calling child class method from parent class file in python

Doing this would only make sense if A is an abstract base class, meaning that A is only meant to be used as a base for other classes, not instantiated directly. If that were the case, you would define methodB on class A, but leave it unimplemented:

class A(object):    def methodA(self):        print("in methodA")    def methodB(self):        raise NotImplementedError("Must override methodB")from parent import Aclass B(A):    def methodB(self):        print("am in methodB")

This isn't strictly necessary. If you don't declare methodB anywhere in A, and instantiate B, you'd still be able to call methodB from the body of methodA, but it's a bad practice; it's not clear where methodA is supposed to come from, or that child classes need to override it.

If you want to be more formal, you can use the Python abc module to declare A as an abstract base class.

from abc import ABCMeta, abstractmethodclass A(object): __metaclass__ = ABCMeta    def methodA(self):        print("in methodA")    @abstractmethod    def methodB(self):        raise NotImplementedError("Must override methodB")

Using this will actually prevent you from instantiating A or any class that inherits from A without overriding methodB. For example, if B looked like this:

class B(A):   pass

You'd get an error trying to instantiate it:

Traceback (most recent call last):  File "<stdin>", line 1, in <module>TypeError: Can't instantiate abstract class B with abstract methods methodB

The same would happen if you tried instantiating A.

You can do something like this:

class A():    def foo(self):        self.testb()class B(A):    def testb(self):        print('lol, it works')b = B()b.foo()

Which would return this of course:

lol, it works

Note, that in fact there is no call from parent, there is just call of function foo from instance of child class, this instance has inherited foo from parent, i.e. this is impossible:

a=A()a.foo()

will produce: AttributeError: A instance has no attribute 'testb'

because

>>> dir(A)['__doc__', '__module__', 'foo']>>> dir(B)['__doc__', '__module__', 'foo', 'testb']

What I've wanted to show that you can create instance of child class, and it will have all methods and parameters from both parent and it's own classes.

There are three approaches/ways to do this ! but I highly recommend to use the approach #3 because composition/decoupling has certain benefits in terms of design pattern. (GOF)

## approach 1 inheritance class A():    def methodA(self):        print("in methodA")    def call_mehtodB(self):        self.methodb()class B(A):    def methodb(self):        print("am in methodb")b=B()b.call_mehtodB()## approach 2 using abstract method still class highly coupledfrom abc import ABC, abstractmethodclass A(ABC):    def methodA(self):        print("in methodA")    @abstractmethod        def methodb(self):       passclass B(A):    def methodb(self):        print("am in methodb")b=B()b.methodb()#approach 3 the recommended way ! Composition class A():    def __init__(self, message):        self.message=message    def methodA(self):        print(self.message)class B():    def __init__(self,messageB, messageA):        self.message=messageB        self.a=A(messageA)    def methodb(self):        print(self.message)    def methodA(self):        print(self.a.message)b=B("am in methodb", "am in methodA")b.methodb()b.methodA()