Question

I’m trying to create a list of objects in Python. The below code should (hopefully) explain what I’m doing:

class Base(object):
    @classmethod
    def CallMe(self):
        out = []
        #another (models) object is initialised here which is iterable
        for model in models:
            #create new instance of the called class and fill class property
            newobj = self.__class__()
            newobj.model = model
            out.append(newobj)
        return out


class Derived(Base):
    pass

I’m trying to initialise the class as follows:

objs = Derived.CallMe()

I want ‘objs’ to contain a list of objects which I can iterate through.

I get the following error in the stacktrace: TypeError: type() takes 1 or 3 arguments on the line that contains newobj = self.__class__()

Is there some way to do this or am I looking at the problem the wrong way?

Answer 1

The issue is that a classmethod doesn’t receive an instance (self) as an argument. Instead, it receives a reference to the class object it was called on. Often this argument is named cls though this convention is somewhat less strong than the one for self (and you’ll occasionally see code with names, like klass).

So rather than calling self.__class__() to create your instances, just call cls(). You’ll get Derived instances if the function was called as Derived.CallMe(), or Base instances if called as Base.CallMe().

@classmethod
def CallMe(cls):     # signature changed
    out = []
    models = something()
    for model in models:
        newobj = cls()    # create instances of cls
        newobj.model = model
        out.append(newobj)
    return out