On Wed, Oct 21, 2015 at 8:56 AM, Carl Meyer carl@oddbird.net wrote:
I think you missed her real point, which applies both to Python and Ruby. In her presentation, it's obscured a bit by too much discussion of "one side or the other" code duplication (which can be "solved" with multiple inheritance). It takes her a couple minutes more to get to the real point, which starts at the slide "inheritance is for specialization, not for sharing code."
Sure, but a dictionary with a default handler _is_ a form of specialization - as is a dictionary that preserves order. Both of them behave absolutely identically to a regular dict when you set something, retrieve it, iterate over them, etc, etc, etc. She recommends a massive superclass that's capable of any form of injection; using inheritance allows the dict type to be broadly unaware of the modified dictionaries that exist.
Suppose I write a SortedDict that behaves exactly the way any other dictionary does, but when you iterate over its keys, you get them in order. I should be able to do this without tampering with the original dict type. In the composition model, I would need to appeal for an "IterationOrder" feature to be added to the base dict; using inheritance, it's as simple as:
class SortedDict(dict): def __iter__(self): yield from sorted(self.keys())
To me, this is the biggest benefit of inheritance: you do NOT have to predict what someone might want to do. I can subclass someone else's object and change how it works.
One symptom of the problem is that using multiple inheritance this way doesn't scale: the number of leaf subclasses you need grows geometrically with the number of behavior knobs. Composition with strategy objects doesn't have this issue.
Sure it does... but nobody needs to know about _all_ the leaf subclasses. How many subclasses of object are there in Python?
len(object.__subclasses__())
122
And that in a bare interactive Py3, without importing anything fancy. ('import decimal' raises that to 159, for instance.)
Composition has its place, don't get me wrong. But inheritance isn't the attractive nuisance she makes it out to be. It works just fine.
ChrisA