#10929: Support a default value for Sum (and possibly other aggregation
functions)
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Reporter: nolan | Owner:
Type: New feature | Status: new
Component: ORM aggregation | Version: SVN
Severity: Normal | Resolution:
Keywords: aggregate annotate | Triage Stage: Accepted
default | Needs documentation: 1
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by lukeplant):
There is a good case for giving `Sum` a default of zero, while leaving
`Max` and `Min` with default of `None`. Essentially, the reason is that
zero is the fixed point of the `+` operator, while max and min have no
fixed points (excluding negative infinity and positive infinity
respectively, which are problematic). Also, it makes the behaviour
analogous to the Python builtins `sum`, `min` and `max` - `sum` returns 0
for an empty list, whereas `min` and `max` throw exceptions. (We don't
want to throw exceptions here, for various obvious reasons, but I think we
should be indicating 'undefined' in ''some'' way under the same
circumstances).
If we do this, we need a note in the release notes about the backwards
incompatibility - if people were relying on Sum returning None for no data
instead of zero. (We can certainly classify the previous behaviour as a
bug, since it was out of line with our docs, and neither expected or
useful behaviour, but should still mention this). We should also have
some tests that ensure that it returns a zero of the right type for
different underlying field types.
This also brings up the possibility of whether the default for `Sum`
should act like the `start` parameter of the `sum` Python builtin. I think
it should. That would make implementation harder, though, I guess.
Alternatively we could have a separate 'start' parameter for `Sum`, which
might be clearer, and would make that a separate feature.
--
Ticket URL: <https://code.djangoproject.com/ticket/10929#comment:6>
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