how to use callback functions with object/method pair

Question

I have a simple method which accepts a function to call this back later:

def SimpleFunc(parm1):
    print(parm1)

class CallMe:
    def __init__(self, func):
        self.func = func

    def Call(self, parm):
        self.func(parm)

caller = CallMe(SimpleFunc)
caller.Call("Hallo")

That works fine!

But I want to use a class method and want to call the method on a defined object as callback:

class WithClassMethod:
    def __init__( self, val ):
        self.val = val 

    def Func(self, parm):
        print( "WithClass: ", self.val, parm )


obj = WithClassMethod(1)
caller = CallMe( ??? )
caller.Call("Next")

How can I bind an object/method pair to a callable object?

Attention: The code from CallMe is not under my control. It comes from a webserver which needs a handler function.

just caller = CallMe(obj.Func) if I'm not misunderstanding things? — Dimitris Fasarakis Hilliard
– Dimitris Fasarakis Hilliard, Commented Jan 23, 2017 at 10:54
@JimFasarakis-Hilliard: Ubs. I am new to python and come from c++ so I search for more difficult solutions :-) make it an answer so I can accept. Thanks! — Klaus
– Klaus, Commented Jan 23, 2017 at 11:05
Done, I added a bit more information to it too if interested. — Dimitris Fasarakis Hilliard
– Dimitris Fasarakis Hilliard, Commented Jan 23, 2017 at 11:13

Dimitris Fasarakis Hilliard · Accepted Answer · 2017-01-23 11:13:31Z

11

You could simply pass the method object to the class:

called = CallMe(obj.Func)

To expand a bit, instance methods are really just the original class function:

>>> obj.Func.__func__
<function __main__.WithClassMethod.Func>

which, during access on an instance (obj.Func) are transformed via a descriptor (__get__) that attaches self (the instance) to them:

>>> obj.Func.__self__
<__main__.WithClassMethod at 0x7fbe740ce588>

so you can pretty much do anything you want with methods as with functions.

answered Jan 23, 2017 at 11:13

Dimitris Fasarakis Hilliard

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1 Comment

Domino Over a year ago

Ah, I see. Coming from a JavaScript background, I expected self to be bound on call, not on get. That's very interesting :)

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