Getting name of subclass from superclass?

Question

I have a base class bc and a number of subclasses based on bc.

class bc(Object):
  def get_subclass_name(self):
      # Retrieve the name of the most derived subclass of this instance
      pass
  def somemethod(self):
      x = self.get_subclass_name()

class sc1(bc):
    pass

class sc2(bc)
    pass

The idea is that when somemethod() is invoked on an instance of a subclass of bc, it will be able to use the name of the most derived subclass of that instance without needing to know in advance what potential subclasses may exist.

I have put together a test case for this:

class base_class(object):
    @classmethod
    def get_subclass_name(cls):
        return cls.__name__

    def somemethod(self):
        print(base_class.get_subclass_name())

class sub_class(base_class):
    pass

sub_class().somemethod()

When this code is run it produces base_class rather than sub_class.

Answer 1

You need a class method.

class bc(Object):
    @classmethod
    def get_subclass_name(cls):
        return cls.__name__

    def somemethod(self):
        x = self.get_subclass_name()

Normal methods, when invoked, get passed the instance as the first parameter. This is why you see self everywhere in Python.

When you invoke a class method, however, the concrete class of the instance is passed to the function. We usually use the name cls, as I have done here.

The classic use case for class methods is alternative constructors. For example, the standard library's dict class provides a fromkeys class method, which constructs a new dict from the supplied keys and a single value. You could construct such a dict manually, but the Python developers made life easy for you!