python - How to run code when a class is subclassed?

Is there a way to trigger code when my class is subclassed?

class SuperClass:
    def triggered_routine(subclass):
        print("was subclassed by " + subclass.__name__)

magically_register_triggered_routine()

print("foo")

class SubClass0(SuperClass):
    pass


print("bar")

class SubClass1(SuperClass):
    print("test")

Should output

foo

was subclassed by SubClass0
bar
test
was subclassed by SubClass1

Answer

Classes (by default) are instances of type.
Just as an instance of a class Foo is created by foo = Foo(...),
an instance of type (i.e. a class) is created by myclass = type(name, bases, clsdict).

If you want something special to happen at the moment of class-creation, then you have to modify the thing creating the class -- i.e. type. The way to do that is to define a subclass of type -- i.e. a metaclass.

A metaclass is to its class as a class is to its instance.

In Python2 you would define the metaclass of a class with

class SuperClass:
    __metaclass__ = Watcher

where Watcher is a subclass of type.

In Python3 the syntax has been changed to

class SuperClass(metaclass=Watcher)

Both are equivalent to

Superclass = Watcher(name, bases, clsdict)

where in this case, name equals the string 'Superclass', and bases is the tuple (object, ). The clsdict is a dictionary of the class attributes defined in the body of the class definition.

Note the similarity to myclass = type(name, bases, clsdict).

So, just as you would use a class's __init__ to control events at the moment of a instance's creation, you can control events at the moment of a class's creation with a metaclass's __init__:

---

class Watcher(type):
    def __init__(cls, name, bases, clsdict):
        if len(cls.mro()) > 2:
            print("was subclassed by " + name)
        super(Watcher, cls).__init__(name, bases, clsdict)

class SuperClass:
    __metaclass__ = Watcher



print("foo")

class SubClass0(SuperClass):
  pass

print("bar")

class SubClass1(SuperClass):
  print("test")

prints

foo
was subclassed by SubClass0
bar
test
was subclassed by SubClass1