Problem with method overriding from base class

bruno.desthuilliers at bruno.desthuilliers at
Mon Mar 31 22:37:33 CEST 2008

On 31 mar, 11:05, <Dominique.Holzwa... at> wrote:
> Hello everyone
> I have defined some sort of 'interface class' and a factory function that creates instance objects of specific classes, which implement that interface:
> Interface definition:
> ***************************************************************************************
> import GUI.webGUI as webGUI
> class EditInterface(webGUI.WebGUI):
>     def addEntry(self, *p):
>         raise 'EditInterface.addEntry(): Interface must not be called directly'

You want:
          raise NotImplementedError('EditInterface.addEntry():
Interface must not be called directly')

And unless you have some pretty good reason to do so (ie: template
methods in EditInterface depending on these methods), you don't even
want to bother with all this - just document which methods must be
implemented, and let Python raise an AttributeError if they are not.


> Factory:
> ***************************************************************************************
> def factory(type, *p):
>     if type == common.databaseEntryTypes[0]:
>         return module1.Class1(*p);
>     elif type == common.databaseEntryTypes[1]:
>         return module2.Class2(*p);
>     elif type == common.databaseEntryTypes[2]:
>         return module3.Class3(*p);
>     elif type == common.databaseEntryTypes[3]:
>         return module4.Class4(*p);

The whole point of polymorphic dispatch in OO is to avoid this kind of
mess. What's wrong with instanciating classes directly ? NB : in
Python, classes are objects too, so you can pass them around as
needed. Also, instanciation is done thu a call to the class object -
which makes it just the same as a function call. IOW, you just don't
need a clumsy dedicated factory function just to make sure the client
code is not too tightly coupled to the exact implementation.

> Implementing Class1:
> ***************************************************************************************
> import editInterface
> class Class1(editInterface.EditInterface):
>     def __init__(self, product, database):
>         # do something here ...
>     def showEntry(self, entry, statustext):
>         # do something here as well, return some string...
> ***************************************************************************************
> Now, when I want to create an Instance of Class1 I do:
> myClass1Instance = factory.factory(common.databaseEntryTypes[1], 'Name', databaseObj )

> Which seems to work fine according to the debugger. But when I do next:
> msg =, '')
> Then the show() method of the class 'EditInterface' is called instead of the show() method of the class 'Class1' !!

Reread your code : class Class1 have no 'show' method !-)

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