#17311: bool(queryset) calls __len__ instead of __nonzero__
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Reporter: adamnelson | Owner: nobody
Type: Uncategorized | Status: closed
Component: Database layer | Version:
(models, ORM) | Resolution: invalid
Severity: Normal | Triage Stage:
Keywords: | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by lukeplant):
* status: new => closed
* needs_better_patch: => 0
* resolution: => invalid
* needs_tests: => 0
* needs_docs: => 0
Comment:
Almost everything about this bug report is incorrect, I'm afraid :-) .
When running `if queryset`, `__nonzero__` is called, not `__len__` (this
is Python behaviour over which we have no control. It is true that
`__len__` is called indirectly in some situations in trunk, but not
always). The special attribute `__len__` does not call `count()`. Finally
`__nonzero__` already exists and does not and should not call `exists()`.
The current behaviour is exactly as intended. The reason for this is to
make the behaviour 1) predictable, and 2) behave nicely with several
common usage patterns, such as:
{{{
#!python
if queryset:
for obj in queryset
}}}
In short, the rule is you only get special queries (like `exists()` and
`count()`) if you ask for them, because we cannot predict if you are going
to want to use the `QuerySet`'s result cache or not. All other constructs
like `bool()` and `len()` and `iter()` evaluate the same query, and
populate/use the same result cache where possible.
--
Ticket URL: <https://code.djangoproject.com/ticket/17311#comment:1>
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