Method assignment and objects

Question

i've got a problem with python: I want to assign a method to an object form another class, but in this method use its own attributes. Since i have many container with different use methods in my project (not in that example) i dont want to use inheritance, thad would force me to create a custom class for each instance.

class container():
    def __init__(self):
        self.info = "undefiend info attribute"

    def use(self):
        print self.info


class tree():
    def __init__(self):

        # create container instance
        b = container()

        # change b's info attribute
        b.info = "b's info attribute"

        # bound method test is set as use of b and in this case unbound, i think
        b.use = self.test

        # should read b's info attribute and print it
        # should output: test: b's info attribute but test is bound in some way to the tree object
        print b.use()

    # bound method test
    def test(self):
        return "test: "+self.info


if __name__ == "__main__":
    b = tree()

Thank you very much for reading this, and perhaps helping me! :)

Unknown · Accepted Answer · 2009-09-15 06:35:42Z

2

Here you go. You should know that self.test is already bound since by the time you are in __init__ the instance has already been created and its methods are bound. Therefore you must access the unbound member by using the im_func member, and binding it with MethodType.

import types

class container():
    def __init__(self):
        self.info = "undefiend info attribute"

    def use(self):
        print self.info


class tree():
    def __init__(self):

        # create container instance
        b = container()

        # change b's info attribute
        b.info = "b's info attribute"

        # bound method test is set as use of b and in this case unbound, i think
        b.use = types.MethodType(self.test.im_func, b, b.__class__)

        # should read b's info attribute and print it
        # should output: test: b's info attribute but test is bound in some way to the tree object
        print b.use()

    # bound method test
    def test(self):
        return "test: "+self.info


if __name__ == "__main__":
    b = tree()

answered Sep 15, 2009 at 6:35

Unknown

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3 Comments

blackforestcowboy Over a year ago

Thank you, i've to learn some more about types and MethodType. But your solution works very well!

Unknown Over a year ago

@blackforestcowboy: This knowledge is black magic so I wouldn't expect you to know about it. It is not documented anywhere last time I checked. I discovered it myself when I was trying to make a plugin system, and all the self-described Python experts told me it couldn't be done and kept questioning me instead of giving answers.

blackforestcowboy Over a year ago

@Unknown: Yea black magic I like! ;) I just googled around for the types.MethodType packacke and could bearly find information about it. I'm also trying to write some kind of PluginSystem, so inheritence would be much more code to write. Thank you very much again.

DevDevDev · Accepted Answer · 2009-09-15 06:34:44Z

1

Looks like you are trying to use inheritance? The tree inherits from the container?

answered Sep 15, 2009 at 6:34

DevDevDev

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Comments

Laurence Gonsalves · Accepted Answer · 2009-09-15 06:35:52Z

1

Use tree.test instead of self.test. The method attributes of an instance are bound to that instance.

answered Sep 15, 2009 at 6:35

Laurence Gonsalves

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Comments

S.Lott · Accepted Answer · 2009-09-15 10:58:52Z

1

Do not move methods around dynamically.

Just Use Delegation. Avoid Magic.

Pass the "Tree" object to the Container. It saves trying to move methods around.

class Container( object ):
    def use( self, context ):
        print context.info
        context.test()

class Tree( object ):
    def __init__( self, theContainerToUse ):
        b= theContinerToUse( self )
        print b.use()
    def test( self ):
        print "test"+self.info

answered Sep 15, 2009 at 10:58

S.Lott

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2 Comments

blackforestcowboy Over a year ago

@S.Lott Thank you for your solution, at the moment i dont get how that helps me, but i'm at work at the moment, so i sadly have to deal with java... But i sure will think about it after work! Thank you!

blackforestcowboy Over a year ago

@S.Lott No no, its a python question, not java!! this question is regarding a private project what i program in python, BUT at work, i have to work with java, there is no question about that. Sorry for the confusion! This is defenitely a python question!

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