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.

Answer 1

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.

• Michael Billingham: 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: @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