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Read-only attributes using properties?

Inyeol Lee inyeol.lee at siimage.com
Wed Dec 18 13:58:14 EST 2002 
On Wed, Dec 18, 2002 at 01:04:29AM -0000, James T. Dennis wrote:
> Pedro RODRIGUEZ <pedro_rodriguez at club-internet.fr> wrote:
> > On Thu, 21 Nov 2002 14:27:55 +0100, Roberto Amorim wrote:
> 
> >> I was thinking about trying to use the new properties on Python 2.2 to
> >> implement read-only attributes. So I tried the following:
> 
> >> class MyException(Exception):
> >>       pass
> 
> >> class TProp(object):
> >>       def __init__(self):
> >>               self.a = 0
> >>       def get_a(self):
> >>               return self.a
> >>       def set_a(self, v):
> >>               raise MyException
> >>       a = property(get_a, set_a, None, "Test a")
> >> 
> >> t = TProp()
> >> print t.a
> >> t.a = 5
> 
> >> I was expecting that the script would fail with an exception on the "t.a
> >> = 5" command. However, I was surprised to see the code fail on the
> >> attribution inside __init__ - that is, there is no "inner class scope",
> >> or direct access within the class itself, and no clear way to initialize
> >> the property. If the property is an alias to the real internal variable
> >> (call it size, for instance), it works, but then if I try to add a
> >> __slots__ list excluding the internal var and only adding the external
> >> reference (__slots__=("a")) it stops working again.
> 
> >> Is there any other way to do that?
> 
> >> Thanks in advance,
> >> Roberto
> 
>  Try: 
>  
>  class TProp(object):
>     def __init__(self):
>         self.__dict__[a] = 0
> 
>  instead?
> -- 
> http://mail.python.org/mailman/listinfo/python-list

Common idiom is to separate public read-only attribute 'a' and private
storage attribute '_a'.

class TProp(object):
      def __init__(self):
              self._a = 0
      def get_a(self):
              return self._a
      def set_a(self, v):
              raise MyException
      a = property(get_a, set_a, None, "Test a")

HTH,
Inyeol Lee...
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