How to make a function usable by a class but not by it's subclasses in python

Question

I remember that there is some sort of syntax that will make a function of a class only callable by that class and not by any subclasses that inherit the functions of that class, but I can't recall exactly what it is and google has been of no help to me. Can anyone remind me how I would go about doing this?

That's not the done thing in python, so there's no simple way to do it. — sapi, Dec 16 '13 at 01:13
Why would you want to do this? If you have a subclass of something, it should have all of the methods of that class. — , Dec 16 '13 at 01:18

score 5 · Accepted Answer · edited May 23 '17 at 10:32

5

You can't enforce the privacy, but starting a method name with double underscores is the idiomatic Python way to indicate that the method should not be used by subclasses.

edited May 23 '17 at 10:32

Community

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answered Dec 16 '13 at 01:14

Sam Mussmann

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Thanks, that's exactly what I was thinking of. If I did ever need to strictly enforce the usage of the function to one class I would indeed just use Hyperboreus' answer, but the underscore syntax is the way I had in mind. – user3002473 Dec 16 '13 at 01:17
@user3002473 Glad to hear that, because my answer may be correct, but I wouldn't advise using it myself. – Hyperboreus Dec 16 '13 at 01:18

Hyperboreus · Answer 2 · 2013-12-16T01:22:58.940

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This works, but I think it is a bit of an anti-pattern:

class A:
    def f(self):
        if type(self) != A:
            raise NotImplementedError('Thou shalt not invoke this here method.')
        print('good')

class B(A): pass

A().f()
B().f()

But actually, please don't use it. Stick to the normal underscore and dunder notation with the implied name-wrangling. We are all adults around here. Especially as my approach can be easily broken from the outside:

type_ = type
type = lambda x: A
B().f()
type = type_

edited Dec 16 '13 at 01:22

answered Dec 16 '13 at 01:13

Hyperboreus

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Just a quibble, but shouldn't you be raising `NotImplementedError()` rather than `NotImplementedError`? – sapi Dec 16 '13 at 01:13
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@SamMussmann Well any answer to a python question involving enforced privacy, will be unidiomatic I fear. – Hyperboreus Dec 16 '13 at 01:17
@Hyperboreus True enough. – Sam Mussmann Dec 16 '13 at 01:18
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@SamMussmann And you can also work around my approach quite easily. Snug in a `type = lambda x: A` before the calls, and all enforced privacy goes down the john. – Hyperboreus Dec 16 '13 at 01:21

score 0 · Answer 3 · answered Dec 16 '13 at 15:04

0

I know, this is not an answer to your question, but you should really consider rethinking your object structure since you are violating the common principles of object oriented programming.

answered Dec 16 '13 at 15:04

OBu

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What in specific would I be violating? – user3002473 Dec 16 '13 at 22:09
Inheritance is semantically a "is a"-relationship - and not a "is a except of"-relationship – OBu Dec 17 '13 at 12:51

How to make a function usable by a class but not by it's subclasses in python

3 Answers3