[Python-ideas] namedtuple literals [Was: RE a new namedtuple]

[Ivan Levkivskyi]

Something probably not directly related, but since we started to talk about
syntactic changes...
I think what would be great to eventually have is some form of pattern
matching.
Essentially pattern matching  could be just a "tagged" unpacking protocol.
For example, something like this will simplify a common pattern
with a sequence of if isinstance() branches:

class Single(NamedTuple):
    x: int

class Pair(NamedTuple):
    x: int
    y: int

def func(arg: Union[Single, Pair]) -> int:
    whether arg:
        Single as a:
            return a + 2
        Pair as a, b:
            return a * b
        else:
            return 0

The idea is that the expression before ``as`` is evaluated, then if ``arg``
is an instance of the result,
then ``__unpack__`` is called on it. Then the resulting tuple is unpacked
into the names a, b, etc.

I think named tuples could provide the __unpack__, and especially it would
be great for dataclasses
to provide the __unpack__ method. (Maybe we can then call it __data__?)

--
Ivan



On 20 July 2017 at 11:39, Clément Pit-Claudel <cpitclaudel at gmail.com> wrote:

> On 2017-07-20 11:30, Paul Moore wrote:
> > On 20 July 2017 at 10:15, Clément Pit-Claudel <cpitclaudel at gmail.com>
> wrote:
> >> On 2017-07-20 11:02, Paul Moore wrote:
> >>>> Also, what's the advantage of (x=1, y=2) over ntuple(x=1, y=2)? I.e.,
> >>>> why does this need to be syntax instead of a library?
> >>>
> >>> Agreed. Now that keyword argument dictionaries retain their order,
> >>> there's no need for new syntax here. In fact, that's one of the key
> >>> motivating reasons for the feature.
> >>
> >> Isn't there a speed aspect?  That is, doesn't the library approach
> require creating (and likely discarding) a dictionary every time a new
> ntuple is created?  The syntax approach wouldn't need to do that.
> >
> > I don't think anyone has suggested that the instance creation time
> > penalty for namedtuple is the issue (it's the initial creation of the
> > class that affects interpreter startup time), so it's not clear that
> > we need to optimise that (at this stage)
>
> Indeed, it's not clear we do.  I was just offering a response to the
> original question, "what's the advantage of (x=1, y=2) over ntuple(x=1,
> y=2)?".
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