[Tutor] a class that may not be instantiated

[Oscar Benjamin]

On 24 November 2015 at 15:36, Albert-Jan Roskam <sjeik_appie at hotmail.com> wrote:
> Hi,
>
> I have two classes with a number of methods that are the same, so I want to define a super class that holds these methods. But the super class (below called _Generic) should not be instantiated, because it serves no purpose other than the DRY principle. I raise a NotImplementedError in case if somebody dares to instantiate _Generic. Below, only one common method is defined in_Generic, namely __repr__ (the other ones are __enter__ and __exit__, maybe more, if you must know). At first I thought I'd need the abc module for this, but this seems to do the trick. I do not want to enforce concrete implementations of abstract methods, which is the goal of abc. Is this the way to do this, or this this quirky code?
>
> import inspect
>
> class _Generic(object):
>
>     def __init__(self, *args, **kwargs):
>         raise NotImplementedError
>
>     def __repr__(self):
>         fmt = []
>         for arg in inspect.getargspec(self.__init__).args[1:]:
>             value = getattr(self, arg)
>             sr = "%r" if isinstance(value, basestring) else "%s"
>             fmt.append(("%s=" + sr) % (arg, value))
>         return self.__class__.__name__ + "(" + ", ".join(fmt) + ")"

I just wouldn't bother with this (personally).

Firstly why are you worried that someone will try to instantiate
_Generic? I don't imagine that I would if I was trying to use your
library since it has an underscore prefix and presumably your
user-facing docs don't mention it. You could make that more explicit
by putting the word Abstract into the class name or something.

Really though that class is just a function:

     def my_repr_function(self):
         fmt = []
         for arg in inspect.getargspec(self.__init__).args[1:]:
             value = getattr(self, arg)
             sr = "%r" if isinstance(value, basestring) else "%s"
             fmt.append(("%s=" + sr) % (arg, value))
         return self.__class__.__name__ + "(" + ", ".join(fmt) + ")"

Then you can do:


 class Concrete(Generic):
    ...
    __repr__ = my_repr_function

I realise that in your actual case there are more methods but then
this can just be a mixin class. Then you can call it something with
Mixin in the name and again people will know not to use it (except in
inheritance).

--
Oscar