BlueBream

zope calls list(iterable) whenever we use tal:repeat="item iterable"

Bug #4819 reported by Guilherme Salgado on 2005-11-24

	Status	Importance	Assigned to
BlueBream	New	Undecided	Unassigned
Launchpad itself	Fix Released	Medium	Unassigned
Zope 3	Won't Fix	Undecided	Unassigned
zope.tal	Invalid	Undecided	Unassigned

Bug Description

I don't see any reason why this needs to be done, and it can be a problem because is some cases the result can be something you don't expect. For instance, with sqlobject, if you do a list(SelectResults), it'll issue a COUNT(*) query with that SelectResult's WHERE clause, which is not issued if you iterate over SelectResults. One can argue this is a bug in sqlobject (and I do agree), but I still think this has to be fixed in zope too.

Tags:

Revision history for this message

Guilherme Salgado (salgado) wrote on 2005-11-24: Patch to print the issued queries on stdout

Patch to print the issued queries on stdout Edit (396 bytes, text/plain)

Apply this patch and go to http://localhost:8086/+portlet-foaf to see the extra COUNT(*) query.

Changed in launchpad:
assignee:	nobody → stevea

Revision history for this message

James Henstridge (jamesh) wrote on 2005-12-01:

The extra COUNT(*) is because of an allocation optimisation when converting an iterable to a list: if the iterable provides a __len__() method, it is called first. That many items are allocated in the new list and then filled by iterating over the list.

For SQLObject, this optimisation hurts performance overall. The additional query could be avoided if SelectResults.__len__() was removed (but that might cause other problems).

Dafydd Harries (daf) on 2005-12-16

Changed in launchpad:
status:	New → Accepted

Francis J. Lacoste (flacoste) on 2007-09-27

Changed in launchpad:
assignee:	stevea → nobody

Revision history for this message

David Allouche (ddaa) wrote on 2007-09-28:

The sqlobject currently used by Launchpad does not provide SelectResults.__len__().

Tres Seaver (tseaver) on 2010-04-15

Changed in zope3:
status:	New → Won't Fix

Fred Drake (fdrake) on 2010-04-15

Changed in zope.tal:
status:	New → Confirmed

Tres Seaver (tseaver) on 2010-04-21

tags:

added: bugday20100424

Revision history for this message

Robert Collins (lifeless) wrote on 2011-09-20:

So the issue here for LP is reasonably paraphrased as 'our ORM result sets had a __len__ method'. We probably want to ensure they don't and add a test for that to prevent regressions. That said, we already havge ditched __len__, so I'm going to close this as fixreleased for LP.