How to convert (inherit) parent to child class?

问题:

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. Thanks

回答1:

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

回答2:

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.

回答3:

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.)

回答4:

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

回答5:

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.