Inheritance and initialization of attributes

Question

class Email():

    def __init__(self, store_number):
        self.store_number = store_number

    def amethod(self):
        pass

What is the correct way to pass variables from a sub-class to a parent-class? should I do:

class MoreSpecificEmail():

    def __init__(self, store_number):
        Email.__init__(self, store_number=store_number)

    def another_method(self):
        pass

or:

class MoreSpecificEmail():

    def __init__(self, store_number):
        self.store_number = store_number
        Email.__init__(self, store_number=self.store_number)

I have just been using different abbreviations of store_number in each sub-class to help clarify what's going on in my head. I am sure that is the wrong way, though.

jonrsharpe · Accepted Answer · 2015-03-18 16:20:38Z

4

What you currently have isn't inheritance; neither of your classes actually inherits from anything! Firstly, Email should be a "new-style class", inheriting from object:

class Email(object):
          # ^ note inheritance from object

    def __init__(self, store_number):
        self.store_number = store_number

    def amethod(self):
        pass

Then MoreSpecificEmail should inherit from Email - as it doesn't have any additional instantiation parameters, it can just use the inherited __init__ and doesn't need to define its own:

class MoreSpecificEmail(Email):
                      # ^ note inheritance from Email

    # note no need to define __init__

    def another_method(self):
        pass

For an example where there are additional __init__ parameters, note that you should use super and rely on the superclass's __init__ to assign the parameters it takes - you only need to assign the attributes that don't get handled by the superclass:

class MoreSpecificEmail(Email):

    def __init__(self, store_number, something_else):
        super(MoreSpecificEmail, self).__init__(store_number)
                                              # ^ pass it straight on
        self.something_else = something_else

    def another_method(self):
        pass

For more information, see the Python class tutorial.

edited Mar 18, 2015 at 16:20

answered Mar 18, 2015 at 16:02

jonrsharpe

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2 Comments

Michael Billingham Over a year ago

Thank you so much. I checked out that tutorial, but in it they dont use SuperClass(object):, they just use SuperClass: Is there a difference?

jonrsharpe Over a year ago

@MichaelBillingham there is in 2.x - the former creates a new-style class, the latter an old-style class. See e.g. python.org/doc/newstyle

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