[Python-ideas] Composition over Inheritance

Koos Zevenhoven k7hoven at gmail.com
Sat Oct 28 21:31:25 EDT 2017

On Sat, Oct 28, 2017 at 11:24 PM, Soni L. <fakedme+py at gmail.com> wrote:

> On 2017-10-28 02:51 PM, Steven D'Aprano wrote:
>> You ignored my question: Is that the sort of thing you mean by
>> composition? If not, then what do you mean by it? This is not a
>> rhetorical question: I'm having difficulty understanding your proposal.
>> It is too vague, and you are using terminology in ways I don't
>> understand.
>> Maybe that's my ignorance, or maybe you're using non-standard
>> terminology. Either way, if I'm having trouble, probably others are too.
>> Help us understand your proposal.
I have to say I'm almost impressed by the constructiveness of the
discussion, even though​ I still don't understand the point of all the
square brackets in the proposal.

> With composition, you can have car.key.turn() call car.ignition.start(),
> without having to add car to key or ignition to key. You just have to put
> both in a car and they can then see eachother!

Here it's a bit confusing that the key is thought of as part of the car.
It's easy to imagine that an owner of two cars would want the same key to
work for both of them. Or that one car could have multiple users with
non-identical keys. I'm not sure if these things already exist in real
life, but if not, it's probably just a matter of time.

But let's ignore this confusion for a moment, and imagine that the example
makes perfect sense. Now, it sounds like you want something like
namespacing for methods and attributes within a complicated class. Maybe
you could implement it using nested classes and decorators to make sure
'self' is passed to to the methods in the way you want. The usage might
look roughly like:

class Car:
    class ignition:
        def start(self):

    class key:
        def turn(self):

Another concern regarding the example, however, is that this seems to make
it unclear what the public API of the car is. It looks like you can just as
easily drive the car without having the key: just call car.ignition.start().

-- Koos

+ Koos Zevenhoven + http://twitter.com/k7hoven +
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