How to call Base Class's __init__ method from the child class? [duplicate]

72

This question already has an answer here:

If I have a python class as:

class BaseClass(object):
#code and the init function of the base class

And then I define a child class such as:

class ChildClass(BaseClass):
#here I want to call the init function of the base class

If the init function of the base class takes some arguments that I am taking them as arguments of the child class's init function, how do I pass these arguments to the base class?

The code that I have written is:

class Car(object):
    condition = "new"

    def __init__(self, model, color, mpg):
        self.model = model
        self.color = color
        self.mpg   = mpg

class ElectricCar(Car):
    def __init__(self, battery_type, model, color, mpg):
        self.battery_type=battery_type
        super(ElectricCar, self).__init__(model, color, mpg)

Where am I going wrong?

Prakhar Mohan Srivastava

출처 python python-2.7 inheritance constructor
  • Your format is wrong, is that a copy-and-paste problem? Python will bark on wrong format. - Mingyu
  • @Mingyu format is wrong? You mean indentation or am I missing something else here? - Prakhar Mohan Srivastava
  • Yea.. I mean indentation. Please look at my answer below. - Mingyu

4 답변

86

You could use super(ChildClass, self).__init__()

class BaseClass(object):
    def __init__(self, *args, **kwargs):
        pass

class ChildClass(BaseClass):
    def __init__(self, *args, **kwargs):
        super(ChildClass, self).__init__(*args, **kwargs)

Your indentation is incorrect, here's the modified code:

class Car(object):
    condition = "new"

    def __init__(self, model, color, mpg):
        self.model = model
        self.color = color
        self.mpg   = mpg

class ElectricCar(Car):
    def __init__(self, battery_type, model, color, mpg):
        self.battery_type=battery_type
        super(ElectricCar, self).__init__(model, color, mpg)

car = ElectricCar('battery', 'ford', 'golden', 10)
print car.__dict__

Here's the output:

{'color': 'golden', 'mpg': 10, 'model': 'ford', 'battery_type': 'battery'}

  • If you pass BaseClass to super, it'll skip over BaseClass and call object.__init__, which is almost certainly not what you want. - abarnert
  • Meanwhile, the only thing wrong with the OP's code is his indentation, explaining super (even if you explained it correctly, which you didn't) doesn't help, especially given that he already used it exactly the same way; in fact, it's character-for-character identical. - abarnert
  • Thanks, @abarnert. Initially, he did not post his code and the question he asked is the one in the title. - Mingyu
23

As Mingyu pointed out, there is a problem in formatting. Other than that, I would strongly recommend not using the Derived class's name while calling super() since it makes your code inflexible (code maintenance and inheritance issues). In Python 3, Use super().__init__ instead. Here is the code after incorporating these changes : 

class Car(object):
    condition = "new"

    def __init__(self, model, color, mpg):
        self.model = model
        self.color = color
        self.mpg   = mpg

class ElectricCar(Car):

    def __init__(self, battery_type, model, color, mpg):
        self.battery_type=battery_type
        super().__init__(model, color, mpg)

Thanks to Erwin Mayer for pointing out the issue in using __class__ with super()

  • Can you give any explanation for the recommendation 'not to use the Derived class's name'? - jwg
  • @jwg I updated the answer to explain why class is a better choice. - Craig Finch
  • @CraigFinch it doesn't seem to be a good idea at all to use self.__class__. In Python 3 you can simply use super().__init__ - Erwin Mayer
  • Yet another reason Py3 is superior to Py2. (As someone who came to python after 3.4 came out) - Marc
9

You can call the super class's constructor like this

class A(object):
    def __init__(self, number):
        print "parent", number

class B(A):
    def __init__(self):
        super(B, self).__init__(5)

b = B()

NOTE:

This will work only when the parent class inherits object

7

If you are using Python 3, it is recommended to simply call super() without any argument:

class Car(object):
    condition = "new"

    def __init__(self, model, color, mpg):
        self.model = model
        self.color = color
        self.mpg   = mpg

class ElectricCar(Car):
    def __init__(self, battery_type, model, color, mpg):
        self.battery_type=battery_type
        super().__init__(model, color, mpg)

car = ElectricCar('battery', 'ford', 'golden', 10)
print car.__dict__

Do not call super with class as it may lead to infinite recursion exceptions as per this answer.

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