How should we interpret this function signature?

It means this method accepts only keyword arguments (because ), specifically one keyword argument named object_list of unknown type with a default value of ..., which could mean anything, plus any number of additional arbitrary keyword arguments (*kwargs).

What the author wants ... to mean is anyone's guess.

The way you have defined method getcontextdata, the only positional argument you have is self; you cannot have additional unnamed positional arguments. For example, you cannot make the call self.getcontextdata(1, objectlist=mylist) because you only have one positional argument (self), but two are being provided.

If you want to have unnamed, positional arguments, then:

class MyObject:
    def getcontextdata(self, args, object_list = ..., *kwargs):
        ...

If you do not specify the objectlist argument on the method invocation, then it will have the default value of Ellipsis. Is this of any value to you given that objectlist is not being referenced in your method's body?