Base class class variable values in Python

Question

If I want to make a base class that other classes will inherit from, but will never use the class variables from, what should I set the class variables to?

For example:

class Spam:
    time_to_cook = None

    def eat(self):
        ...

class SpamWithEggs(Spam):
    time_to_cook = Minutes(5)

    def eat(self):
        ...

All subclasses will have their own time_to_cook, so its value in Spam has no effect on the functioning of the program.

class Spam:
    time_to_cook = None

looks good to me, but it doesn't give any information about the type of time_to_cook. Therefore I'm leaning towards

class Spam:
    time_to_cook = Minutes(0)

but that could give the misleading impression that the value of time_to_cook is actually used.

I'm also considering missing it out entirely, and mentioning it in the docstring instead.

You could also set it to None or zero or whatever placeholder value you want, and mention in the docstring what kind of value it should be set to. — BrenBarn
– BrenBarn, Commented Jun 14, 2014 at 18:17
Note that SpamWithEggs is not a subclass of Spam (because the latter isn't listed as a base class of the former). — martineau
– martineau, Commented Jun 14, 2014 at 19:50
Can you set it to Minute(0), then put a doc string saying it is not used in the base class? — Hai Vu
– Hai Vu, Commented Jun 14, 2014 at 23:42

Patrick Collins · Accepted Answer · 2014-06-16 01:16:38Z

1

I like shavenwarthog's answer, but it is a little strange to use an exception directly as the value of an attribute. So, instead, you could pass it something that will raise an error no matter what method you call:

class Complainer:
    def __getattr__(self, name):
        raise NotImplementedError

and use it like so:

class Spam(object):
    time_to_cook = Complainer()

then, whenever you try to call any of Minute's methods, you'll get an exception. Alternatively, if you're never going to be instantiating Spam directly, you can just make it entirely abstract and not worry about it.

answered Jun 16, 2014 at 1:16

Patrick Collins

10.7k5 gold badges36 silver badges74 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

johntellsall · Accepted Answer · 2014-06-15 21:35:00Z

0

I suggest NotImplementedError. It doesn't give any hint as to what the type should be, but it's very clear that the value should not be used directly.

class Spam(object):
    time_to_cook = NotImplementedError

    def eat(self):
        ...

class SpamWithEggs(Spam):
    time_to_cook = Minutes(5)

edited Jun 15, 2014 at 21:35

answered Jun 14, 2014 at 18:35

johntellsall

15.2k4 gold badges48 silver badges40 bronze badges

2 Comments

martineau Over a year ago

Except that NotImplementedError is a type, so using it as value this way is a little weird IMHO, even though (obviously) legal.

johntellsall Over a year ago

@martineau: true. NotImplementedError() is a little more proper, as it's an instance vs a type, but in neither case does it signify the type of the variable. It's a hack :)

Collectives™ on Stack Overflow

Base class class variable values in Python

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related