----
>From: Ezio Melotti <report@bugs.python.org>
>Sent: Nov 18, 2012 2:07 PM
>To: ttcooper@indy.net
>Subject: [issue16498] Unwanted link between volatile and shelve storage
>
>
>Ezio Melotti added the comment:
>
>> They are obviously not the same record
>
>Have you tried printing gList1 and dBasis[163], their repr and their ids?
>
>----------
>nosy: +ezio.melotti
>
>_______________________________________
>Python tracker <report@bugs.python.org>
><http://bugs.python.org/issue16498>
>_______________________________________

If it "got it" from dBasis[163] via item assignment (say gList1[i1][1] = dBasis[163]), then yes, Python remembers that.  Names just hold pointers to objects, so after that assignment gList1[i1][1] points to the same object as dBasis[163] does.  If that object is mutable, and you mutate it (as += will do, if the object is mutable), both names will still reference the same object, the one that has been changed.  

As Serhiy suggested, you are more likely to get help with this on python-list.

Got your message before I got his.  Thanks for the reference to python-list.

-----Original Message-----
>From: "R. David Murray" <report@bugs.python.org>
>Sent: Nov 18, 2012 9:39 PM
>To: ttcooper@indy.net
>Subject: [issue16498] Unwanted link between volatile and shelve storage
>
>
>R. David Murray added the comment:
>
>If it "got it" from dBasis[163] via item assignment (say gList1[i1][1] = dBasis[163]), then yes, Python remembers that.  Names just hold pointers to objects, so after that assignment gList1[i1][1] points to the same object as dBasis[163] does.  If that object is mutable, and you mutate it (as += will do, if the object is mutable), both names will still reference the same object, the one that has been changed.  
>
>As Serhiy suggested, you are more likely to get help with this on python-list.
>
>----------
>
>_______________________________________
>Python tracker <report@bugs.python.org>
><http://bugs.python.org/issue16498>
>_______________________________________