Mixin class error

[Ed Leafe]

In Dabo, we create cursor classes that combine the backend-specific
dbapi cursor class with our own mixin class that adds framework-
specific behaviors. This has been working well for a couple of years
now with many different backends, but today I'm getting errors with
our Firebird class. I've checked the kinterbasdb site, and found
nothing there that was helpful. The error reads:

TypeError: Error when calling the metaclass bases
type 'kinterbasdb.Cu rsor' is not an acceptable base type

Here's some simple code that will generate the error:

import kinterbasdb
KCursor = kinterbasdb.Cur sor

class TestMixin(objec t): pass
# This next line will raise the error.
class CombinedCursor( TestMixin, KCursor): pass
myCursor = CombinedCursor( )

I'm not sure exactly what this error message means, so I don't know
how to go about fixing it. Here's my setup:

Windows 2000
Python v. 2.4.1
kinterbasdb 3.2.0a1

-- Ed Leafe
-- http://leafe.com
-- http://dabodev.com

[Kent Johnson]

Re: Mixin class error

Ed Leafe wrote:[color=blue]
> In Dabo, we create cursor classes that combine the backend-specific
> dbapi cursor class with our own mixin class that adds framework-
> specific behaviors. This has been working well for a couple of years
> now with many different backends, but today I'm getting errors with our
> Firebird class. I've checked the kinterbasdb site, and found nothing
> there that was helpful. The error reads:
>
> TypeError: Error when calling the metaclass bases
> type 'kinterbasdb.Cu rsor' is not an acceptable base type
>
> Here's some simple code that will generate the error:
>
> import kinterbasdb
> KCursor = kinterbasdb.Cur sor
>
> class TestMixin(objec t): pass
> # This next line will raise the error.
> class CombinedCursor( TestMixin, KCursor): pass
> myCursor = CombinedCursor( )
>
> I'm not sure exactly what this error message means, so I don't know
> how to go about fixing it.[/color]

I have no clue but googling 'type is not an acceptable base type' finds
this thread

http://groups.google.com/group/comp.lang.python/browse_thread/thread/628b8ad34a36db17/579f716b143f4967%23579f716b143f4967?sa=X&oi=groupsr&start=0&num=3

which points to the Py_TPFLAGS_BASE TYPE. This example

http://www.python.org/doc/2.3.5/ext/node22.html

shows that flag being set to indicate that an extension class may be
subclassed; the API docs confirm this:

http://docs.python.org/api/type-structs.html#l2h-968

So it looks like kinterbasdb.Cur sor is a C extension class that may not
be subclassed because Py_TPFLAGS_BASE TYPE is not set. Whether this is by
design or accident would be a question for the kinterbasdb developers.

One workaround might be to use delegation instead of subclassing...

Kent

[Ed Leafe]

Re: Mixin class error

On Mar 6, 2006, at 8:08 PM, Kent Johnson wrote:
[color=blue]
> I have no clue but googling 'type is not an acceptable base type'
> finds
> this thread
> http://groups.google.com/group/comp....browse_thread/
> thread/628b8ad34a36db1 7/579f716b143f496 7%23579f716b143 f4967?
> sa=X&oi=groupsr &start=0&num =3[/color]

I spent about an hour Googling and never found that thread!
[color=blue]
> So it looks like kinterbasdb.Cur sor is a C extension class that may
> not
> be subclassed because Py_TPFLAGS_BASE TYPE is not set. Whether this
> is by
> design or accident would be a question for the kinterbasdb developers.[/color]

OK, I'll ask them.
[color=blue]
> One workaround might be to use delegation instead of subclassing...[/color]

Yeah, but that would involve a lot more work at this point. The
mixin approach has been working quite well up until this problem.

-- Ed Leafe
-- http://leafe.com
-- http://dabodev.com

[Kent Johnson]

Re: Mixin class error

Ed Leafe wrote:[color=blue]
> On Mar 6, 2006, at 8:08 PM, Kent Johnson wrote:[color=green]
>> One workaround might be to use delegation instead of subclassing...[/color]
>
> Yeah, but that would involve a lot more work at this point. The
> mixin approach has been working quite well up until this problem.[/color]

Automatic delegation is pretty simple, just define __getattr__() and
__setattr__() to delegate to the wrapped class. For example

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52295

Kent