__init_subclass__ in Python

Python, a famous and flexible programming language, boasts an array of effective functions that cater to developers' varying wishes. One such function, the lesser-recognized yet extraordinarily beneficial init_subclass, enables builders to exercise greater manipulation over the subclass advent procedure. This article delves deep into the complexity of the init_subclass technique, discussing its reason, use of instances, and high-quality practices.

Section 1: The Fundamentals of Init_subclass

1.1: Defining Init_subclass

Introduced in Python 3.6, init_subclass is a category method that permits customization of subclass advent. It serves as a hook that triggers at some stage in the introduction of a new subclass, making it possible for developers to adjust or amplify the subclass's behavior without converting the base class.

1.2: The Purpose Behind Init_subclass

The main objective of init_subclass is to enhance the flexibility of object-oriented programming in Python. By providing a clean and straightforward way to manage an inheritance, init_subclass simplifies the process of enforcing coding standards, modifying subclasses, and maintaining large codebases with numerous classes.

Section 2: A Practical Example of Init_subclass

To have a clear understanding of using init_subclass, let’s have a look at a simple example. Imagine a base class, Animal, with diverse subclasses, along with Cat, Dog, and Fish. We want to ensure that each time a new Animal subclass is created, a category attribute called 'species' is assigned.

  • Define the base class Animal with the __init_subclass__ method.

  • Check if the 'species' attribute exists in the subclass.

  • Raise a TypeError if the attribute is missing.

  • Define the subclasses Cat, Dog, and Fish with the required 'species' attribute.

class Animal:
   def __init_subclass__(cls, **kwargs):
      if not hasattr(cls, 'species'):
         raise TypeError(f"{cls.__name__} must have a 'species' attribute")
class Cat(Animal):
   species = 'Feline'

class Dog(Animal):
   species = 'Canine'

class Fish(Animal):  # This will raise a TypeError

In this case, we've got a base class Animal and subclasses Cat, Dog, and Fish. The __init_subclass__ method assesses if the 'species' attribute is found in each subclass. If it's no longer, a TypeError is raised.

Section 3: Advanced Use Cases

3.1: Registering Subclasses

init_subclass is especially useful when creating a registry of subclasses. This registry can resource in object serialization and deserialization, amongst different programs. Consider the following instance −

  • Define the base class Plugin with an empty dictionary named _registry.

  • Implement the __init_subclass__ method in the Plugin class.

  • Add each subclass to the _registry dictionary using the subclass name as the key and the subclass itself as the value.

  • Define the subclasses PluginA and PluginB.

  • Print the _registry to see the registered subclasses.

class Plugin:
   _registry = {}
   def __init_subclass__(cls, **kwargs):
      cls._registry[cls.__name__] = cls

class PluginA(Plugin):

class PluginB(Plugin):



{'PluginA': <class 'main.PluginA'>, 'PluginB': <class 'main.PluginB'>}

This instance shows the way to create a registry of subclasses using the __init_subclass__ method. The registry may be helpful for object serialization and deserialization.

3.2: Enforcing Class Attributes

init_subclass can also implement the presence of class attributes in subclasses, making sure that unique attributes are present whilst developing a subclass. This is beneficial for handling the nature of the code and imposing design styles. Below is an example −

  • Define the base class Vehicle with the __init_subclass__ method.

  • List the required attributes engine_type and wheels in required_attributes.

  • Iterate through the required attributes, checking for their presence in the subclass.

  • Raise an AttributeError if a required attribute is missing.

  • Define the subclasses Car, Motorcycle, and Boat with the required attributes.

class Vehicle:
   def __init_subclass__(cls, **kwargs):
      required_attributes = ['engine_type', 'wheels']

      for attribute in required_attributes:
         if not hasattr(cls, attribute):
            raise AttributeError(f"{cls.name} must have a '{attribute}' attribute")

         class Car(Vehicle):
            engine_type = 'combustion'
            wheels = 4

         class Motorcycle(Vehicle):
            engine_type = 'combustion'

# wheels attribute is missing, this will raise an AttributeError
class Boat(Vehicle):
   engine_type = 'electric'
   wheels = 0

This example shows how to enforce class attributes in subclasses using __init_subclass__.

Section 4: Best Practices for Using __init_subclass__

4.1: Always Invoke super().__init_subclass__()

When enforcing __init_subclass__, it is important to name the super().__init_subclass__(**kwargs) technique. This ensures that the base class's __init_subclass__ method is achieved, permitting the right inheritance and the ideal behavior of the method in multiple inheritance situations.

4.2: Consider Using Class Decorators

Before opting for __init_subclass__, evaluate whether a class decorator might be a more suitable solution. Class decorators offer a way to modify a class's behavior or attributes and can be more explicit and easier to comprehend than __init_subclass__. However, __init_subclass__ is more appropriate when the desired behavior must be applied to an entire class hierarchy.

4.3: Maintain Simplicity in __init_subclass__

__init_subclass__ is an incredibly powerful tool, but it can become complex and challenging to maintain if not used with care. Keep the logic in __init_subclass__ as simple as possible, and avoid overusing it. When in doubt, consider alternative solutions, such as class decorators or metaclasses, to achieve the desired outcome.


The __init_subclass__ method in Python is an underutilized yet powerful feature that allows developers to customize the creation of subclasses. By understanding its purpose and use cases, as well as following best practices, developers can harness its full potential to enhance the flexibility of their Python applications.

Updated on: 09-May-2023


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