Generic metaclass to keep track of subclasses?
Python does this automatically for new-style classes, as mentioned in this answer to the similar queston How to find all the subclasses of a class given its name? here.
Here's something I've been playing around with (that works):
def sublass_registry():
''' Create a metaclass to register subclasses '''
class SublassRegistryMeta(type):
def __init__(cls, name, bases, classdict):
if classdict.get('__metaclass__') is SublassRegistryMeta:
SublassRegistryMeta.lineage = [cls] # put root class at head of a list
else:
# sublclasses won't have __metaclass__ explicitly set to this class
# we know they're subclassees because this ctor is being called for them
SublassRegistryMeta.lineage.append(cls) # add subclass to list
type.__init__(cls, name, bases, classdict)
return SublassRegistryMeta
def subclasses(cls):
''' Return a list containing base and subclasses '''
try:
if cls.__metaclass__.lineage[0] is cls: # only valid for a root class
return cls.__metaclass__.lineage
except AttributeError:
pass
return None
class Car(object): # root class
__metaclass__ = sublass_registry()
class Audi(Car): # inherits __metaclass__
pass
class Ford(Car): # inherits __metaclass__
pass
class Audi2(Audi): # sub-subclass also inherits __metaclass__
pass
print subclasses(Car)
# [<class '__main__.Car'>, <class '__main__.Audi'>, <class '__main__.Ford'>, <class '__main__.Audi2'>]
print subclasses(Audi)
# None
I think you want something like this (untested):
class SubclassTracker(type):
def __init__(cls, name, bases, dct):
if not hasattr(cls, '_registry'):
cls._registry = []
print('registering %s' % (name,))
cls._registry.append(cls)
super(SubclassTracker, cls).__init__(name, bases, dct)
Then, for Python 2, you can invoke it like:
class Root(object):
__metaclass__ = SubclassTracker
for Python 3
class Root(object, metaclass=SubclassTracker):
Note that you don't need to stick the _registry
attribute on there because stuff like that is what metaclasses are for. Since you already happen to have one laying around... ;)
Note also that you might want to move the registration code into an else
clause so that the class doesn't register itself as a subclass.