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Re: Composition instead of inheritance

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  Re: Composition instead of inheritance Carl Banks <pavlovevidence@gmail.com> - 2011-04-28 15:35 -0700
    Re: Composition instead of inheritance Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-29 02:31 +0000
      Re: Composition instead of inheritance Ben Finney <ben+python@benfinney.id.au> - 2011-04-29 13:50 +1000
      Re: Composition instead of inheritance Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-04-28 20:57 -0700

#4277 — Re: Composition instead of inheritance

On Thursday, April 28, 2011 10:15:02 AM UTC-7, Ethan Furman wrote:
> For anybody interested in composition instead of multiple inheritance, I 
> have posted this recipe on ActiveState (for python 2.6/7, not 3.x):
> 
> http://code.activestate.com/recipes/577658-composition-of-classes-instead-of-multiple-inherit/
> 
> Comments welcome!

That's not what we mean by composition.  Composition is when one object calls upon another object that it owns to implement some of its behavior.  Often used to model a part/whole relationship, hence the name.

The sorts of class that this decorator will work for are probably not the ones that are going to have problems cooperating in the first place.  So you might as well just use inheritance; that way people trying to read the code will have a common, well-known Python construct rather than a custom decorator to understand.

If you want to enforce no duplication of attributes you can do that, such as with this untested metaclass:

class MakeSureNoBasesHaveTheSameClassAttributesMetaclass(type):
    def __new__(metatype,name,bases,dct):
        u = collections.Counter()
        for base in bases:
            for key in base.__dict__.keys():
                u[key] += 1
        for key in dct.keys():
            u[key] += 1
        if any(u[key] > 1 for key in u.keys()):
            raise TypeError("base classes and this class share some class attributes")
        return type.__new__(metatype,name,bases,dct)
 

Carl Banks

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#4288

On Thu, 28 Apr 2011 15:35:47 -0700, Carl Banks wrote:

> On Thursday, April 28, 2011 10:15:02 AM UTC-7, Ethan Furman wrote:
>> For anybody interested in composition instead of multiple inheritance,
>> I have posted this recipe on ActiveState (for python 2.6/7, not 3.x):
>> 
>> http://code.activestate.com/recipes/577658-composition-of-classes-
instead-of-multiple-inherit/
>> 
>> Comments welcome!
> 
> That's not what we mean by composition.  Composition is when one object
> calls upon another object that it owns to implement some of its
> behavior.

I thought that was delegation. As in, when one object delegates some or 
all of it's behaviour to another object:

http://code.activestate.com/recipes/52295


> Often used to model a part/whole relationship, hence the name.

In mathematics, composition means to make a function by applying a 
function to the output of another. E.g.:

f o g(x) is equivalent to f(g(x))

(Strictly speaking, the "o" in the f o g should be a little circle rather 
than lowercase "O".)



-- 
Steven

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#4289

Steven D'Aprano <steve+comp.lang.python@pearwood.info> writes:

> On Thu, 28 Apr 2011 15:35:47 -0700, Carl Banks wrote:
>
> > That's not what we mean by composition. Composition is when one
> > object calls upon another object that it owns to implement some of
> > its behavior.
>
> I thought that was delegation. As in, when one object delegates some or 
> all of it's behaviour to another object:
>
> http://code.activestate.com/recipes/52295

The difference being that, with delegation, the object being called is
unrelated to this one. With composition, the object being called is not
wholly separate, but is instead an attribute on the current object.

The “another object that it owns” was, I presume, meant to communicate
this “an object which is an attribute of the current one” relationship.

> In mathematics, composition means to make a function by applying a 
> function to the output of another.

The term “composition” in the programming sense comes from OO design,
not functional nor mathematical terminology.

-- 
 \          “I don't like country music, but I don't mean to denigrate |
  `\          those who do. And for the people who like country music, |
_o__)                        denigrate means ‘put down’.” —Bob Newhart |
Ben Finney

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#4292

On 29 Apr 2011 02:31:15 GMT, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> declaimed the following in
gmane.comp.python.general:

> 
> I thought that was delegation. As in, when one object delegates some or 
> all of it's behaviour to another object:
>

	"Delegation" sounds more like functional behavior whereas
"composition" is more the static organization -- that is, which objects
are components of the overall object.

	Just a gut feeling...

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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