On Tue, Oct 19, 2021 at 05:09:42PM -0700, Michael Selik wrote:
> None and its ilk often conflate too many qualities. For example, is it
> missing because it doesn't exist, it never existed, or because we never
> received a value, despite knowing it must exist?
Does it matter if different functions have different semantic
interpretations for None?
> The languages SAS and R
> support at least 27 varieties of NA, allowing un-tagged, and tagged with
> the letters A-Z to help someone create distinctions between different kinds
> of nothingness. IEEE-754 allows about 16 million possible NaNs, which I
> believe was intended to allow floating point instructions to pass error
> messages along.
Yes, and after something like 30-40 years of IEEE-754 supporting NAN
payloads, the number of systems that actually use them can probably be
counted on the fingers of one hand :-(
Ironically, one of those systems is R, which -- so I have been lead to
believe -- uses distict NANs to represent those 27 tagged NA values.
Back in the 1980s, one of the earliest systems which supported IEEE-754
maths was the Apple Numeric Toolkit. Apple's maths routines generated
NANs with documented payloads for certain errors, e.g:
* NAN(1) invalid sqrt
* NAN(2) invalid addition such as INF + -INF
* NAN(34) invalid argument to inverse trig functions
In a complex computation, it was sometimes useful to see why a NAN was
generated. Alas, when Apple moved their maths routines into hardware,
the MC68881 coprocessor always generated NANs with payload 255, and that
useful debugging information was lost.
30+ years later, and we cannot easily, reliably or portably use NAN
payloads. Most people don't care. If we offerred them a dozen or a
thousand distinct sentinels for all the various kinds of missing data,
how many people would use them and how many would just stick to plain
old None?
> If the motivation for this operator is chained lookups, how about adding a
> feature to the operator module, first? It seems natural to add a
> keyword-only argument to `attrgetter`, and it's a lighter touch than
> implementing a new operator. If use becomes widespread, that gives more
> weight to PEP 505.
I agree that this is a nice way forward, and a useful function in its
own right. The only thing is that I would argue for a different colour
of the bikeshed:
def getattr_chain(obj, *attrs, default):
# like obj.a.b.c.d
# if any attribute is missing,
# raises if default is not given
# otherwise returns default
getattr is far more commonly used than attrgetter.
--
Steve
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