Dynamically bind method to class instance in python

Question

Let\'s say that I have a class defined in moduleA.py which I want to add a method to, using some sort of loader method that takes a the name of a second module

Answer 1

Since meth2() is a function, it is a descriptor and you can bind it by calling the __get__() method.

def meth2(self):
    return self.a + self.b

class ClassA(object):
    def __init__(self,config):
        super(ClassA, self).__init__()
        self.a = 1
        self.b = 2
        self.meth1 = config.__get__(self, ClassA)

c = ClassA(meth2)
print c.meth1() #correctly prints 3 

Answer 2

There's actually a much simpler way to do this:

class ClassA(object):
    def __init__(self,config):
        super(ClassA, self).__init__()

        self.a = 1
        self.b = 2

    from moduleB import meth2 as meth1

    def calling_method():
        return self.meth1()

Answer 3

import sys
import types

def getobj(astr):
    """
    getobj('scipy.stats.stats') returns the associated module
    getobj('scipy.stats.stats.chisquare') returns the associated function
    """
    try:
        return globals()[astr]
    except KeyError:
        try:
            return __import__(astr, fromlist=[''])
        except ImportError:
            modname, _, basename = astr.rpartition('.')
            if modname:
                mod = getobj(modname)
                return getattr(mod, basename)
            else:
                raise

class ClassA(object):
    def __init__(self, methpath):
        super(ClassA, self).__init__()
        self.a = 1
        self.b = 2
        self.meth1 = types.MethodType(getobj(methpath), self)

a = ClassA('moduleB.meth2')
print(a.meth1())
# 3

Answer 4

Skipping the config stuff which wasn't clear to me, the binding itself would look like this:

from moduleB import meth2
ClassA.meth1 = meth2

The important part is that you're binding to the class, not to an instance. This way if you call meth1 on an instance, it will automatically receive the instance as the first argument.