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From: Roald de Vries on 14 Jul 2010 04:32
On Jul 14, 2010, at 3:53 AM, Steven D'Aprano wrote:
> On Tue, 13 Jul 2010 19:26:34 +0200, Roald de Vries wrote:
>
>> Hi all,
>>
>> I have two objects that should both be able to alter a shared
>> float. So
>> i need something like a mutable float object, or a float reference
>> object. Does anybody know if something like that exists? I know
>> it's not
>> hard to build, but I have a feeling that there should be a standard
>> solution to it.
>
> One standard solution would be to wrap the float in a class as an
> attribute.

I think the nicest solution is to do this with a wrapping descriptor
class.

Thanks for your input!

Cheers, Roald

From: Christian Heimes on 14 Jul 2010 05:37
> I know, I just wondered if there is a *standard* solution.

Yeah, you have to reset your brain and switch to Python mode. *scnr*

Seriously, your inquiry sounds like you are trying to code C in Python.
I'm using Python for more than seven years and I've never felt the need
for a mutable float ref or something similar. Well, except for the first
year when my brain was still in C pointer mode. :)

From: Tino Wildenhain on 14 Jul 2010 16:22
Am 13.07.2010 19:26, schrieb Roald de Vries:
> Hi all,
>
> I have two objects that should both be able to alter a shared float.
> So i need something like a mutable float object, or a float reference
> object. Does anybody know if something like that exists? I know it's
> not hard to build, but I have a feeling that there should be a
> standard solution to it.

Nice question to reflect our understanding of the basics of this language.

From what I know, we have the terms:

- reference
- name

(pointer is not really used beside from indices into lists or such)

I see it this way: you have named and unnamed references to objects.
This means you
can bind any number of names to an object as well as reference the
object in a list, tupe or set:

>>> (1,2,3) # unnamed references to int objects 1,2 and 3

>>> a=(1,2,3)[1] # binds a to tuple element #1 (int 2 object)

>>> b=a # now a and b are bound to the same int 2 object

So back to your question:

>>> a=b=1.0 # binds the names a and b to the same float object 1.0

>>> a,b
(1.0, 1.0)

now, there is no way to alter the float object since it is immutable (as
there are str, unicode, int, long, tuple...)

>>> a+=0.5
>>> a,b
(1.5, 1.0)

now you see even the "in place" addition creates a new object and
binds the name to it.

What you'd need to do would probably wrapping the float value into a
class where you can simulate a mutable float by providing a constant
name or reference for it within the class.

For example:

>>> a=b=[1.0]
>>> a[0]+=0.5
>>> a,b
([1.5], [1.5])

HTH
Tino




From: John Nagle on 15 Jul 2010 03:14
On 7/14/2010 12:56 AM, Roald de Vries wrote:
> I know, I just wondered if there is a *standard* solution.

Yes, there is.

class floatref(object) :
def __init__(self, initialval) :
self.__dict__["_val"] = initialval
def __getattr__(self, attrname) :
if attrname == 'val' :
return(self._val)
else :
raise AttributeError(attrname)
def __setattr__(self, attrname, val) :
if attrname == 'val' :
self.__dict__["_val"] = val
else :
raise AttributeError(attrname)


x = floatref(1.0)
y = x

print(x.val)
x.val = 10.0
print(x.val)
print(y.val)


Are you happy now?

John Nagle
From: Tino Wildenhain on 15 Jul 2010 05:48
Hi John,

Am 15.07.2010 09:14, schrieb John Nagle:
> On 7/14/2010 12:56 AM, Roald de Vries wrote:
>> I know, I just wondered if there is a *standard* solution.
>
> Yes, there is.
>
> class floatref(object) :
> def __init__(self, initialval) :
> self.__dict__["_val"] = initialval
> def __getattr__(self, attrname) :
> if attrname == 'val' :
> return(self._val)
> else :
> raise AttributeError(attrname)
> def __setattr__(self, attrname, val) :
> if attrname == 'val' :
> self.__dict__["_val"] = val
> else :
> raise AttributeError(attrname)

Uhm. How would that be different to:

class floatref(object):
def __init__(self,val):
self.val=val


?

>
> x = floatref(1.0)
> y = x
>
> print(x.val)
> x.val = 10.0
> print(x.val)
> print(y.val)


Regards
Tino

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