dictionary containing instances of classes behaving oddly

[Ben]

Ah - I have been very silly indeed!

I had not realised that by creating my lists outside of the
def__init___() constructor they would always be class rather than
instance variables, and so there was only one of them when I referred
to it. Thanks to Erik and Chris for your help!

Ben


Ben wrote:

> Ah - ok. In fact I simply had:
>
> class record:
>         my_list =[]
>         mops=[]
>
>         def __init__(self,mops):
>                 self.mops=mops
>
> Where mops is something I pass in when i create the instance. I had
> thought that then each time I created an instance of the record class
> as an element of my dictionary:
>
> self.mop_list[record_number]=record(self.mops[:])
>
> I would create a brand new instance of the record class.It would have a
> mops list initialized with mops (because the def__init__ constructor is
> called when the class is instantiated), and an empty, individual list
> my_list.
>
> I could then access each individual list by doing:
>
> self.mop_list[x].my_list[y]=something
>
> But in fact the same my_list is being accessed for all values of x, so
> a change to one list is in fact a change to them all. I think you might
> be right, but can't quite work it out myself! I'll keep trying :-)
>
> Thanks for your help,
>
> Ben
>
>
>
>
>
> Erik Johnson wrote:
>
> > "Ben" <Benjamin.Barker at gmail.com> wrote in message
> > news:1167326178.386764.300200 at n51g2000cwc.googlegroups.com...
> >
> > <snip>
> >
> > > This seems to work without any errors. But bizzarely I find that
> > > whatever my record number, the instance of "my_class" is appended to
> > > every list. So in this case
> > >
> > > self.mop_list[0].my_list.append(my_class(Some data for the
> > > constructor))
> > >
> > > I would expect to append an instance of my_class to
> > > self.mop_list[0].my_list
> > >
> > > But annoyingly
> > >
> > > self.mop_list[0].my_list
> > > self.mop_list[3].my_list
> > > self.mop_list[7].my_list
> > >
> > > all have an instance of my_class created and appended to them. This is
> > > really confusing and quite annoying - I don't know whether anyone out
> > > there can make head or tail of what I'm doing wrong?
> >
> > Well, it's a little bit difficult, but I think I actually know what's going
> > on. You probably need some code that looks something like this, to ensure
> > each object has it's own, independent list:
> >
> > class record:
> >     def __init__(self, init_list=None):
> >         self.my_list = []
> >         if init_list is not None:
> >             self.my_list.extend(init_list)
> >
> >
> > Here's what I think you are doing, and below should make it clear why that
> > doesn't work:
> >
> > class record:
> >     def __init__(self, init_list=[]):
> >
> > That list above, the default initializer is constructed just once (when the
> > def statement executes)!
> >
> > >>> class record:
> > ...   def __init__(self, init_list=[]):
> > ...     self.my_list = init_list
> > ...
> > >>> r1 = record()
> > >>> r1.my_list
> > []
> > >>> r2 = record()
> > >>> r2.my_list
> > []
> > >>> r2.my_list.append('boo!')
> > >>> r1.my_list
> > ['boo!']
> > >>>
> > >>> l1 = range(1, 4)
> > >>> l1
> > [1, 2, 3]
> > >>> r1 = record(l1)
> > >>> r2 = record(l1)
> > >>> r1.my_list
> > [1, 2, 3]
> > >>> r2.my_list
> > [1, 2, 3]
> > >>> r1.my_list.append(42)
> > >>> l1
> > [1, 2, 3, 42]
> > >>> r1.my_list
> > [1, 2, 3, 42]
> > >>> r2.my_list
> > [1, 2, 3, 42]
> > >>>