On 2010-08-08, Tim Harig <user...@ilthio.net> wrote: > On 2010-08-08, Tim Harig <user...@ilthio.net> wrote: >> On 2010-08-08, Costin Gament <costin.gam...@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.
And to complete that thought, when you assign variables directly to the instance, as I did using the constructor's reference to self, each instance receives a brand new reference that is not shared among any of the other instances. -- http://mail.python.org/mailman/listinfo/python-list
