class scope questions

[Duncan Booth]

"Ben de Luca" <c941520 at alinga.newcastle.edu.au> wrote in
<3a83bea8$0$16388$7f31c96c at news01.syd.optusnet.com.au>: 

>if i do this
>
>class a:
>         dog='woof'
>
>          def blah():
>                    something
>
>
>
>how do i call a
>how do i get soemthing in blah to refernce dog? either to read it or
>write to it
>
class a:
    dog='woof'
    def blah(self):
        print self.dog
    def update(self, value):
        self.dog = value
>>> a1 = a()
>>> a1.blah()
woof
>>> a1.update('bow wow')
>>> a2 = a()
>>> a2.blah()
woof
>>> a1.blah()
bow wow
>>> 

But, although this works fine, there is a problem with it. dog is
actually a variable in the class a until it is set by the update method
when it becomes  a variable in the instance a1. This works fine for
strings, but if you used a mutable object such as a list or dictionary
you would see that dog was shared between all instances of the class: 
class B: 	list = []
	def show(self):
		print self.list
	def append(self, value):
		self.list.append(value)

		
>>> b1 = B()
>>> b1.append('hello')
>>> b1.show()
['hello']
>>> b2 = B()
>>> b2.show()
['hello']
>>> b2.append('world')
>>> b1.show()
['hello', 'world']

To avoid this you should generally define an __init__ method in your
class and use that to initialise any instance variables.