[ctypes-users] Extending a ctype to put into a structure

[Guillaume P. <gui...@sc...>]

Hi,

I wanted to do the following thing: Create a subclass of c_int that 
could be used to transparently get a formatted string from the integer 
when accessing the value property. However it does not work as expected 
if used from a structure.


class usdollars(c_int):
    def valueget(self):
        intval=super(usdollars,self).value
        return self._format(intval)
    def valueset(self,newvalue):
        setattr(super(usdollars,self),'value',newvalue)
    value=property(valueget,valueset)
    def _format(self,value):
        s='%03d'%value
        decpart=s[-2:]
        if int(decpart)==0:
            decpart=''
        else:
            decpart='.'+decpart
       
        integerpart=s[:-2]
        parts=[]
        while len(integerpart)>3:
            parts.append(integerpart[-3:])
            integerpart=integerpart[:-3]
       
        parts.append(integerpart)
        parts.reverse()
        return '$'+','.join(parts)+decpart           
    def __repr__(self):
        return "usdollars(%s)"%self.valueget()


Then I use it with
 >>> a=usdollars(100000)
 >>> a.value
'$1,000'

but if i do:
 >>> class test(Structure):
...     _fields_ = [ ('money',usdollars) ]
...
 >>> b=test()
 >>> b.money=100000
 >>> b.money
100000

And not '$1,000' as I hoped.
also assigning the attribute value does not work as hoped
 >>> a.value=1000
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 6, in valueset
TypeError: 'super' object has only read-only attributes (assign to .value)


How do ctypes Structure implement its properties?

It is not a big issue because easy to workaround. It would be fantastic 
though to inherit from ctypes native types!

Regards,

Guillaume

-- 
Guillaume Proux　（ギヨム・プル)
アジア事業部　シニア　マネージャー 
Scala Inc. -- http://www.scala.com/