Variable inheritance

[Roman Suzi]

On Tue, 22 May 2001, Clarence Gardner wrote:

>On Tue, 22 May 2001, kosh wrote:
>>
>>Yes that is a great use of MI. Using mixins for behavior is extremely
>>common in zope. It works very nicely be able to gain ACL capability by
>>inherting from the RoleManager or gain full persistance in the ZODB by
>>inheriting from Persist. Mixins work very nicely and I have used that
>>feature fairly commonly since it allows a better abstraction of various
>>workhorse functions from the design.
>
>I may be able to clarify my particular situation by including more
>context.  Here are some comments that have been made about my use
>of multiple inheritance [though not about my question about how to
>do the inheritance in a Pythonically-variable way :)]
>
>[Roman Suzi]
>>This example [not my printer scenario -- CG] is also artificial, because
>>in reality we do not have so orthogonal properties.
>In fact, the two base classes involved in my example are *completely*
>orthogonal.
>
>[Roman Suzi]
>>Multiple inheritance says it bad design.
>>Probably, you want a third container class, which
>>will hold your Printer and OutputDevice classes
>>and provide communication between them.
>There *is* no communication between them...
>
>[Magnus Lycka]
>>The reasonable use of multiple inheritance is
>>for so-called mixin-classes, that gives some
>>particular behavior to a class.
>>Both Printer.PSPrinter and OutputDevice.ToLP and
>>are obviously real world objects.
>Unless I've got my terminology wrong, my use of this *is* a mix-in class.
>And the classes defined in module Printer are not real world objects.
>My use of the name Printer may have thrown Magnus off.
>
>Here's the deal.  I've got programs that want to produce printed output.
>Depending on the invocation they may generate PostScript or plain text
>(and now, PCL).  Also depending on the invocation, the output may in
>fact be printed on paper, may be faxed, or emailed, or just stored in
>a file.
>
>The Printer module provides classes that implement an interface for
>putting data on a virtual page; the OutputDevice module provides classes
>that implement an interface for sending pages to devices.
>
>Thus, (before the introduction of PCL), I have the following mix-in
>classes that provide various combinations of these two independent notions:
>    class TextToEmail(Printer.TextPrinter, OutputDevice.ToEmail):
>    class TextToFile(Printer.TextPrinter, OutputDevice.ToFile):
>    class TextToLP(Printer.TextPrinter, OutputDevice.ToLP):
>    class PSToEmail(Printer.PSPrinter, OutputDevice.ToEmail):
>    class PSToFile(Printer.PSPrinter, OutputDevice.ToFile):
>    class PSToLP(Printer.PSPrinter, OutputDevice.ToLP):
>    class PSToFax(Printer.PSPrinter, OutputDevice.ToFax):
>
>Except for PSToLP, which I showed in my original post, the entire
>implementation of each of these looks like this:
>    class PSToEmail(Printer.PSPrinter, OutputDevice.ToEmail):
>        def __init__(self, From, To, Relay, OptHdrs={}):
>            OutputDevice.ToEmail.__init__(self, From, To, Relay, OptHdrs)
>            Printer.PSPrinter.__init__(self)
>
>The programs that use this have a small section at the top to decide
>which mix-in to instantiate, then just get on with their printing to
>that object.  This seems to me to be a very reasonable use of mix-in
>(and hence, multiple inheritance).

Maybe multimethods could help you?
(Multimethod.py)

>-- Clarence
>
>

Sincerely yours, Roman Suzi
-- 
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