Extending base class methods

[henrikpierrou@hotmail.com]

Hi,

I am trying to extend an overridden base class method (printer) by
printing some extra fields and then calling the base class method.
Extract from the python tutorial:

'An overriding method in a derived class may in fact want to extend
rather than simply replace the base class method of the same name.
There is a simple way to call the base class method directly: just call
"BaseClassName. methodname(self , arguments)". '


Any ideas why this does not work? I get the error "TypeError: unbound
method printer() must be called with Field_Collectio n instance as first
argument (got MSD instance instead)"):


#============== =============== =============== =============== =============== ===========
class Field_Collectio n:
fieldList = []

def add(self, name, size, compression, responseValue, value,
description):
self.fieldList. append( Field(name, size, compression,
responseValue, value, description) )

def update(self):
print "updating field"

def get(self):
print "getting field"

def printer(self):
for x in self.fieldList:
x.printer()


#============== =============== =============== =============== =============== ===========


class MSD(Field_Colle ction):
standard = ""
decField = ""

def printer(self):
print "Standard: " + self.standard
print "decField: " + self.decField
Field_Collectio n.printer(self)

#============== =============== =============== =============== =============== ===========

w2k, python 2.3

[Steve Juranich]

Re: Extending base class methods

On 19 Apr 2005 07:01:10 -0700, henrikpierrou@h otmail.com
<henrikpierrou@ hotmail.com> wrote:[color=blue]
> Any ideas why this does not work? I get the error "TypeError: unbound
> method printer() must be called with Field_Collectio n instance as first
> argument (got MSD instance instead)"):[/color]

My suggestion would be to make Field_Collectio n inheirit from
"object". Then you can make use of the "super" call inside of MSD.
It would look something like this:

class Field_Collectio n(object):
# Some stuff.

class MSD(Field_Colle ction):
# More stuff
def printer(self):
print "Standard: " + self.standard
print "decField: " + self.decField
super(MSD, self).printer()


HTH

--
Steve Juranich
Tucson, AZ
USA

[henrikpierrou@hotmail.com]

Re: Extending base class methods

Ok, i'll try that. But what about the recommendation in the tutorial,
is that not possible?

/H

[Steve Juranich]

Re: Extending base class methods

On 19 Apr 2005 08:27:28 -0700, henrikpierrou@h otmail.com
<henrikpierrou@ hotmail.com> wrote:[color=blue]
> Ok, i'll try that. But what about the recommendation in the tutorial,
> is that not possible?[/color]

In the new (2.4) version of the Tutorial, that statement has been
removed. What you're using has been called "old-style" classes for
quite a while now (since 2.0?). Any new Python code should really be
using the "new-style" classes (inherit from "object" and use things
like "super".

But FWIW, it appears to still work with Python 2.4:

#---------- <snip FILE="Foo.py"> -----------------
class Foo:
def foo(self):
print "foo"

class Bar(Foo):
def foo(self):
print "bar"
Foo.foo(self)
#----------- </snip> ------------


# Now in the Python interpreter:
[color=blue][color=green][color=darkred]
>>> from Foo import *
>>> f = Foo()
>>> f.foo()[/color][/color][/color]
foo[color=blue][color=green][color=darkred]
>>> b = Bar()
>>> b.foo()[/color][/color][/color]
bar
foo

So nothing jumps out to me that you did obviously wrong, sorry. But
in general my advice would be to switch to using "new-style" classes.

HTH
--
Steve Juranich
Tucson, AZ
USA

[Sizer]

Re: Extending base class methods

henrikpierrou@h otmail.com wrote in
news:1113919270 .802463.54310@l 41g2000cwc.goog legroups.com:[color=blue]
> Any ideas why this does not work? I get the error "TypeError: unbound
> method printer() must be called with Field_Collectio n instance as
> first argument (got MSD instance instead)"):
>
>
> #============== =============== =============== =============== ===========
> =============== class Field_Collectio n:
> fieldList = []
>
> def add(self, name, size, compression, responseValue, value,
> description):
> self.fieldList. append( Field(name, size, compression,
> responseValue, value, description) )
>
> def update(self):
> print "updating field"
>
> def get(self):
> print "getting field"
>
> def printer(self):
> for x in self.fieldList:
> x.printer()
>
>
> #============== =============== =============== =============== ===========
> ===============
>
>
> class MSD(Field_Colle ction):
> standard = ""
> decField = ""
>
> def printer(self):
> print "Standard: " + self.standard
> print "decField: " + self.decField
> Field_Collectio n.printer(self)
>
> #============== =============== =============== =============== ===========
> ===============
>
> w2k, python 2.3
>[/color]


I added these lines to your code:

foo = MSD()
foo.printer()

And it worked perfectly (python 2.3.3). You would get that error if you
accidentally did something like:

foo = MSD()
MSD.printer() # oops, should be foo.printer()