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

[Andrew Barnert]

On Nov 21, 2013, at 5:34, 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.

Is it really that hard to name a plot "p"? Is typing "p.", or reading it, more work than ".."?

The FAQ already suggests the "just give it a short name" answer (http://docs.python.org/3.3/faq/design.html#why-doesn-t-python-have-a-with-statement-for-attribute-assignments). Nick's suggestion does that plus an indent for readability. Do we actually need more than that?

> 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).

Cascading has to be a statement. It has statements inside it. It uses indentation. If it's an expression it will have all of the same ambiguity problems that a naive multiline lambda proposal does, and break the simplicity of the language.

(That also means it can't be an operator. It has to be special syntax, like =.)

So if this is any part of the argument for the proposal, I'm -1.


> 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!’;);
> 
> 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”. 
> 
> 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.
> 
> 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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