CODEWITHSUNDEEP
pythonclassinheritance
Tomas
Tomas

Reputation: 343

How to convert (inherit) parent to child class?

I would like to know how to convert parent object that was return by some function to child class.

class A(object):
    def __init__():
        pass

class B(A):
    def functionIneed():
        pass

i = module.getObject() # i will get object that is class A
j = B(i) # this will return exception
j.functionIneed()

I cannot change class A. If I could I would implement functionIneed to class A, but it is impossible because of structure of code.

Upvotes: 14

Views: 32321

Answers (7)

charelf
charelf

Reputation: 3825

This comment shares a link, I copy-pasted the contents here in case of link-rot. Please consider visiting the original link if possible.

import copy

class Base(object):
    def __init__(self):
        self.base_init = True

    def foo(self):
        return 'base foo'

    def bar(self):
        return 'base bar'

class Child(Base):
    def __new__(cls, other):
        if isinstance(other, Base):
            other = copy.copy(other)
            other.__class__ = Child
            return other
        return object.__new__(cls)

    def __init__(self, other):
        self.child_init = True

    def bar(self):
        return 'child bar'

b = Base()
assert b.base_init == True
assert b.foo() == 'base foo'
assert b.bar() == 'base bar'
assert b.__class__ == Base

c = Child(b)
assert c.base_init == True
assert c.child_init == True
assert c.foo() == 'base foo'
assert c.bar() == 'child bar'
assert c.__class__ == Child
assert b.__class__ == Base

Upvotes: 0

Ben Law
Ben Law

Reputation: 58

Just thinking outside the box:

Instead of a new class with the function you want, how about just adding the function to the class or instance you already have?

There is a good description of this in Adding a Method to an Existing Object Instance

Upvotes: 0

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798716

Python does not support "casting". You will need to write B.__init__() so that it can take an A and initialize itself appropriately.

Upvotes: 26

John Moore
John Moore

Reputation: 181

You did not correctly define your classes. Should be like this:

class A(object):
    def __init__(self):
        pass

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

    def functionIneed(self):
        pass

Then you can 

j=B()
j.fuctionIneed()

as expected

You forgot to refer to the ins

Upvotes: 0

sureshvv
sureshvv

Reputation: 4422

How about:

i = module.getObject() # i will get object that is class A
try:
    i.functionIneed()
except AttributeError:
    # handle case when u have a bad object

Read up on duck typing.

Upvotes: -1

wkschwartz
wkschwartz

Reputation: 3877

You said you want to implement something like this:

class B(A):
    def functionIneed():
        pass

But really what you would be making is something more like this (unless you had intended on making a class or static method in the first place):

class B(A):
    def functionIneed(self):
        pass

Then you can call B.functionIneed(instance_of_A). (This is one of the advantages of having to pass self explicitly to methods.)

Upvotes: 5

A. Jesse Jiryu Davis
A. Jesse Jiryu Davis

Reputation: 24007

I have a strong suspicion, nay, conviction, that there is something horribly wrong with your program design that it requires you to do this. In Python, unlike Java, very few problems require classes to solve. If there's a function you need, simply define it:

def function_i_need(a):
     """parameter a: an instance of A"""
     pass # do something with 'a'

However, if I cannot dissuade you from making your function a method of the class, you can change an instance's class by setting its __class__ attribute:

>>> class A(object):
...     def __init__(self):
...         pass
... 
>>> class B(A):
...     def functionIneed(self):
...         print 'functionIneed'
... 
>>> a = A()
>>> a.functionIneed()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'functionIneed'
>>> a.__class__ = B
>>> a.functionIneed()
functionIneed

This will work as long as B has no __init__ method, since, obviously, that __init__ will never be called.

Upvotes: 7

Related Questions