> On Feb 23, 2017, at 4:40 PM, MRAB <pyt...@mrabarnett.plus.com> wrote: > > On 2017-02-24 00:19, 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.) >> > Apart from renaming Base to _Base as a hint, you could put Base's > initialisation code in, say, '_init' and have Base's __init__ just raise an > exception. > > ClassA and ClassB would then call Base's _init instead of its __init__. > > -- > https://mail.python.org/mailman/listinfo/python-list >
MRAB and Peter: Thank you very much for your solutions. I had considered both of these, but was wondering if there was another way. Peter's solution does exactly what I want, but I'm not ready to get into metaclasses. So I decided to go with MRAB's approach. I've modified my code and it seems to work great. Thank you both. Irv -- https://mail.python.org/mailman/listinfo/python-list
