I'm with Shai, against changing bool() and len() behaviour. I think the
"fetch the whole queryset" behaviour is normally helpful for beginners, who
write things like:
if qs:
for x in qs:
...
# really common for anyone new to Python:
for i in len(qs):
do_something(qs[i])
We have highlighted this behaviour for a long time in the 'database access
optimization' page really well:
https://docs.djangoproject.com/en/3.0/topics/db/optimization/#use-queryset-count
. There are alternatives if you want different behaviour - exists() and
count() .
+1 to .contains() since it would follow that same pattern.
On Fri, 5 Jun 2020 at 07:03, Shai Berger <[email protected]> wrote:
> On Tue, 2 Jun 2020 20:57:54 +0200
> Aymeric Augustin <[email protected]> wrote:
>
> >
> > We're talking about a trade-off between preserving optimisations in
> > existing code bases and expertise of advanced users versus doing the
> > right thing by default for less experienced users.
>
> I disagree.
>
> The suggestion is to make
>
> if obj in qs:
> # do things which don't fetch qs
>
> more performant, at the expense of
>
> if obj in qs:
> # do things that fetch qs
>
> As well as
>
> for obj in container:
> if obj in qs:
> # whatever
>
> So, it is not a net optimization even in terms of its own.
>
> Also, changing the performance characteristics of thousands, if not
> millions of existing lines of code, and making a decade of online
> comments and advice wrong (not just obsolete "there's better ways now"
> but wrong "this points you away from truth and good"), is IMO a
> non-starter.
>
> Strong -1 on Aymeric's option 1. Strong +1 on adding ".contains()".
>
> My 2 cents,
> Shai.
>
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