[Tutor] Overriding a method in a class

[David Knupp]

I think the problem is this bit about overriding an object's internal 
method with one that is defined externally. My gut feeilng is that you'd 
now have to explicitly pass the object (i.e., self) as well as the string, 
i.e.:

B.addstuff(B, "WXYZ")

...which seems clunky.

I'm not familiar with this particular technique. Is it common to do 
something like this?


On Fri, 13 May 2011, Terry Carroll wrote:

> I have a pretty basic point of confusion that I'm hoping I can have explained 
> to me.  I have a class in which I want to override a method, and have my 
> method defined externally to the class definition invoked instead. But when I 
> do so, my external method is invoked with a different argument signature than 
> the method it overrides.
>
> (I'll illustrate with a toy example named toy.py that maintains a list of 
> strings; the actual use case is a wxPython drag-and-drop shell that I find I 
> keep re-using over and over, so I decided to try to turn it into a 
> general-purpose module for my own use.)
>
> ### example 1 begin
>
> class Thing(object):
>    def __init__(self):
>        self.stuff = []
>    def addstuff(self, text):
>        self.add_the_stuff(text)
>    def add_the_stuff(self, s1):
>        self.stuff.append(s1)
>
> A = Thing()
> A.addstuff("ABCDEFG")
> print A.stuff
>
> ### example 1 end
>
> So far, this works as expected.  addstuff invokes add_the_stuff; and the line 
> "print A.stuff" prints out as ['ABCDEFG'], as expected.
>
> Now, here's where I am getting befuddled, with the following additional 
> lines:
>
> ### example, continued
> def addlower(self, s2):
>    self.stuff.append(s2.lower()) # add it as lower case
>
> B = Thing()
> B.add_the_stuff=addlower
> B.addstuff("WXYZ")
> print B.stuff
> ### end
>
> My *intent* here is to patch the Thing object named B so that the B's 
> add_the_stuff method is replaced with this additional addlower method that I 
> define external to the object.  My expectation would be that, just as 
> add_the_stuff method was called with two arguments (self and the string), the 
> patched-in addlower would also get called the same way.
>
> What I *expect* is to see ['abcdefg'] printed.  What I get is:
>
> Traceback (most recent call last):
>  File "E:\Personal\py\DragDrop\toy.py", line 22, in <module>
>    B.addstuff("WXYZ")
>  File "E:\Personal\py\DragDrop\toy.py", line 7, in addstuff
>    self.add_the_stuff(text)
> TypeError: addlower() takes exactly 2 arguments (1 given)
>
> I'm assuming I'm missing some fundamental concept.  What is it?
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