[Tutor] problem with class - get message that self is not defined

[Alan Gauld]

> When I try to use the class listed below, I get the statement that 
> self
> is not defined.
>
> test=TriggerMessage(data)
> var = test.decode(self.qname)
>
> I would have thought that self would have carried forward when I 
> grabbed
> an instance of TriggerMessage.

self is a special varianble that is only defined inside of a
method of an object. It refers to the object itself (hence the name!)
Thus you cannot pass self.qname into a method unless you are
already inside another object's method.

You can read about this in more edetail in the OOP topic
of my tutor but I'll try to illustrate here:

class C:
    def method(self,x):
         self.var = x
    def g(self, anObj):
         anObj.f(self.var)

class D:
    def f(self, val):
       print val

c = C()
c.method(42)

creates an object called c and executes its method with a
value of 42. This executes the function

C.method(c,42)

where self takes the values c and x 42
The end result is that object c has a data member
called var with the value 42

d = D()
c.g(d)

creates a new object d
calls the g method of c passing object d as argument
executes the function

C.g(c,d)

where self takes on the value c and anObj takes on d
C.g() then calls d.f(self.var) which executes

D.f(d,c.var)

that is, the self in D.f() is d and the self in C.g() = c

I hope that makes sense.

Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld