Using a Method in a Derived Python Class Only if it is Defined, with a Fallback Otherwise

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What will you learn?

In this tutorial, you will learn how to conditionally use a method in a subclass of a Python class based on whether it is defined or not. This technique allows for dynamic method invocation with fallback options when necessary.

Introduction to the Problem and Solution

When working with inheritance in Python, custom implementations in subclasses often override methods from parent classes. However, situations may arise where a method is not implemented in the subclass, requiring a fallback mechanism.

To address this issue, we can leverage Python’s dynamic nature and introspection capabilities to check if a method is defined in the subclass before invoking it. If the method exists, we can call it; otherwise, we can gracefully handle the absence by using an alternative implementation.

Code

class MyBaseClass:
    def my_method(self):
        print("Base class method")

class MyDerivedClass(MyBaseClass):
    def my_method(self):
        print("Derived class method")

def conditional_method_call(obj):
    if 'my_method' in dir(obj):  # Check if 'my_method' is defined in obj
        obj.my_method()
    else:
        print("Fallback: Method not found")

# Usage example
obj1 = MyBaseClass()
obj2 = MyDerivedClass()

conditional_method_call(obj1)  # Output: Base class method
conditional_method_call(obj2)  # Output: Derived class method


# Copyright PHD

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Explanation

In the provided code: – We define two classes MyBaseClass and MyDerivedClass where MyDerivedClass inherits from MyBaseClass. – The function conditional_method_call takes an object as input and checks if the object has a specific method called my_method. – By using dir(obj), we dynamically inspect the attributes of an object at runtime. – Depending on the presence of my_method, either the subclass’s implementation is invoked or a fallback message is displayed.

    How does Python handle inheritance?

    Python supports single inheritance where each derived class inherits from only one base (parent) class. In case multiple inheritances are needed, they can be achieved using mixins or by inheriting from multiple classes.

    What does dir() function do?

    The built-in function dir() returns all attributes of an object – both properties and methods. It helps us inspect what functionalities are available for that particular object dynamically.

    Can I directly check for methods without using dir()?

    Yes, another way to check for methods would be through hasattr(object, ‘method_name’). This approach directly checks if an object has a specified attribute (method).

    Is it common practice to use conditional method calls like this?

    Conditional method calls are often used when dealing with optional behaviors that certain objects may implement. It provides flexibility while ensuring graceful handling when methods are absent.

    What happens if I try calling conditional_method_call on an object without my_method implemented?

    If the given object does not have its own implementation of my_method nor inherits it from any parent classes, “Fallback: Method not found” will be printed as per our code logic.

    Conclusion

    By incorporating dynamic attribute checking and conditional statements, this technique enables flexible handling of varying behaviors/methods among different subclasses. It promotes code robustness by offering fallback mechanisms when required, enhancing adaptability within your codebase.

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