Python: replacing a function within a class of a module
Question:
I’m trying to replace a function defined within a class in order to modify its function (as in inner workings) without changing the actual code.
I’ve never done this before, and, hence, am having some problems while replacing it.
Changing the code will have me accessing the package within my Python library which is not much of an option.
For example, if the module was called testMOD,
class testMOD(object):
def testFunc(self, variable):
var = variable
self.something = var + 12
Then I would import testMOD, define a class (mytest = testMOD()), and access the defined function within the class, testFunc, and change it to already defined function.
For instance,
from somemodule import testMOD
mytest = testMOD()
def alternativeFunc(self, variable):
var = variable
self.something = var + 1.2
# A problem here
mytest.testFunc = alternativeFunc
As you can see, if I just manually overwrite(?) the function in the class with my defined function it won’t work properly.
It doesn’t give any syntax errors, however, the problem is that the replaced function thinks that the ‘self’ is another variable for the function and says that it requires another argument for the ‘variable’ variable (I guess that wasn’t a good name).
I want to make the replacing function to be exactly the same thing as the replaced function, but with additional code or some minor modifications. However, the ‘self’ is pretty much not working as it should be in a class.
Would there be a way to properly implement a defined function to replace a function of an imported class?
Answers:
Check class inheritance in Python to create your own custom class:
from somemodule import TestMOD
class YourCustomClass(TestMOD):
# change the function
def test_func(self, variable):
#
#
your_class = YourCustomClass()
your_class.test_func(x)
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
I suggest 4 solutions, from the worst to the best (IMHO), but of course it also depends on your specific constraints:
-
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
-
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
-
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
-
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
This is kind of a hack, but you can use the lambda function:
mytest.testFunc = lambda *args, **kwargs: alternativeFunc(mytest, *args, **kwargs)
Since the original question asked for a way to call the function in the parent class and then do something extra also, I thought I’d point out that simply replacing the function can be problematic; if the parent class is modified in any way (the module it is part of is updated), then you may have to modify the code accordingly. Also, they may not want to recreate the original function just to add a bit to the end.
I definitely agree that creating a class inherited from testMod is the best option, I would just suggest calling the function from testMod and then modifying the result.
class testMOD(object):
def testFunc(self, variable):
var = variable
return var + 12
class testMODNew(testMOD):
def testFunc(self, variable):
return testMOD.testFunc(self,variable) - 10.8
mytest4 = testMODNew()
print('Alternative4:', mytest4.testFunc(10)) # Alternative4: 11.2
One could make other changes, for example if you wanted objects with that class to keep track of how many times the method was called:
class testMODNew(testMOD):
__testFuncCount__ = 0
def testFunc(self, variable):
self.__testFuncCount__ += 1
return testMOD.testFunc(self,variable)
def getTestFuncCount(self):
return self.__testFuncCount__
mytest5 = testMODNew()
print('Original:',mytest5.testFunc(10)) #Original: 10
print('Original:',mytest5.testFunc(10)) #Original: 10
print('testFunc was called', mytest5.getTestFuncCount(), 'times.')
#testFunc was called 2 times
I’m trying to replace a function defined within a class in order to modify its function (as in inner workings) without changing the actual code.
I’ve never done this before, and, hence, am having some problems while replacing it.
Changing the code will have me accessing the package within my Python library which is not much of an option.
For example, if the module was called testMOD,
class testMOD(object):
def testFunc(self, variable):
var = variable
self.something = var + 12
Then I would import testMOD, define a class (mytest = testMOD()), and access the defined function within the class, testFunc, and change it to already defined function.
For instance,
from somemodule import testMOD
mytest = testMOD()
def alternativeFunc(self, variable):
var = variable
self.something = var + 1.2
# A problem here
mytest.testFunc = alternativeFunc
As you can see, if I just manually overwrite(?) the function in the class with my defined function it won’t work properly.
It doesn’t give any syntax errors, however, the problem is that the replaced function thinks that the ‘self’ is another variable for the function and says that it requires another argument for the ‘variable’ variable (I guess that wasn’t a good name).
I want to make the replacing function to be exactly the same thing as the replaced function, but with additional code or some minor modifications. However, the ‘self’ is pretty much not working as it should be in a class.
Would there be a way to properly implement a defined function to replace a function of an imported class?
Check class inheritance in Python to create your own custom class:
from somemodule import TestMOD
class YourCustomClass(TestMOD):
# change the function
def test_func(self, variable):
#
#
your_class = YourCustomClass()
your_class.test_func(x)
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
I suggest 4 solutions, from the worst to the best (IMHO), but of course it also depends on your specific constraints:
-
Replace the instance method (1): I use the fact that functions are descriptors in Python, so that I can use the
__get__
method onAlternativeFunc
to get it as a method of the instancemytest
and overwrite thetestFunc
method of the instancemytest
(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
-
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
-
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
-
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
This is kind of a hack, but you can use the lambda function:
mytest.testFunc = lambda *args, **kwargs: alternativeFunc(mytest, *args, **kwargs)
Since the original question asked for a way to call the function in the parent class and then do something extra also, I thought I’d point out that simply replacing the function can be problematic; if the parent class is modified in any way (the module it is part of is updated), then you may have to modify the code accordingly. Also, they may not want to recreate the original function just to add a bit to the end.
I definitely agree that creating a class inherited from testMod is the best option, I would just suggest calling the function from testMod and then modifying the result.
class testMOD(object):
def testFunc(self, variable):
var = variable
return var + 12
class testMODNew(testMOD):
def testFunc(self, variable):
return testMOD.testFunc(self,variable) - 10.8
mytest4 = testMODNew()
print('Alternative4:', mytest4.testFunc(10)) # Alternative4: 11.2
One could make other changes, for example if you wanted objects with that class to keep track of how many times the method was called:
class testMODNew(testMOD):
__testFuncCount__ = 0
def testFunc(self, variable):
self.__testFuncCount__ += 1
return testMOD.testFunc(self,variable)
def getTestFuncCount(self):
return self.__testFuncCount__
mytest5 = testMODNew()
print('Original:',mytest5.testFunc(10)) #Original: 10
print('Original:',mytest5.testFunc(10)) #Original: 10
print('testFunc was called', mytest5.getTestFuncCount(), 'times.')
#testFunc was called 2 times