Class Variable Question

[Alex Martelli]

"Robert Johnson" <rjohnson at exotic-eo.com> wrote in message
news:3ad1c7e9$0$196$e2e8da3 at nntp.cts.com...
> I am new to Python and I just read something that I thought was peculiar.
> Is it true that a user can add to the variables of a class just by naming
> the new variable?

Perfectly true, depending on what you mean by "user" (any code
having access to the class-object can do that).

> Like:
>
> ClassObject.var1=5

Yes, or, equivalently, setattr(ClassObject, 'var1', 5).

> Would this create a variable "var1" inside the class even though the
creator
> of the class never intended it to be there (there was no var1 originally)?

Exactly.  This is THE way any object's attributes spring into existence,
by the way: they "were not there originally", then they get bound to
something, and, voila -- they now ARE there.  Assignment syntax and
getattr are just two handy ways to perform such a binding (there are
others, e.g., def and class statements also bind an attribute in the
namespace in which they are executed).

> If I mistype the variable in my code, a new variable inside the class is
> created rather than flagging the error?

Yep.  E.g., if you write:

classObj.variable = 23
classObj.variabile = 2323

there is no way for the compiler to know whether the second
binding is a mistake, or just a perfectly correct new binding
with a name that is very similar to a previous one.  Python
doesn't start out ASSUMING all you do is an error...:-).


> Is there a way to prevent this so
> users cannot add variables?

Nope (how would one distinguish 'users' from other pieces
of code for this purpose?).  You can wrap objects up into
a Bastion if you have security worries.

> It seems to me that the user could just
> override a class with unrelated data.

Sure, just like he could say x=y-z when actually meaning
x=y+z, and a zillion other horrible programming errors.

Hopefully the user will then run some _tests_ (what other
ways would you suggest for the "plus oops I meant minus"
kind or errors to be caught...?), fix the mistakes he then
finds as a result of his testing, and have code with fewer
errors -- including code free from accidental, erroneous
rebindings of all kinds.


Alex