property decorator?

[Rob Gaddi]

On 12/20/2017 03:56 PM, Irv Kalb wrote:
> I am trying  to work through the concept of the @property decorator with respect to object oriented programming.
> 
> I believe that I understand how the @property and @<variableName>.setter work - and that they are used to turn what looks like direct access to instance variables into method calls in an object.    It seems like these two decorators are essentially fancy substitutes for traditional getter and setter methods.  But, I'm having a hard time understanding why these names were chosen.
>
 > [snip]
> 
> My questions about this are really historical.  From my reading, it looks like using an @property decorator is a reference to an older approach using a built in "property" function.  But here goes:
> 
> 1) Why were these decorator names chosen?  These two names @property and @<name>.setter don't seem to be very clear to me.  At a minimum, they don't match.  Wouldn't decorator names like @<name>.getter and @<name>.setter have been better - more explicit?
> 
> 2)  Alternatively, if the getter was going to use the @property decorator, then it would seem complimentary  to have the decorator name for the associated setter function to have the word "property" in its also, something like @propertySetter.
> 
> 3)  If I create a method with the @property decorator, is there anything else that is implied about the name of the method other than you can now refer to the <objectName>.<name> - which calls the appropriate method?  My guess/understanding is that in my example above, "salary" is considered the property name, which is how you would refer to it outside of the object. Inside the class, you use the property name as the name of both the setter and the getter methods.  Is that the right way to think about it?
> 
> 
> Finally, it seems very odd to me that when you use the @property decorator and the @<variableName>.setter, that both of the methods that are decorated need to have the same name (but of course different set of parameters.)  As a teacher, this seems like it would be an extremely difficult concept to get across to students, as this does not work the same way as other Python functions (and methods).  Without a decorator, the second function of the same name overrides an earlier function of the same name, as in this simple example:
>
 > [snip]
 >
> Irv
> 

It's because of how Python implements decorators.  When I write

class C:
   def foobar(self):
     pass

I create a binding in the local scope of class C with the name "foobar" 
that holds a function object.  We call this a "local variable" in 
keeping with other languages, but it's not really the same as it would 
be in like C, it's a reference in the local namespace to some other 
object.  When I decorate that foobar instead:

class C:
   @property
   def foobar(self):
     pass

That variable now, instead of referencing a function object, references 
an instance of type 'property' that holds a reference to the original 
foobar method as it's "_getter".  When I expand that to

class C:
   @property
   def foobar(self):
     return self.__foobar

   @foobar.setter
   def foobar(self, value):
     self.__foobar = value

The second decoration calls the 'setter' method of the property instance 
stored in variable name foobar.  That method assigns the new method to 
the "_setter" of that property instance and returns the updated 
property, which is assigned redundantly to the variable name foobar.


-- 
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order.  See above to fix.