Forward declaration of classes?

Write a decorator that adds it to the registry in Base.

class Base(object):
  subs = []

  @classmethod
  def addsub(cls, scls):
    cls.subs.append(scls)

 ...

@Base.addsub
class Sub1(Base):
  pass

class Sub2(Base):
  pass

@Base.addsub
class Sub3(Base):
  pass

Here's essentially a hybrid version of @Ignacio Vazquez-Abrams' and @aaronasterling's answers which preserves the order of the subclasses in the list. Initially the desired subclass names (i.e. strings) are manually placed in the subs list in the desired order, then as each subclass is defined, a class decorator causes the corresponding string to be replaced with the actual subclass:

class Base(object):  # New-style class (i.e. explicitly derived from object).

    @classmethod
    def register_subclass(cls, subclass):
        """ Class decorator for registering subclasses. """

        # Replace any occurrences of the class name in the class' subs list.
        # with the class itself.
        # Assumes the classes in the list are all subclasses of this one.
        # Works because class decorators are called *after* the decorated class'
        # initial creation.
        while subclass.__name__ in cls.subs:
            cls.subs[cls.subs.index(subclass.__name__)] = subclass

        return cls  # Return modified class.

    subs = ['Sub3', 'Sub1']  # Ordered list of subclass names.


@Base.register_subclass
class Sub1(Base): pass

@Base.register_subclass
class Sub2(Base): pass

@Base.register_subclass
class Sub3(Base): pass

print('Base.subs: {}'.format(Base.subs))
# Base.subs: [<class '__main__.Sub3'>, <class '__main__.Sub1'>]

Update: Metaclasses

Exactly the same thing can also be done using a metaclass—which has the advantage that it eliminates the need to explicitly decorate each subclass as shown in my original answer above (which you accepted), however it makes it all happen automagically.

Note that even though the metaclass' __init__() is called for the creation of every subclass, it only updates the subs list if the subclass' name appears in it—so the initial Base class' definition of the contents of the subs list still controls what gets replaced in it (maintaining its order).

class BaseMeta(type):

    def __init__(cls, name, bases, classdict):
        if classdict.get('__metaclass__') is not BaseMeta:  # Metaclass instance?
            # Replace any occurrences of a subclass' name in the class being
            # created the class' sub list with the subclass itself.
            # Names of classes which aren't direct subclasses will be ignored.
            while name in cls.subs:
                cls.subs[cls.subs.index(name)] = cls

        # Chain to __init__() of the class instance being created after changes.
        # Note class instance being defined must be new-style class.
        super(BaseMeta, cls).__init__(name, bases, classdict)


# Python 2 metaclass syntax.
class Base(object):  # New-style class (derived from built-in object class).
    __metaclass__ = BaseMeta
    subs = ['Sub3', 'Sub1']  # Ordered list of subclass names.

# Python 3 metaclass syntax.
#class Base(metaclass=BaseMeta):
#    subs = ['Sub3', 'Sub1']  # Ordered list of subclass names.


# Note: No need to manually register the (direct) subclasses.
class Sub1(Base): pass
class Sub2(Base): pass
class Sub3(Base): pass

print('Base.subs: {}'.format(Base.subs))

Output:

Base.subs: [<class '__main__.Sub3'>, <class '__main__.Sub1'>]

It important to note that there's at least one subtle difference between these two answers—namely that the first will work with any class name that is registered via @Base.register_subclass(), whether or not its actually a subclass of Base (although that might be possible to change/fix.)

I'm pointing this out for a couple of reasons: First because in your comments you said that subs was a "bunch of classes in a list, some of which might be its subclasses", and more importantly, because that's not the case with the code in my update, which only works for Base subclasses since they effectively get "registered" automatically via the metaclass—but will leave anything else in the list alone. This could be considered a bug or a feature. ;¬)

Update: Python 3.6+

In Python 3.6 a new object method was added named __init_subclass__() which provides an even simpler way implement things that also doesn't require decorating all the subclasses or defining a metaclass:

#!/usr/bin/env python3.6

class Base:
    subs = ['Sub3', 'Sub1']  # Ordered list of subclass names.

    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        while cls.__name__ in cls.subs:
            cls.subs[cls.subs.index(cls.__name__)] = cls


# Note: No need to manually register the subclasses.
class Sub1(Base): pass
class Sub2(Base): pass
class Sub3(Base): pass

print('Base.subs: {}'.format(Base.subs))