Class initialization

[Tim Harig]

On 2010-08-08, Tim Harig <usernet at ilthio.net> wrote:
> On 2010-08-08, Costin Gament <costin.gament at gmail.com> wrote:
>> So you're saying I should just use __init__? Will that get me out of
>> my predicament?
>> No, I don't quite understand the difference between my exemple and
>> using __init__, but I will read the docs about it.
>
> It is not so much using __init__() that makes the difference as it what
> scope the variables are assigned to.  If you define them as you where, then
> the variables are associated with the class object itself.  If the variable
> is a mutable type, and you change it in one instance, it actually changes
> it in the class object which means it also changes for all of the
> instances.
>
> I used the constructor because it gives me a reference to the newly created
> instance object "self".  I then assign the variables to self, which
> assignes them to the newly created instance object.  Then each instance has
> its own separate a and b variables that will not change when the variables
> are changed inside of another instance object.

Maybe I can make that a little clearer yet.  When you define a class in
python you actually create a class object.  This object is basically used
as a template to create instance objects.  When you define a variable
attached to the class object that is mutable, the instance objects receive
the exact same reference that was given to the instance object.  Since it
is mutable, any changes made using that reference will affect all of the
instances that also point to that reference.  You wouldn't have seen this
effect using your simplified examle because number are immutable objects.
When you change the value for one of the instance objects, it receives a
new reference, rather then making the change in place.  The other instances
do not reflect this change because their variables still point back to the
original reference given to the class.