Python callback working with functions but not methods

Question

I have a third-party Python library that allows me to register a callback function that it called later.

While the code works okay with functions, when I tried to pass a method it fails, the callback is never called.

I have no control over the third party library (source code not available).

  def old_callbackFunction(param, data):
       print data

  class MyClass():
      def callbackFunction(self, param, data):
          print data

  myObj = MyClass()
  # old_setCallback(myObj.callbackFunction, param = "x") # this would work
  setCallback(myObj.callbackFunction, param = "x") # this is never called

I don't think I could give a precise answer without more information, but have you considered turning the method call into a function by wrapping it with a lambda expression or functools.partial? — HardlyKnowEm
– HardlyKnowEm, Commented Dec 19, 2011 at 14:58
I second @mlefavor's recommendation on investigating functools.partial. — PaulMcG
– PaulMcG, Commented Dec 19, 2011 at 15:01

HardlyKnowEm · Accepted Answer · 2011-12-20 16:08:16Z

1

Sorin actually figured this out himself, with help from my comment, but he indicated that he wanted me to post the original comment as an answer. I was reluctant to post this originally because I'm unsure of the precise behavior of the setCallback and callbackFunction code; use at your own risk and modify as reason dictates.

The best way to wrap a function is to use functools.partial:

from functools import partial
setCallback(partial(myObj.callbackFunction), param="x")

You may also use a lambda (but you'll lose style points with the pythonistas):

setCallback(lambda param, data: myObj.callbackFunction(param, data), param="x")

answered Dec 20, 2011 at 16:08

HardlyKnowEm

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