Prevent sharing of class variables with base class in Python

Question

I have the following situation in Python 3:

class A:
    d = {}

class B(A):  pass
class C(A):  pass

I work with the classes only, no instances get created. When I access B.d I will get a shared reference to A.d. That's not what I want. I would like to have each class which inherits A have its own d which is set to a dictionary. d is an implementation detail to A. All access to d is done in the code of A. Just B's d should not be identical to A's d.

With instances, I would create that dictionary in the __init__() function. But in my case I work with the classes only.

Is there a standard way to achieve this (EDIT: without changing the implementation of the subclasses B and C)? Is there something analog to __init__() which gets called for (or in) each derived class at the time of deriving?

Well, you should be able to define d in the other classes which would overwrite the d variable from the base class for B and C. Or is that not working? — meissner_
– meissner_, Commented Jun 1, 2018 at 8:31
Looks like a design issue. Why are you not using instances of your classes, and all your problems go away? — timgeb
– timgeb, Commented Jun 1, 2018 at 8:54
@timgeb Because the "and all problems go away" is not correct. It raises other problems I did not address in my question. Using classes is the "right way" in some cases; let's just assume I have such a case. — Alfe
– Alfe, Commented Jun 1, 2018 at 9:03
@meissner_ The existence of d in A is an implementation detail of A. When deriving A I want to avoid having to mention d at all. — Alfe
– Alfe, Commented Jun 1, 2018 at 9:18

Alfe · Accepted Answer · 2018-06-01 10:47:58Z

1

I found a solution myself using a metaclass:

class M(type):
  def __init__(self, *args, **kwargs):
    super().__init__(*args, **kwargs)
    self.d = {}

class A(metaclass=M): pass

Then one can create subclasses of A which all have their own d attribute:

class B(A): pass

B.d is A.d
False

answered Jun 1, 2018 at 10:47

Alfe

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Comments

Patrick Artner · Accepted Answer · 2018-06-01 09:15:43Z

0

Simply testing @meissner_ comment: you can easily replace the d inside your subcalsses by other ones.

class BaseA():
    """Provides basic initialization for dicts used in A() and deriveds."""
    @classmethod
    def InitDefaults(cls):
        return { 1: "this", 2:"are", 3:"some", 4:"defaults"}

class A():
    d = BaseA.InitDefaults()

class B(A):
    d = BaseA.InitDefaults()

class C(A):
    d = BaseA.InitDefaults()


print(A.d)
print(B.d)
print(C.d)

print("\n")
B.d[99] = "Not a default"
C.d[42] = "Not THE answer"

print(A.d)
print(B.d)
print(C.d)

Output:

{1: 'this', 2: 'are', 3: 'some', 4: 'defaults'}
{1: 'this', 2: 'are', 3: 'some', 4: 'defaults'}
{1: 'this', 2: 'are', 3: 'some', 4: 'defaults'}


{1: 'this', 2: 'are', 3: 'some', 4: 'defaults'}
{1: 'this', 2: 'are', 3: 'some', 4: 'defaults', 99: 'Not a default'}
{1: 'this', 2: 'are', 3: 'some', 4: 'defaults', 42: 'Not THE answer'}

This is kinda ugly but would work. You have a central place for code modifying the "base-default" and A,B,C get distinct dicts - initted the same - that can develop into different directions.

Probably better would be to use Instances and init() though ...

edited Jun 1, 2018 at 9:15

answered Jun 1, 2018 at 8:34

Patrick Artner

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6 Comments

Alfe Over a year ago

Of course I want to avoid copying the creation into each derived class. That this thing d even exists is an implementation detail.

Patrick Artner Over a year ago

@Alfe If your dicts are partially the same, create a classmethod (in A) that returns a newly build dict with the defaults. Thats one is used by B and C to create it's initial dict once. Code creation placed once, seperate dicts afterwards. If you want diffenrent dicts you need to create them trice for three classes - or copy or w/e. Or use instances.

Alfe Over a year ago

Sounds promising! :) Could you elaborate in your answer?

Alfe Over a year ago

As I said, this doesn't solve my issue because it still mentions the implementation detail d in the subclasses. There are lots of inheriting classes which are not under my control (users of my library providing A) which are supposed not to change. So having to mention d in each subclass is no option.

Alfe Over a year ago

You might be interested in the solution I found using a meta class.

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