Disallowing instantiation of super class
Peter Otten
__peter__ at web.de
Thu Feb 23 19:58:45 EST 2017
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__
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