[Python-ideas] Dart-like method cascading operator in Python

[Perešíni Peter]

On Thu, Nov 21, 2013 at 2:34 PM, Masklinn <masklinn at masklinn.net> wrote:

> On 2013-11-21, at 13:40 , Nick Coghlan <ncoghlan at gmail.com> wrote:
>
> > On 21 November 2013 20:55, Perešíni Peter <ppershing at gmail.com> wrote:
> >> gnuplot.newPlot()
> >> ..set("xrange [0:5]")
> >> ..set("yrange [0:20]")
> >> ..newPlot()
> >>     ..addSeries("Linear", [1,2,3])
> >>     ..addSeries("Quadratic", [1,4,6])
> >> ..run()
> >
> > If you just want structural grouping of some code, you can already
> > define an appropriate context manager:
> >
> > @contextlib.contextmanager
> > def value(x)
> >    yield x
> >
> > with value(gnuplot.newPlot()) as p:
> >    p.set("xrange [0:5]")
> >    p.set("yrange [0:20]")
> >    with value(p.newPlot()) as n:
> >        n.addSeries("Linear", [1,2,3])
> >        n.addSeries("Quadratic", [1,4,6])
> >    p.run()
> >
> > It doesn’t define a new scope
>
> And it requires naming things.
>

Exactly, point of the proposal is to avoid repetitive naming


>
> An other drawback is that it is a statement, where the cascading
> operator yields an expression (at least in Smalltalk it did) (well of
> course more or less everything was an expression in smalltalk so that
> helped).
>
> I really liked message cascading when I played with it in Smalltalk, but:
>
> 1. I find Dart’s cascading syntax rather noisy, and examples on The
>    Internets seem to show it regularly used in single-line expression
>    which I find a detriment to readability:
>
>        document.body.children.add(new
> ButtonElement()..id='awesome'..text='Click Me!’;);
>

Agree, this example is extra bad and unreadable. If we want cascading (and
especially nested cascading), I would force cascading operator to be the
first token on a new (and indented) line as in my examples


>
> 2. Unless I missed it, the original suggestion failed to specify what
>    the overall expression returns. In Smalltalk, it returns the value of
>    the last message of the cascade, and Smalltalk has a `yourself` message
>    which returns, well, its self. IIRC, Python has no such message “built
>    in”.
>
>
It would return the result of the expression before cascading, e.g.

o = MyObject()
  ..set(x)
  ..set(y)

is a syntactic sugar for

tmp = MyObject()
tmp.set(x)
tmp.set(y)
o = tmp

Note that we need a get the priority right though, e.g. cascading operator
takes precedence over assignment in order to avoid surprises

3. It encourages and facilitates APIs based on object mutation and
>    incompletely initialised objects, the former being a slight dislike and
>    the latter being something I loathe.
>
>
Agree. It may lead to a bit more sloppier API design than usual.


> 4. I doubt OP’s (a) would be fixed, it’s not an issue of attribute deref,
> so
>    a cascading operator would have the exact same behaviour.
>
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