Velocity Reviews > simple integer subclass

# simple integer subclass

Andreas Pfrengle
Guest
Posts: n/a

 08-02-2010
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?

samwyse
Guest
Posts: n/a

 08-03-2010
On Aug 2, 6:52*pm, Andreas Pfrengle <(E-Mail Removed)> 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.

Carl Banks
Guest
Posts: n/a

 08-03-2010
On Aug 2, 4:52*pm, Andreas Pfrengle <(E-Mail Removed)> 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

rantingrick
Guest
Posts: n/a

 08-03-2010
On Aug 2, 6:52*pm, Andreas Pfrengle <(E-Mail Removed)> 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)

Is zero based indexing that bad for you? I also think this is a very
bad idea unless you can offer a sensible use case -- for which i
cannot imagine.

Andreas Pfrengle
Guest
Posts: n/a

 08-03-2010
On 3 Aug., 03:22, Carl Banks <(E-Mail Removed)> 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).
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
-.-

rantingrick
Guest
Posts: n/a

 08-03-2010
> On Aug 3, 5:15*am, Andreas Pfrengle <(E-Mail Removed)> wrote:
>
> Seems I end up with your suggestion - if noone else has an idea

START_COUNTING_FROM_HERE = 1

Jean-Michel Pichavant
Guest
Posts: n/a

 08-03-2010
Andreas Pfrengle wrote:
> On 3 Aug., 03:22, Carl Banks <(E-Mail Removed)> 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).
> 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
> -.-
>

The level of an item is attribute of this item, not an index in a list.
You may have an issue with your design, using an improper structure.

In a more general manner, given your example I don't see why you should
expose the index of an element in an internal list to the user.

JM

Jean-Michel Pichavant
Guest
Posts: n/a

 08-03-2010
Jean-Michel Pichavant wrote:
> Andreas Pfrengle wrote:
>> On 3 Aug., 03:22, Carl Banks <(E-Mail Removed)> 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).
>> 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
>> -.-
>>

> The level of an item is attribute of this item, not an index in a list.
> You may have an issue with your design, using an improper structure.
>
> In a more general manner, given your example I don't see why you
> should expose the index of an element in an internal list to the user.
>
> JM
>

"If there didn't already exist lots of code, I would redesign the whole
data-structure"

sorry I didn't get this one at first read, my comment is pretty much
useless.

JM

Roald de Vries
Guest
Posts: n/a

 08-03-2010
Hi Andreas,

On Aug 3, 2010, at 1:52 AM, Andreas Pfrengle 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?

Maybe you could use:
1) a dict with keys 1..n
2) a simple list (or iterable) subclass with 1-based indices.

class list1(list):
def __new__(cls, *args, **kwargs):
return list.__new__(cls, *args, **kwargs)

def __getitem__(self, key):
return list.__getitem__(self, key-1)

... etcetera

Cheers, Roald

Stefan Schwarzer
Guest
Posts: n/a

 08-05-2010
Hi Andreas,

On 2010-08-03 12:15, Andreas Pfrengle wrote:
> On 3 Aug., 03:22, Carl Banks <(E-Mail Removed)> wrote:>
> 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).

Yes, that way your problem may shift from inconvenient to
outright hairy.

> 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

def _index_to_level(index):
return index + 1

with this or a different name? This admittedly is longer
than writing `something + 1` but in the latter case you
might wonder what the addition is for, i. e. if it's really
a level offset calculation or something else.

Stefan