Copy constructing a base class from an instance in Python

Question

Consider the following piece of code:

class ABC:
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

class ABCD(ABC):
    def __init__(self, abc, d):
        # How to initialize the base class with abc in here?
        self.d = d

abc = ABC(1, 2, 3)
abcd = ABCD(abc, 4)
print(abcd.a)

What is a Pythonic way of initializing the base class with abc? If I used

super().__init__(abc.a, abc.b, abc.c)

I would have to change ABCD everytime I add someting to ABC. What I could do is to use

self.__dict__.update(abc.__dict__)

However, this feels clumsy and will break when ABC uses a different underlying implementation than dict (e.g. __slots__). Are there some alternative ways?

If you put the arguments specific to ABCD at the beginning, you could do: __init__ (self, d, *args, **kargs) and then super().__init__(*args, **kargs). — Holt
– Holt, Commented May 10, 2014 at 11:23
@Holt: Then I would need to use abcd = ABCD(4, abc.a, abc.b, abc.c) instead of abcd = ABCD(4, abc), thus forcing the user of ABCD to unpack abc manually. — s3rvac
– s3rvac, Commented May 10, 2014 at 11:27
Sorry, I didn't read the end of your code, I though you wanted to do ABCD(4, 1, 2, 3) directly. — Holt
– Holt, Commented May 10, 2014 at 11:30

juanchopanza · Accepted Answer · 2015-10-09 05:41:36Z

1

If you pass an object of type abc to the constructor, maybe you should have abc as field and not inheritance.

e.g. maybe:

class ABC:
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

class ABCD:
    def __init__(self, abc, d):
        # How to initialize the base class with abc in here?
        self.abc = abc

abc = ABC(1, 2, 3)
abcd = ABCD(abc, 4)
print(abcd.abc.a)

Per your comment, I would write method to copy the ABC part. This way this method would be "responsibility" of ABC.

class ABC:
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

    def copy_init(other):
        self.a = other.a
        self.b = other.b
        self.c = other.c

class ABCD(ABC):
    def __init__(self, abc, d):
        self.copy_init(abc)
        self.d = d

edited Oct 9, 2015 at 5:41

juanchopanza

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answered May 10, 2014 at 11:23

eran

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3 Comments

s3rvac Over a year ago

Yes, this is often the case. However, in my particular case, I need ABCD to inherit from ABC so it behaves like a subclass of ABC and the users can access all of ABC's attributes directly.

eran Over a year ago

In that case, I would write a method (of ABC) to do this copying. See EDIT above

Rainn Over a year ago

@eran Copying ABC is not very practical if you are inheriting a giant class from an external library. It's hard to ensure all things get copies in the correct way.

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