Attribute definition, WHY?

[bhellao at my-deja.com]

One of the ways they are different is in inheritance.
For example:

# the original script
class A:
	color = "Blue"
	def __init__(self):
		pass
class B:
	def __init__(self):
		self.color = 'Blue'
>>> a = A(); b = A()
>>> a.color; b.color
'Blue'
'Blue'
>>> a.color = 'Red'
>>> b.color
'Blue'  <-- did not change the 'color' value of instance 'b'

>>> a = B(); b = B()
>>> a.color; b.color'Blue'
'Blue'
'Blue'
>>> a.color = 'Red'
>>> b.color
'Blue'  <-- did not change the 'color' value of instance 'b'

However,
class D(A): # subclassing A
	def __init__(self):
		pass
class B:
	def __init__(self):
		self.color = 'Blue'
class E(B): # subclassing B
	def __init__(self):
		pass
>>> d = D(); e = E()
>>> d.color
'Blue'
>>> e.color # the init method of class B was not automatically called,
            # so the 'color' never became an instance variable.
AttributeError: 'E' instance has no attribute 'color'

Of course you could invoke the init method by
class E(B): # subclassing B
	def __init__(self):
		B.__init__(self)

which would give the result
>>> e.color
'Blue'

Hope this helps.

In article <slrn8rssnb.on.matt at happy-hour.mondoinfo.com>,
  matt at mondoinfo.com (Matthew Dixon Cowles) wrote:
> On Tue, 12 Sep 2000 17:17:42 GMT, bragib at my-deja.com
> <bragib at my-deja.com> wrote:
>
> >Why would you want to do the following:
> >
> >class A:
> >   attr1 = [1,2]
> >   def __init__(self,name):
> >       self.name = name
> >
> >and not
> >
> >class A:
> >   def __init__(self,name):
> >       self.name = name
> >       self.attr1 = [1,2]
> >
> >Are there benefits to the first definition?
>
> Bragi,
> In the first definition, the value of attr1 is shared by the whole
> class. That's useful for counters and whatnot. So, given the first
> definition, code like this:
>
> foo=A('foo')
> bar=A('bar')
> A.attr1=[3,4]
> print foo.attr1
> print bar.attr1
>
> would produce
>
> [3, 4]
> [3, 4]
>
> Regards,
> Matt
>


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