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Overloading =

jhefferon at my-deja.com jhefferon at my-deja.com
Fri Feb 18 10:16:40 EST 2000 
In article <m3bt5fo1lk.fsf at atrus.jesus.cam.ac.uk>,
  Michael Hudson <mwh21 at cam.ac.uk> wrote:
> jhefferon at my-deja.com writes:
>
> > I have a class, rclass.  It has an attribute value.  Can I arrange
> > so that the string
> >     r1=r2
> > sets r1.value to equal r2.value (where r1 and r2 are class
instances,
> > of course) without changing anything else about r1?
>
> No. Assignment rebinds references, rather than affecting objects. This
> is quite a difference from C++ (say), probably one of the harder ones
> to get one's head around.
>
> >
> > And if so, can I also have r1=7 set r1.value to be 7?
> >
> Hmm... do you know about the "exec <blah> in globals,locals" style of
> doing things? Though `exec'ing user input sounds deeply unwise to me.
> What is it you are trying to do? Maybe we can come up with a better
> solution (no promises though).

Thanks.  I *am* reluctant to allow students to accidently exec something
that erases all files, or something.

I am simulating a computer.  It is made up of registers and each
register (memory or CPU) may have more than one attribute (for instance,
whether to print it out in binary or hex, or how many times it has
been referenced in this program, or what source line compiled to this
register, etc.).  That's why each register is a class instance rather
than a simple variable.

I want to have the obvious GUI, that allows a person to scroll through
memory, for instance.  I also want to allow them to do things like
set a register `rA=7' or `rA=memory[52]'.

In the register class I can use __setattr__ to let me write either
  `rA.value=7' or `rA.value=memory[52]',
right (I look at the type of the thing on the right and if it is a
class instance then I see if, say, memory[52].classtype==``register'')?
But I can't eliminate the `.value' on the left?  I thought maybe I could
mess with the top-level dictionary (I don't know what I mean by that!).

Thanks again,
Jim Hefferon jim at joshua.smcvt.edu


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