[Python-ideas] PEP 505: None-aware operators

[Michael Lee]

Hi -- I'm a new voice here, though I've been lurking for a while now.

How do people feel about removing "??=" and "foo?[bar]" from the PEP and
sticking with just "foo?.bar" and "foo ?? bar"?

Reasons for not implementing "??=":

   1. I think the other operators are useful in that they can be chained
   together to shrink many different lines of code. In contrast, ??= can
   shrink at most one line of code: we get to remove a single 'if foo is
   None'. If the rest of the PEP is implemented, we can also just express
   this by doing "foo = foo ?? some_expr" which I think is still relatively
   concise.

   2. None of the other short-circuiting operators have an augmented
   assignment form -- e.g. we can do "foo = foo or bar", but there's no
   such thing as "foo or= bar". I don't really see why ?? in particular
   deserves to have an augmented assignment form.

   3. We already have two different kinds of assignment operators with the
   inclusion of PEP 572 -- in the interests of keeping Python as simple as
   possible, I think it'd be a good idea not to add a third one.


Reasons for not implementing "foo?[bar]":

   1. It seems like "foo?[bar]" could be easily confused with "foo??[bar]".
   I don't think it's immediately obvious to a newcomer which spelling
   corresponds to which usage, and getting the two mixed up seems like the
   sort of thing that could cause all sorts of subtle bugs.

   2. We can already easily get the same functionality using standard
   Python. E.g., instead of doing foo?["bar"]?[0]?["baz"], we could do
lookup(foo,
   "bar", 0, "baz") where lookup is a function that looks roughly like this:

       def lookup(item, *parts):
           for part in parts:
               if item is None:
                   return None
               item = item[parts]
           return item

   Of course, we could probably create the same sort of function to replace
   foo?.bar (albeit less ergonomically), but unlike foo?[bar], there's no
   possibility for confusion: doing foo??.bar will never be a valid Python
   expression.

   3. One counter-argument against removing foo?[bar] is that it would make
   expression that need both safe index and attribute lookups look weird --
   you'd sometimes be using the "lookup" function described above (or
   something similar) and sometimes using ".?". However, I'd argue that these
   sorts of scenarios are relatively rare in practice, and that if you really
   need to deal with a bunch of code that requires you to use both forms of
   safe navigation, your code is likely already becoming pretty unreadable and
   should be rewritten -- maybe split up into multiple lines or something, or
   maybe just redesigned from scratch so you don't need to constantly manage a
   mess of Nones.


More broadly, I think I agree with the sentiment some other people have
that Python has acquired a lot of new features in a relatively short period
of time, and that it would be nice to have some cooldown to let tooling and
other implementations catch up. In that regard, I'd personally be happy if
we didn't implement this PEP or just deferred it again. But if we *are*
moving forward with it, I think it's worth trying to simplify it as much as
possible/try and make it orthogonal with existing Python features.

(But of course, I'm just some random person on the internet, so IDK if my
opinion counts for much.)

Regards,
-- Michael


On Thu, Jul 19, 2018 at 5:06 PM, Chris Angelico <rosuav at gmail.com> wrote:

> On Fri, Jul 20, 2018 at 10:03 AM, Greg Ewing
> <greg.ewing at canterbury.ac.nz> wrote:
> > Rhodri James wrote:
> >>
> >> If anyone can think of a good word for "if it isn't None, otherwise",
> I'd
> >> be all for it :-)
> >
> >
> > I don't think there's any single Engish word that captures
> > all of that, so we'd have to invent one.
> >
> > Some suggestions:
> >
> > inno (If Not None, Otherwise)
> >
> > oft  (Or, Failing That)
>
> Iunno, that'd oft be confusing.
>
> Kappa
>
> ChrisA
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