Question

Long time reader, first time asker. Anyway, Here’s the code I’m working with:

class Person(object):
  def __init__(self, s):
    self.name = s
    self.secret = 'I HAVE THE COOKIES'

  @classmethod
  def shout(self):
    print self.name.upper()

class Kid(Person):
  def __init__(self, s):
    super(Kid,self).__init__(s)
    self.age = 12

b = Person('Bob')
k = Kid('Bobby')
print b.name
print k.name
print k.age
print k.secret
k.shout()

Which results in this output and error:

Bob
Bobby
12
I HAVE THE COOKIES
Traceback (most recent call last):
File "a.py", line 22, in <module>
  k.shout()
File "a.py", line 8, in shout
  print self.name.upper()
AttributeError: type object 'Kid' has no attribute 'name'

I assumed that Kid would be able to use the Person’s shout method substituting its (the kid’s) “self” for parent (where the method lives). Apparently, that’s not the case. I know I could declare name outside of init, but that’s both unable to accomodate inputted data and a no-no. Another alternative would be to redefine shout for every child of Person, but that’s a lot of repeated code that I’m trying to avoid.

Thanks very much in advance!

Answer 1

The issue is that @classmethod is a method on a class. It does not have access to an instance’s attributes. Specifically the method is actually passed the class object, thus self is misnamed. You should really call shout‘s argument cls. If you remove the @classmethod then this would all make sense and your code would work as expected.

As it is, you can think of k.shout() as equivalent to Kid.shout().