simple integer subclass [Python]

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From: Andreas Pfrengle on 2 Aug 2010 19:52

I'm trying to define a subclass of int called int1. An int1-object
shall behave exactly like an int-object, with the only difference that
the displayed value shall be value + 1 (it will be used to display
array indices starting at 1 instead of 0). Right now I have:

class int1(int):
def __str__(self):
return int.__str__(self + 1)

However, if I calculate with int1 and int- (or other number) objects,
the result is always coerced to an int (or other number object), e.g:
a = int1(5)
b = 5
print a # "6"
print a+b #"10"

How can I tell int1 to be the "default integer object"? Do I need to
overload *every* mathematical operation method of int, or is there an
easier way?

From: samwyse on 2 Aug 2010 20:24

On Aug 2, 6:52 pm, Andreas Pfrengle <a.pfren...(a)gmail.com> wrote:
> I'm trying to define a subclass of int called int1. An int1-object
> shall behave exactly like an int-object, with the only difference that
> the displayed value shall be value + 1 (it will be used to display
> array indices starting at 1 instead of 0). Right now I have:
>
> class int1(int):
> def __str__(self):
> return int.__str__(self + 1)
>
> However, if I calculate with int1 and int- (or other number) objects,
> the result is always coerced to an int (or other number object), e.g:
> a = int1(5)
> b = 5
> print a # "6"
> print a+b #"10"
>
> How can I tell int1 to be the "default integer object"? Do I need to
> overload *every* mathematical operation method of int, or is there an
> easier way?

I had a similar problem a few years ago, and couldn't find a solution
then. The thread from back then may shed some light on your problem.
http://groups.google.com/group/comp.lang.python/browse_thread/thread/10cfe2affc265ac/2ad03b121c1c6489

From: Carl Banks on 2 Aug 2010 21:22

On Aug 2, 4:52 pm, Andreas Pfrengle <a.pfren...(a)gmail.com> wrote:
> I'm trying to define a subclass of int called int1. An int1-object
> shall behave exactly like an int-object, with the only difference that
> the displayed value shall be value + 1 (it will be used to display
> array indices starting at 1 instead of 0). Right now I have:
>
> class int1(int):
> def __str__(self):
> return int.__str__(self + 1)
>
> However, if I calculate with int1 and int- (or other number) objects,
> the result is always coerced to an int (or other number object), e.g:
> a = int1(5)
> b = 5
> print a # "6"
> print a+b #"10"
>
> How can I tell int1 to be the "default integer object"? Do I need to
> overload *every* mathematical operation method of int, or is there an
> easier way?

(Preface: I normally don't offer recommendations without answering the
question as asked, but once in a while it has to be done.)

I **highly** recommend against this approach.

You are creating an object that differs from a built-in, int, in a
highly misleading way that only makes sense in a very limited context,
and this object's modified behavior gives no clue that it's been
modified in such as way. (That is, it's not possible to tell if the
object's not a regular int just by looking at __str__()'s return
value.) To make matters worse, you want to program this object to
coerce other integers, so there's a risk of these objects escaping
from the context where they make sense.

This is just a bad idea. The type is not the place to implement
behavior that makes sense only in a limited context. Instead, do
something like this:

print "Item %d is %s." % (i+1, s[i])

Carl Banks

From: rantingrick on 2 Aug 2010 23:36

From: Andreas Pfrengle on 3 Aug 2010 06:15

On 3 Aug., 03:22, Carl Banks <pavlovevide...(a)gmail.com> wrote:>
> You are creating an object that differs from a built-in, int, in a
> highly misleading way that only makes sense in a very limited context,
> and this object's modified behavior gives no clue that it's been
> modified in such as way. (That is, it's not possible to tell if the
> object's not a regular int just by looking at __str__()'s return
> value.) To make matters worse, you want to program this object to
> coerce other integers, so there's a risk of these objects escaping
> from the context where they make sense.
>
> This is just a bad idea. The type is not the place to implement
> behavior that makes sense only in a limited context. Instead, do
> something like this:
>
> print "Item %d is %s." % (i+1, s[i])

I see your concerns. I started with the approach to add +1 directly
before displaying the int. However, since there are some variables
that shall be displayed normally and others that are indices I want to
show starting at 1, I thought the easiest way would be to define a
type that does the job, then I would only need to define it once and
not take care everywhere whether I have a normal variable or a
displayed index.
Thinking about it, it might really be dangerous to coerce always to
int1, since sometimes I might want a normal int as result (I can't
tell yet for sure).
I'm just thinking about only overloading the operations if the int1 is
on the left-hand side (so __op__ coerces to int1, while __rop__
doesn't). This would make operations non-commutative - but I also
would need to put more brains in every calculation, which could
finally take more effort than only "upgrading" the display :-???
Seems I end up with your suggestion - if noone else has an idea ;-)

The application will be a browsergame, and most gamers start counting
at 1, so they would probably wonder about a "level 0 item" ;-)
If there didn't already exist lots of code, I would redesign the whole
data-structure - I think that's "lessons learned" for the next project
-.-

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