Python: replacing a function within a class of a module

I suggest 4 solutions, from the worst to the best (IMHO), but of course it also depends on your specific constraints:

1. Replace the instance method (1): I use the fact that functions are descriptors in Python, so that I can use the __get__ method on AlternativeFunc to get it as a method of the instance mytest and overwrite the testFunc method of the instance mytest (without overwriting the class method):

   class testMOD(object):
       def testFunc(self, variable):
           var = variable
           self.something = var + 12
           print('Original:', self.something)
   
   def alternativeFunc1(self, variable):
       var = variable
       self.something = var + 1.2
       print('Alternative1:', self.something)
   
   mytest1 = testMOD()
   mytest1.testFunc(10)   # Original: 22
   
   mytest1.testFunc = alternativeFunc1.__get__(mytest1, testMOD)
   mytest1.testFunc(10)   # Alternative1: 11.2
   mytestX = testMOD()
   mytestX.testFunc(10)   # Original: 22
   

2. Replace the instance method (2): This time, I use types.MethodType which is a bit more readable than the first solution:

   import types
   
   class testMOD(object):
       def testFunc(self, variable):
           var = variable
           self.something = var + 12
           print('Original:', self.something)
   
   def alternativeFunc1(self, variable):
       var = variable
       self.something = var + 1.2
       print('Alternative1:', self.something)
   
   mytest1 = testMOD()
   mytest1.testFunc(10)   # Original: 22
   
   funcType = types.MethodType
   mytest1.testFunc = funcType(alternativeFunc1, mytest1)
   mytest1.testFunc(10)   # Alternative1: 11.2
   mytestX = testMOD()
   mytestX.testFunc(10)   # Original: 22
   

3. Perform a monkey patching of the class method. Differently from the first method, it changes the behavior of any instance of the class:

   class testMOD(object):
       def testFunc(self, variable):
           var = variable
           self.something = var + 12
           print('Original:', self.something)
   
   def alternativeFunc2(self, variable):
       var = variable
       self.something = var + 1.2
       print('Alternative2:', self.something)
   
   mytest2 = testMOD()
   mytest2.testFunc(10)   # Original: 22
   
   testMOD.testFunc = alternativeFunc2
   mytest2.testFunc(10)   # Alternative2: 11.2
   mytestX = testMOD()
   mytestX.testFunc(10)   # Alternative2: 11.2
   

4. Create a class inherited from testMOD to override the method:

   class testMODNew(testMOD):
        def testFunc(self, variable):
            var = variable
            self.something = var + 1.2
            print('Alternative3:', self.something)
   
   mytest3 = testMODNew()
   mytest3.testFunc(10) # Alternative3: 11.2
   

You can monkey patch this method as follows:

class TestMOD(object):

    def testFunc(self, variable):
        var = variable
        self.something = var + 12
        print(f'original {self.something}')


def alternativeFunc(self, variable):
    var = variable
    self.something = var + 1.2
    print(f'alternative {self.something}')


if __name__ == '__main__':

    test_original = TestMOD()
    test_original.testFunc(12)

    TestMOD.testFunc = alternativeFunc

    test_alternate = TestMOD()
    test_alternate.testFunc(12)

Output:

original 24
alternative 13.2