Understanding super(type, obj): Fixing 'obj must be an instance or subtype of type' Error in Python

In this guide, we will learn how to fix the TypeError: obj must be an instance or subtype of type error, which can occur when using the super() function in Python. We will also provide a step-by-step solution and an FAQ section to address common questions.

Table of Contents

• What is the super() function?
• Understanding the error: 'obj must be an instance or subtype of type'
• Step-by-step solution
• Step 1: Identify the cause
• Step 2: Fix the error
• FAQ

What is the super() function? {#what-is-the-super-function}

In Python, the super() function is used to call a method from the parent class. This allows us to avoid using the parent class's name explicitly, making the code more maintainable. It is commonly used within the __init__() method of a class to ensure proper initialization of the parent class.

Read more about the super() function in Python

Understanding the error: 'obj must be an instance or subtype of type' {#understanding-the-error-obj-must-be-an-instance-or-subtype-of-type}

The TypeError: obj must be an instance or subtype of type error occurs when the super() function is called with arguments that do not meet its requirements. The super() function can be called with two arguments, type and obj. However, the obj argument must be an instance or a subtype of the specified type.

Read more about the TypeError in Python

Step-by-step solution {#step-by-step-solution}

Let's dive into a step-by-step solution to fix this error.

Step 1: Identify the cause {#step-1-identify-the-cause}

Consider the following code snippet:

class Parent:
    def __init__(self, x):
        self.x = x

class Child(Parent):
    def __init__(self, x, y):
        super(Parent, self).__init__(x)
        self.y = y

c = Child(1, 2)

This code will raise the TypeError: obj must be an instance or subtype of type error. The cause of the error is the incorrect usage of the super() function in the Child class. The super() function is called with the Parent class as the first argument and self as the second argument.

Step 2: Fix the error {#step-2-fix-the-error}

To fix the error, we need to call the super() function without any arguments. This will automatically pass the appropriate arguments and ensure proper initialization of the parent class.

Here's the corrected code:

class Parent:
    def __init__(self, x):
        self.x = x

class Child(Parent):
    def __init__(self, x, y):
        super().__init__(x)
        self.y = y

c = Child(1, 2)

Now the code will run without any errors.

FAQ {#faq}

1. Can I use the super() function in a single inheritance scenario? {#faq-1}

Yes, the super() function can be used in single inheritance scenarios as well. It is especially useful when the parent class's name is changed, as it avoids the need for updating the child class.

2. Is it necessary to use the super() function when inheriting from multiple classes? {#faq-2}

It is not strictly necessary, but using the super() function is recommended when inheriting from multiple classes because it ensures proper initialization of all parent classes and follows the method resolution order (MRO).

Read more about multiple inheritance and MRO in Python

3. Can I use the super() function in Python 2? {#faq-3}

Yes, but the syntax is different. In Python 2, you need to call the super() function with the current class and self as its arguments, like this:

class Child(Parent):
    def __init__(self, x, y):
        super(Child, self).__init__(x)
        self.y = y

4. Can I use the super() function with class methods? {#faq-4}

Yes, the super() function can also be used with class methods. Just make sure to pass the class and the instance as arguments, like this:

class Parent:
    @classmethod
    def some_method(cls):
        pass

class Child(Parent):
    @classmethod
    def some_method(cls):
        super(Child, cls).some_method()

5. Can the super() function be used outside of class methods? {#faq-5}

No, the super() function is meant to be used only within class methods. Using it outside of class methods will likely result in errors.