Re: Question about properties
Frank Millman wrote: > I thought that the main point of using property was to prevent direct > access to the attribute. Not "prevent access to" as much as "add behaviour to". Is this a valid comment, or does it come under the category of 'we are all adults here'? The latter. And the "__" doesn't provide much protection, really (as we'll see below). While experimenting, I came across the following curiosity. I know that prefixing a class attribute with a double-underscore makes it difficult to access the attribute externally. Here is a simple example - class Test(object): ...def __init__(self,x): ...self.x = x ...self.__y = 123 ...def get_y(self): ...return self.__y t = Test(99) t.x 99 t.get_y() 123 t.__y AttributeError: 'Test' object has no attribute '__y' I was surprised that I could do the following - t.__y = 456 t.__y 456 t.get_y() 123 It's not important, but I am curious to know what is going on internally here. hint: >>> dir(t) ['_Test__y', ..., '__y', 'get_y', 'x'] >>> t._Test__y 123 >>> t.__y 456 -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
king kikapu wrote: >On Aug 10, 1:33 pm, Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote: > > >>On Fri, 10 Aug 2007 03:21:29 -0700, king kikapu wrote: >> >> >>>Hi, >>> >>> >>>i read in a book the following code snippet that is dealing with >>>properties: >>> >>> >>>class ProtectAndHideX(object): >>>def __init__(self, x): >>>assert isinstance(x, int), '"x" must be an integer!"' >>>self.__x = ~x >>> >>> >>>def get_x(self): >>>return ~self.__x >>> >>> >>>x = property(get_x) >>> >>> >>>Can anyone please help me understand what the symbol "~" does here ?? >>> >>> >>This has nothing to do with properties. For integer objects ``~`` is the >>bitwise negation or invertion operator. >> >>Ciao, >>Marc 'BlackJack' Rintsch >> >> > >Xmmm...ok then but what is actually doing there ?? I removed it and >things seems to work the same way... > > > I guess it is the `Hide' part of the Protectand*Hide* class. Gerardo -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
On Aug 10, 5:31 am, [EMAIL PROTECTED] wrote: > On Aug 10, 12:21 pm, king kikapu <[EMAIL PROTECTED]> wrote: > > > > > Hi, > > > i read in a book the following code snippet that is dealing with > > properties: > > > class ProtectAndHideX(object): > > def __init__(self, x): > > assert isinstance(x, int), '"x" must be an integer!"' > > self.__x = ~x > > > def get_x(self): > > return ~self.__x > > > x = property(get_x) > > > Can anyone please help me understand what the symbol "~" does here ?? > > > Thanks for any help! > >>> help(2) > > > | __invert__(...) > | x.__invert__() <==> ~x > > hth. > Duikboot http://docs.python.org/ref/unary.html The unary ~ (invert) operator yields the bit-wise inversion of its plain or long integer argument. The bit-wise inversion of x is defined as -(x+1). It only applies to integral numbers. -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
> Hi, > > i read in a book the following code snippet that is dealing with > properties: > > class ProtectAndHideX(object): > def __init__(self, x): > assert isinstance(x, int), '"x" must be an integer!"' > self.__x = ~x > > def get_x(self): > return ~self.__x > > x = property(get_x) > > > Can anyone please help me understand what the symbol "~" does here ?? My guess is that the example tries to show that it does not matter how the property computes the value. You can -- if you want -- to store integers as their bit-inverted versions (the ~ operator) and then do the conversion when getting the property value. Assume you initialized the object with ProtectAndHideX(4). Outside the object you don't have access to the original __x. And! Even if you changed the name of the variable name to y, you'd have hidden_x.y == -5 instead of 4. The example is very contrived. There might be some security related cases where you need to hide what you store in memory, though. (Hopefully they do more than just invert the bits! :) NB: I don't know what the original author was thinking here -- my telepathy isn't what it used to be. -- [ Antti Rasinen <*> [EMAIL PROTECTED] ] -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
king kikapu wrote: > On Aug 10, 1:33 pm, Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote: >> On Fri, 10 Aug 2007 03:21:29 -0700, king kikapu wrote: >>> Hi, >>> i read in a book the following code snippet that is dealing with >>> properties: >>> class ProtectAndHideX(object): >>> def __init__(self, x): >>> assert isinstance(x, int), '"x" must be an integer!"' >>> self.__x = ~x >>> def get_x(self): >>> return ~self.__x >>> x = property(get_x) >>> Can anyone please help me understand what the symbol "~" does here ?? >> This has nothing to do with properties. For integer objects ``~`` is the >> bitwise negation or invertion operator. >> >> Ciao, >> Marc 'BlackJack' Rintsch > > Xmmm...ok then but what is actually doing there ?? I removed it and > things seems to work the same way... > Observe the name of the class. I believe the integer value is inverted merely as a demonstration that the value can be "obscured" somehow - in a more complex example the author might have insisted in string values, the encrypted them. It's not essential to the example, it merely shows that the value retrieved from the property can be computed from underlying attributes. regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --- Asciimercial -- Get on the web: Blog, lens and tag the Internet Many services currently offer free registration --- Thank You for Reading - -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
Maybe is just a writers' "play" and nothing else. -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
On Aug 10, 1:33 pm, Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote: > On Fri, 10 Aug 2007 03:21:29 -0700, king kikapu wrote: > > Hi, > > > i read in a book the following code snippet that is dealing with > > properties: > > > class ProtectAndHideX(object): > > def __init__(self, x): > > assert isinstance(x, int), '"x" must be an integer!"' > > self.__x = ~x > > > def get_x(self): > > return ~self.__x > > > x = property(get_x) > > > Can anyone please help me understand what the symbol "~" does here ?? > > This has nothing to do with properties. For integer objects ``~`` is the > bitwise negation or invertion operator. > > Ciao, > Marc 'BlackJack' Rintsch Xmmm...ok then but what is actually doing there ?? I removed it and things seems to work the same way... -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
On Fri, 10 Aug 2007 03:21:29 -0700, king kikapu wrote: > Hi, > > i read in a book the following code snippet that is dealing with > properties: > > class ProtectAndHideX(object): > def __init__(self, x): > assert isinstance(x, int), '"x" must be an integer!"' > self.__x = ~x > > def get_x(self): > return ~self.__x > > x = property(get_x) > > > Can anyone please help me understand what the symbol "~" does here ?? This has nothing to do with properties. For integer objects ``~`` is the bitwise negation or invertion operator. Ciao, Marc 'BlackJack' Rintsch -- http://mail.python.org/mailman/listinfo/python-list
Re: Question about properties.
On Aug 10, 12:21 pm, king kikapu <[EMAIL PROTECTED]> wrote: > Hi, > > i read in a book the following code snippet that is dealing with > properties: > > class ProtectAndHideX(object): > def __init__(self, x): > assert isinstance(x, int), '"x" must be an integer!"' > self.__x = ~x > > def get_x(self): > return ~self.__x > > x = property(get_x) > > Can anyone please help me understand what the symbol "~" does here ?? > > Thanks for any help! >>> help(2) | __invert__(...) | x.__invert__() <==> ~x hth. Duikboot -- http://mail.python.org/mailman/listinfo/python-list
