Python's object model

[Florent Heyworth]

Francois Bedard wrote:

> Hello,
>
> I'm new at Python. In the following program, "stack_1 pops stack_2's
> content" (that's actually what it prints), which is obviously not what
> I'm after - nor would expect. Is this how Python's object model really
> works (kindly explain), is there some arcane rule that I've missed, or
> is there some obvious mistake I just can't see?
>
> Aside from a few such quirks, it's quite an interesting language (I'm
> using 1.5.1 - on both Linux and NT).
>
> Thanks,
>
> Francois
>
> -------------------------------------
>
> class Stack:
>         stack = []
>
>         def push(self, value):
>                 self.stack.append(value)
>
>         def pop(self):
>                 result = self.stack[-1]
>                 del self.stack[-1]
>                 return result
>
> class System:
>         stack_1 = Stack()
>         stack_2 = Stack()
>
>         def __init__(self):
>                 self.stack_1.push('stack_1\'s content')
>                 self.stack_2.push('stack_2\'s content')
>                 print 'stack_1 pops', self.stack_1.pop()
>
> # 'main'
>
> system = System()

Hi Francois

what you're seeing is the difference between a class and an
instance variable. To see the behaviour you want you need to
modify your stack class as follows:

class Stack:
        def __init__(self):
            self.stack = []

        def push(self, value):
                self.stack.append(value)

        def pop(self):
                result = self.stack[-1]
                del self.stack[-1]
                return result

Otherwise the stack is a class variable which can be referred as
Stack.stack() (this would then act as a class global).

Cheers
Florent Heyworth