Using eval, or something like it...

[r0g]

Scott David Daniels wrote:
> r0g wrote:
>> ...
>> A class is like a template which combines a complex data type (made from
>> a combination of other data types) and the methods that operate on that
>> data type.
>>
>> You generally don't work with classes directly but you make instances of
>> them, each instance has it's own internal state and methods, initially
>> these are the same as the templates but can be changed or overridden
>> without affecting the state of any other instances you might have.
> Take the tutorial and do experiments.
> The attribute lookup checks the class _and_ the instance (with the
> instance over-riding the class).  Make sure you can explain the output
> from this:
> 
>     class Demo(object):
>         non_template = 43
> 
>     d = Demo()
>     print d.non_template
>     Demo.non_template = 44
>     print d.non_template
>     d.non_template = 45
>     print d.non_template
>     Demo.non_template = 46
>     print d.non_template
> 
> Once you can do that, explain this:
>     class Demo2(object):
>         holder = []
> 
>     e = Demo2()
>     print e.holder
>     Demo2.holder.append(44)
>     print e.holder
>     e.holder.append(45)
>     print e.holder
>     Demo2.holder.append(46)
>     print e.holder
> 
>     # clue:
>     print d.holder is Demo.holder
> 


Well that all makes sense what with Pythons' 'only copy explicity' and
lists being mutable and indeed, as you intuited, I did run into issues
with this within days of starting to learn Python! Ever since I have
been using copy.deepcopy() when I need to get the results described
above i.e. to ensure each instance is fully independent of all the others.

I hadn't really appreciated the consequences of this till now though
e.g. that an instance might do a = a + 1 without affecting it's siblings
but that b.append("fish") would affect b for everyone. I don't know if I
will find any uses for that kind of behaviour but it doesn't hurt to
understand it :-)

Isn't Python's behaviour a little peculiar in this respect though,
compared to classes in other languages? i.e. Are instances in other OO
languages like Smalltalk, C++ fully independent copies or do their
attribute names just point to one common object until reassigned like in
python? (Or have I still not it at all?!)


>> Is this correct enough for me to avoid the aforementioned bug pile?
>>
>> Also then, what _is_ an "instance variable" ?
> 
> Well, when you use the term variable, I suspect that you think it
> represents storage.  OK, it kind of does, but only in the sense

Well you used the term "instance variable" in your reply so I was just
asking if there _is_ an actual thing known as "an instance variable" in
Python and if so what it is/does.

Thanks for your reply and examples BTW (I think) they are helping me
clarify my understanding of python classes and the language used to
describe them.


Roger.

> --Scott David Daniels
> Scott.Daniels at Acm.Org