Class Variable Inheritance

[Ken Rowlands]

On Wed, 08 Dec 2004 15:55:09 -0900, Brian Jones wrote:

> I'm sure the solution may be obvious, but this problem is driving me 
> mad.  The following is my code:
> 
> class a(object):
> 
> 	mastervar = []
> 
> 	def __init__(self):
> 		print 'called a'
> 
> class b(a):
> 
> 	def __init__(self):
> 		print 'called b'
> 		self.mapvar()
> 
> 	def mapvar(self):
> 		self.mastervar.append(['b'])
> 
> class c(b):
> 
> 	def __init__(self):
> 		print 'called c'
> 		self.mapvar()
> 
> 	def mapvar(self):
> 		super(c, self).mapvar()
> 		self.mastervar.append(['c'])
> 
> if __name__ == '__main__':
> 
> 	a1 = a()
> 	b1 = b()
> 	c1 = c()
> 	d1 = c() # Call C again
> 
> 	print a1.mastervar
> 	print b1.mastervar
> 	print c1.mastervar
> 	print d1.mastervar
> 
> What I don't understand is why mastervar gets modified by each _seperate 
> instance_ of classes that happen to extend the base class 'a'. 
> Shouldn't mastervar be contained within the scope of the inheriting 
> classes?  Why is it being treated like a global variable and being 
> modified by the other instances?
> 
> 
> Thanks,
> 
> Brian "bojo" Jones


Brian,

This is the first time I've responded to a post in this newsgroup so bear
with me.

First of all I believe that the place you declare mastervar is important.
As you have placed it above (or outside) any class methods/functions it
becomes a class variable. This gives it class scope and ALL instances of
that class (and sub-classes I think) will reference the SAME variable. If,
instead, you declare it inside the __init__ function of the class a object
it will be an instance variable and inheritable.

Secondly, in order to inherit methods and functions from base classes it
is important to remember to call the base class __init__ function in the
sub-class __init__ function.

If you do both these things, that is declare mastervar as 'self.mastervar
= []' inside class a.__init__ and call a.__init__ from b.__init__ (and
likewise for b in c) then you should get the expected result. I hope.

All I can say at this point is that I and not what you would call a
programmer, and I could be quite wrong in my explanation. However if this
doesn't help then there are many folk here who are better qualified to
give you the correct solution.


Regards