Irv Kalb wrote: > Hi, > > I have built a set of three classes: > > - A super class, let's call it: Base > > - A class that inherits from Base, let's call that: ClassA > > - Another class that inherits from Base, let's call that: ClassB > > ClassA and ClassB have some code in their __init__ methods that set some > instance variables to different values. After doing so, they call the the > __init__ method of their common super class (Base) to set some other > instance variables to some common values. This all works great. > Instances of ClassA and ClassB do just what I want them to. > > I would like to add is some "insurance" that I (or someone else who uses > my code) never instantiates my Base class, It is not intended to be > instantiated because some of the needed instance variables are only > created in the __init__ method of ClassA and ClassB. I am looking for > some way in the Base's __init__ method to determine if the method was > called directly: > > instanceOfBase = Base(... some data ...) # I want this case to > generate an error > > I tried using "isinstance(self, Base)", but it returns True when I > instantiate an object from ClassA, from ClassB, or from Base. > > If I can find a way to determine that the caller is attempting to > instantiate Base directly, I will raise an exception. > > Thanks, > > Irv > > (If it makes a difference, I am doing this currently in Python 2.7 - > please don't beat me up about that.)
>>> import abc >>> class Base: ... __metaclass__ = abc.ABCMeta ... @abc.abstractmethod ... def __init__(self): ... self.z = self.x + self.y ... >>> class A(Base): ... def __init__(self): ... self.x = 2 ... self.y = 3 ... super(A, self).__init__() ... >>> A().z 5 >>> Base() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: Can't instantiate abstract class Base with abstract methods __init__ -- https://mail.python.org/mailman/listinfo/python-list
