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Adding method to object

Hi!

How can I add a method to an object.
This code does not work:

class Foo:
def __init__(self):
self.counter=0

f=Foo()

def incr(self):
self.counter+=1

f.incr=incr

f.incr()

===> python extend.py
Traceback (most recent call last):
File "extend.py", line 12, in ?
f.incr()
TypeError: incr() takes exactly 1 argument (0 given)

Jul 18 '05 #1
8 1372
On Wed, 03 Dec 2003 10:44:12 +0100, "Thomas Guettler"
<gu*****@thomas-guettler.de> wrote:
Hi!

How can I add a method to an object.
This code does not work:

class Foo:
def __init__(self):
self.counter=0

f=Foo()

def incr(self):
self.counter+=1

f.incr=incr

f.incr()

===> python extend.py
Traceback (most recent call last):
File "extend.py", line 12, in ?
f.incr()
TypeError: incr() takes exactly 1 argument (0 given)


You have to call it like

f.incr(f)

If you want to leave out the f as arg (call it like an instance
method) then
import new
help(new.instancemethod)

Help on class instancemethod in module __builtin__:

class instancemethod(object)
| instancemethod(function, instance, class)
|
| Create an instance method object.
|
....

And

new.instancemethod(incr, f, f.__class__)

Should do the trick.

With my best regards,
G. Rodrigues
Jul 18 '05 #2
Thomas Guettler wrote:
How can I add a method to an object.


self is not passed automatically, if you call an instance attribute.
You can provide a default value instead:
class Foo: .... def __init__(self):
.... self.counter = 0
.... f = Foo()
def incr(self=f): .... self.counter += 1
.... f.incr = incr
f.incr()
f.counter 1
If you have more than one instance with the same incr() method, you can wrap
it into a lambda:
g = Foo()
g.incr = lambda self=g: incr(self)
g.incr()
g.incr()
g.counter

2

Peter

Jul 18 '05 #3
Is it possible to do the same thing for an attribut, instead of a method ?

i'd like to wrap an newAttribute to an oldAttribute one :
example:
i've got an instance "n" of an "xmlNode" class
i'd like to use "n.parentNode" instead of "n.parent" ...
"Gonçalo Rodrigues" <op*****@mail.telepac.pt> a écrit dans le message de
news: 72********************************@4ax.com...
On Wed, 03 Dec 2003 10:44:12 +0100, "Thomas Guettler"
<gu*****@thomas-guettler.de> wrote:
Hi!

How can I add a method to an object.
This code does not work:

class Foo:
def __init__(self):
self.counter=0

f=Foo()

def incr(self):
self.counter+=1

f.incr=incr

f.incr()

===> python extend.py
Traceback (most recent call last):
File "extend.py", line 12, in ?
f.incr()
TypeError: incr() takes exactly 1 argument (0 given)


You have to call it like

f.incr(f)

If you want to leave out the f as arg (call it like an instance
method) then
import new
help(new.instancemethod)

Help on class instancemethod in module __builtin__:

class instancemethod(object)
| instancemethod(function, instance, class)
|
| Create an instance method object.
|
...

And

new.instancemethod(incr, f, f.__class__)

Should do the trick.

With my best regards,
G. Rodrigues

Jul 18 '05 #4
"marco" <ma********@ctrceal.caisse-epargne.fr> wrote in message
news:bq**********@s1.read.news.oleane.net...
Is it possible to do the same thing for an attribut, instead of a method ?

i'd like to wrap an newAttribute to an oldAttribute one :
example:
i've got an instance "n" of an "xmlNode" class
i'd like to use "n.parentNode" instead of "n.parent" ...

[snip]

# try this ...
n.parentNode = n.parent

HTH
Sean
Jul 18 '05 #5
> > Is it possible to do the same thing for an attribut, instead of a method
?

i'd like to wrap an newAttribute to an oldAttribute one :
example:
i've got an instance "n" of an "xmlNode" class
i'd like to use "n.parentNode" instead of "n.parent" ...

[snip]

# try this ...
n.parentNode = n.parent


i'll try,
but ... it s not dynamic ....

if n.parent change .... n.parentNode will not change ... it must be
re-affected ... not ?

Jul 18 '05 #6

"marco" <ma********@ctrceal.caisse-epargne.fr> wrote in message
news:bq**********@s1.read.news.oleane.net...
Is it possible to do the same thing for an attribut, instead of a
method
?
i'd like to wrap an newAttribute to an oldAttribute one :
example:
i've got an instance "n" of an "xmlNode" class
i'd like to use "n.parentNode" instead of "n.parent" ...

[snip]

# try this ...
n.parentNode = n.parent


i'll try,
but ... it s not dynamic ....

if n.parent change .... n.parentNode will not change ... it must be
re-affected ... not ?


You're right. Sorry about that. Maybe you could use a property?

n.__class__.parentNode = property(lambda self: self.parent, ... etc ... )

This way you can get/set/del n.parent using n.parentNode, and changes
to n.parent will be reflect in n.parentNode.

There's probably another way, but I can't think of it at the moment...

Hope that's a little more helpful than the last suggestion,
Sean

Jul 18 '05 #7
Sean Ross wrote:
"marco" <ma********@ctrceal.caisse-epargne.fr> wrote in message
news:bq**********@s1.read.news.oleane.net...
Is it possible to do the same thing for an attribut, instead of a


method
?
i'd like to wrap an newAttribute to an oldAttribute one :
example:
i've got an instance "n" of an "xmlNode" class
i'd like to use "n.parentNode" instead of "n.parent" ...

[snip]

# try this ...
n.parentNode = n.parent


i'll try,
but ... it s not dynamic ....

if n.parent change .... n.parentNode will not change ... it must be
re-affected ... not ?

You're right. Sorry about that. Maybe you could use a property?

n.__class__.parentNode = property(lambda self: self.parent, ... etc ... )

This way you can get/set/del n.parent using n.parentNode, and changes
to n.parent will be reflect in n.parentNode.

There's probably another way, but I can't think of it at the moment...

Hope that's a little more helpful than the last suggestion,
Sean


perfect ! it works like a charm
thanx a lot !
Jul 18 '05 #8
In article <72********************************@4ax.com>, Gonçalo Rodrigues wrote:
If you want to leave out the f as arg (call it like an instance
method) then
import new
help(new.instancemethod)

Help on class instancemethod in module __builtin__:

class instancemethod(object)
| instancemethod(function, instance, class)
|
| Create an instance method object.
|
...

And

new.instancemethod(incr, f, f.__class__)

Should do the trick.


I tried to bring this up several weeks ago but nobody replied, so I'm
bringing it up again. I still see people recommending "new.instancemethod",
yet "help(new)" says that the "new" module is deprecated. The seemingly
identical "types.MethodType" ought to be its replacement, even though I
think "new.instancemethod" is more clear. If you stringify types.MethodType,
it says "<type 'instancemethod'>". The help for instancemethod, above, says
that instancemethod is in the __builtin__ module, but it is neither a
builtin nor available in the __builtin__ module. This is confusing. Can we
decide on a community standard for the appropriate way to create new
instance methods, and resolve the documentation discrepancies?

Thankya kindly,
Dave

--
..:[ dave benjamin (ramenboy) -:- www.ramenfest.com -:- www.3dex.com ]:.
: d r i n k i n g l i f e o u t o f t h e c o n t a i n e r :
Jul 18 '05 #9

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