__init_subclass__
- Python course
- Expert
- Lesson 3/47
- 5 min
Learn subclass hooks with __init_subclass__.
What is __init_subclass__?
__init_subclass__ lets a parent class be notified when a subclass is created. It is the lightweight alternative to writing a metaclass.
You define def __init_subclass__(cls, **kwargs) inside class Base. When class Dog(Base) is written, Python automatically calls Base.__init_subclass__ with cls=Dog. The super().__init_subclass__(**kwargs) call keeps the chain going. You can register or validate subclasses this way.
Configuration with kwargs
You can pass key-value pairs to __init_subclass__. Writing class Dog(Base, tag="pet") puts tag in the kwargs dict.
This gives you declarative configuration. The parent reads values from kwargs and can attach them as attributes on the subclass. The super().__init_subclass__(**kwargs) call forwards unhandled keys down the chain, so each class handles only the keys it cares about.
Practice Register subclasses with Base
Write a Base class with a class-level list registry = []. Define __init_subclass__(cls, **kwargs): first call super().__init_subclass__(**kwargs), then append cls.__name__ to cls.registry. Define a Dog(Base) subclass. Print Base.registry.
Starter code
# Add __init_subclass__ to Base
# record subclasses in a list
# define Dog and print the listSample solution
# Add __init_subclass__ to Base
class Base:
registry = []
def __init_subclass__(cls, **kwargs):
super().__init_subclass__(**kwargs)
cls.registry.append(cls.__name__)
class Dog(Base):
pass
print(Base.registry)Expected output
['Dog']
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match __init_subclass__ concepts
| Term | Meaning |
|---|---|
__init_subclass__ |
Subclass hook |
cls |
The new subclass |
super() |
Keeps the chain |
**kwargs |
Extra config |
2 When is __init_subclass__ called?
class Base:
def __init_subclass__(cls):
print("subclass")
class Dog(Base):
pass- When Dog() is called
- When class Dog is defined Correct answer
- When Base() is called
- When the program exits
Explanation: __init_subclass__ fires the moment class Dog is defined; it has nothing to do with creating instances.
3 Complete the __init_subclass__ signature
def __init_subclass__(cls, ____):
super().__init_subclass__(**kw)def __init_subclass__(cls, **kwargs):
super().__init_subclass__(**kw)Explanation: **kwargs captures extra key-value pairs.