ct at gocept
Jul 22, 2008, 12:37 AM
Post #2 of 3
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Re: Ordering of fields with schema interface subclassing
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On Sun, 2008-07-20 at 18:22 +0100, Martin Aspeli wrote:
> Hi,
>
> Suppose I have an interface IDerived that's derived from an interface
> IBase, overriding a field from it.
>
> >>> from zope.interface import Interface
> >>> from zope.schema import TextLine
>
> >>> class IBase(Interface):
> ... a = TextLine(title=u"a")
> ... b = TextLine(title=u"b")
> ... c = TextLine(title=u"c")
>
> >>> class IDerived(IBase):
> ... b = TextLine(title=u"b", default=u"B")
>
> In this case, the field ordering of zope.schema means that I'll get
> something like this:
>
> >>> from zope.schema import getFieldNamesInOrder
> >>> getFieldNamesInOrder(IDerived)
> ['a', 'c', 'b']
>
> I had rather expected the order to be ['a', 'b', 'c']. That is, I would
> expect a field that overrides a field from a base interface, to retain
> the base interface's order.
>
> One way to fix this is to do:
>
> >>> IDerived['b'].order = IBase['b'].order
>
> Now both will have the same order, of course.
>
> So, I'm wondering:
>
> - Is it harmful to have two fields with the same order like this when
> they share a name?
"Sharing" a name sounds weird. The attribute get's overriden and the
field from the base class isn't considered anymore.
> - Should this be the default behavior when deriving interfaces from
> one another like this?
I'm not sure about that. IIRC the general issue here is that there's a
*global* counter involved when determining the order of fields.
> If the latter is desirable, I'd be willing to help find a way to
> implement it. The field order is just an ever-increasing int that's set
> in Field.__init__(), so we'd probably need some after-the-fact fixing up
> of fields.
Ah. Sounds like my memories are right.
> One way to do that would be to fire an event at the end of
> InterfaceClass.__init__() and have an event handler to fix the order,
> though that'd make zope.interface dependent on zope.event which is
> probably not desirable.
>
> We could simulate the event handler in another way, of course, e.g. by
> having the interface initialiser loop over its attributes and see if
> they support e.g. an IOrderAware interface and call a method on it, and
> have Field implement this.
Another way to to that would be to derive the `order` attribute of a
using the MRO (is that the mechanism relevant here?) of the class it
belongs to find inheritance ancestors and use their `order` attribute
instead. That behaviour could also easily be controlled using a flag of
the field.
Christian
--
Christian Theune · ct[at]gocept.com
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http://gocept.com · tel +49 345 1229889 7 · fax +49 345 1229889 1
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