Create Derived object given a base Object

Question

I have a base class which has a __init__ that takes two parameters. I want to create a derived class object, given that I have a base class object. I want to do the following,

a_obj = A(10, 'f') 
## given that I have a_obj, create b_obj
b_obj = B(a_obj)

and get a result such that

type(b_obj) => B

print(b_obj.age) => 10
print(b_obj.sex) => "f"

I tried the following, where the __new__ of B would call A's __new__ and ask it to create an object of type B(an empty object). But at this point I dont know how to set B to have all the functionalities of A.

class A:
  def __init__(self, age, sex):
    self.age = age
    self.sex = sex

class B(A):
  def __new__(cls, a_obj):
    a_class = a_obj.__class__
    b_obj = a_class.__new__(cls)
    a_class.__init__(b_obj, a_obj) #doesnt work since A's init takes 2 params

Is there a better way than to typecast the given A's object to B? ( i.e a_obj.__classs__ = b_obj )

Edit: B is a dynamically created class(using type). So B wouldnt know beforehand what are the parameters that A's __init__ would take.

FHTMitchell · Accepted Answer · 2018-06-18 15:14:34Z

2

The simple way, by passing a's attributes in a call to super

class A:
  def __init__(self, age, sex):
      self.age = age
      self.sex = sex

class B(A):
    def __init__(self, a):
         """a is an instance of A"""
         super().__init__(a.age, a.sex)

In answer to "what if B was a dynamically generated class? B doesnt know what A's params are", that is a lot harder, and requires some true python hacking.

class B(A):
    def __init__(self, a):
        self.__dict__.update(a.__dict__)

I do not endorse doing this. It is bad coding practise and possibly error prone.

edited Jun 18, 2018 at 15:14

answered Jun 18, 2018 at 15:01

FHTMitchell

12.2k2 gold badges40 silver badges50 bronze badges

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

1 Comment

bruno desthuilliers Over a year ago

Not hard at all actually and does not require any hack - just use *args and **kwargs like you would for a decorator or any generic callable.

bruno desthuilliers · Accepted Answer · 2018-06-19 06:53:20Z

1

Quite simply, pass a_object.age and a_object.sex to the parent constructor.

class A(object):
  def __init_(self, age, sex):
    self.age = age
    self.sex = sex

class B(A):
  def __init__(self, a):
      super(B, self).__init__(a.age, a.sex)

Now note that one usually expects subclasses to be compatible with their parents, which is not the case here.

Edit: to answer the additional question "what if B is dynamically generated and doesn't know beforehand what are the parameters that A's init would take", the canonical solution is to use varargs and keyword args:

class B(A):
    def __init__(self, *args, **kwargs):
        super(B, self).__init__(*args, **kawargs)

edited Jun 19, 2018 at 6:53

answered Jun 18, 2018 at 15:02

bruno desthuilliers

78.3k6 gold badges103 silver badges129 bronze badges

5 Comments

bruno desthuilliers Over a year ago

@FHTMitchell chill out - your answer wasn't posted when I started writing mine, I only saw it after posting (else I wouldn't have bothered answering).

FHTMitchell Over a year ago

Sorry. I'm chill +1

user2626431 Over a year ago

@FHTMitchell and bruno, what if B was a dynamically generated class? B doesnt know what A's params are.

bruno desthuilliers Over a year ago

@user2626431 use *args and **kwargs, cf my edited answer.

Clément Over a year ago

Your edit doesn't answer the question, I think: the __init__ of B is supposed to take only one parameter, and instance of a.

popzxc · Accepted Answer · 2018-06-18 15:11:00Z

Well, with the example it's not clear whether inheritance will be used for any purpose other than a properties access. If getting all the object fields is only thing you want, I guess, it's not necessary to use inheritance.

Since every object in python has dict property, you can just copy it:

class A:
    def __init__(self, age, sex):
        self.age = age
        self.sex = sex

class B:
    def __init__(self, a_obj):
        self.__dict__ = a_obj.__dict__.copy()

if __name__ == "__main__":
    a_obj = A(10, "f")
    b_obj = B(a_obj)

    print(type(b_obj)) # B

    # Works as expected.
    print(b_obj.age)
    print(b_obj.sex)

Although provided example will work as-well with inheritance, but in that case I'd hardly suggest to use parent constructor as described above.

Bubble Bubble Bubble Gut · Accepted Answer · 2018-06-18 15:00:02Z

-2

Maybe using getattr, within class B,

class B:
    def __init__(self, a_object):
        self._a = a_object

    def __getattr__(self, item):
        return getattr(self._a, item)

So if b_obj doesn't have an age attribute, it will refer to the age within a_object.

edited Jun 18, 2018 at 15:00

answered Jun 18, 2018 at 14:58

Bubble Bubble Bubble Gut

3,37617 silver badges30 bronze badges

1 Comment

bruno desthuilliers Over a year ago

The OP wants B to derive from A.

Collectives™ on Stack Overflow

Create Derived object given a base Object

4 Answers 4

1 Comment

5 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

5 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related