[Tutor] defining __init__

[Danny Yoo]

On Fri, 13 Jan 2006, S. D. Rose wrote:

> One thing about classes that really tripped me up when I first started
> using classes was 'self' before the variable.  When you have '
> self.variable ' that is a variable that can be referenced by the parent.
> If there's no 'self' before the variable, you can not.
>
> For instance, if you have a routine:
>
> class Dog:
>     def __init__(self, color):
>         self.color = color
>         age = 16

Hi Dave,

'age' here is a local variable, just like any other local variable that
we've used in other function definitions.


> It will error out, because you can not reference age without pre-pending
> a 'self.'

There might be some confusion here --- you mention parents and children,
when there aren't any involved --- so let's talk about this in more
detail.

The key observation we need to make is that the first assignment:

>         self.color = color

makes a state change to the class instance.


But:

>         age = 16

makes an assignment to a local variable 'age', and that variable has no
connection to the state of the Dog.


Let's get away, for a moment, from class stuff, and let's take a quicky
example with just functions:

###
def initializeThing(thing):
    thing['color'] = 'blue'
    age = 42

d = {}
initializeThing(d)
print d
###

Do we expect to see 'age' in the 'd' dictionary?


If you have more questions, please feel free to ask.