[Python-ideas] OrderedCounter and OrderedDefaultDict

[Carl Meyer]

On 10/20/2015 03:40 PM, Chris Angelico wrote:
> On Wed, Oct 21, 2015 at 5:14 AM, Sven R. Kunze <srkunze at mail.de> wrote:
>> Not saying that inheritance is a bad thing but to me It seems to me that
>> ordering and default values should be orthogonal aspects of the standard
>> dict.
>>
>> Just as Sandi described it here: https://www.youtube.com/watch?v=29MAL8pJImQ
> 
> Yeah... that video is absolutely correct... for Ruby. In Python, you
> can use multiple inheritance to do that in the exactly obvious way.
> 
> class EchoRandomHouse(EchoHouse, RandomHouse): pass
> 
> When I got to the bit in that video where she says that inheritance
> paints you into a corner, I went and reimplemented her example code in
> Python, and it's flawless. It doesn't even matter which order you put
> the two superclasses, as there's no conflict.

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."

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.

Carl

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