#29215: Document potential change to behaviour of QuerySet methods when
upgrading
to Python 3.6
-------------------------------------+-------------------------------------
Reporter: Matt Fisher | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Documentation | Version: 2.0
Severity: Normal | Resolution:
Keywords: documentation | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Simon Charette):
* cc: Simon Charette (added)
Comment:
This behavior has little to do with Python 3.6 itself.
If you had query performing better on Python 2.7 than on Python 3 it's
probably because you weren't using
[https://docs.python.org/2/using/cmdline.html?highlight=pythonhashseed#cmdoption-r
a random hash seed] which is the default on Python 3. In other words, you
were relying on a stable hash seed to build queries in a way that
PostgreSQL query planner's would correctly optimize. If you had updated
from 2.7 to Python 3.5 instead you would have hit the slow query issue
from time to time when the interpreter chose an hash seed that made
`list({'b__in', 'a'}) == ['b__in', 'a']`. In some way Python 3.6 allows
you to reliably build your `WHERE` statement through kwargs ordering while
it wasn't possible before.
I guess we could add a section about the fact Python 3 has random hash
seeding on by default and that may break code making assumptions about
unordered data structures ordering. Maybe we should also suggest running
Python 2.7 with the `-R` flag on when porting code?
--
Ticket URL: <https://code.djangoproject.com/ticket/29215#comment:1>
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