property decorator?

[Irv Kalb]

Thanks to Rob, Cameron, Ian, Chris and Kirill for the detailed explanations.

Very helpful,

Irv

> On Dec 20, 2017, at 3:56 PM, Irv Kalb <Irv at furrypants.com> 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. 
> 
> I have the following example code:
> 
> 
> class Employee():
> 
>    def __init__(self, name, salary):
>        self.__name = name
>        self.__salary = salary
> 
>    @property
>    def salary(self):
>        print('Getting salary of', self.__name, 'which is:', self.__salary)
>        return self.__salary
> 
>    @salary.setter
>    def salary(self, newSalary):
>        print('Setting salary of', self.__name, 'to:', newSalary)
>        self.__salary = newSalary
> 
> 
> # Test code:
> employee1 = Employee('Joe Schmoe', 99999)
> employee2 = Employee('Jane Smith', 123456)
> 
> print(employee1.salary)
> print(employee2.salary)
> 
> employee1.salary = 100000
> employee2.salary = 222222
> 
> print(employee1.salary)
> print(employee2.salary)
> 
> 
> When it runs, I get the results I expect:
> 
> Getting salary of Joe Schmoe which is: 99999
> 99999
> Getting salary of Jane Smith which is: 123456
> 123456
> Setting salary of Joe Schmoe to: 100000
> Setting salary of Jane Smith to: 222222
> Getting salary of Joe Schmoe which is: 100000
> 100000
> Getting salary of Jane Smith which is: 222222
> 222222
> 
> 
> 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:
> 
> def myFunction( ):
>    print('In first version of myFunction')
> 
> def myFunction( ):
>    print('In second version of myFunction')
> 
> myFunction()
> 
> Which prints:  In second version of myFunction
> 
> 
> 
> 
> My most recent language before Python was ActionScript 3 (the language of Flash).  It implemented a similar concept of being able to use what appeared to be an explicit data reference which turns into a method call, like this example (from StackOverflow):
> 
> private var _loggy:String;
> 
> public function get loggy ():String
> {
>  return _loggy;
> }
> 
> public function set loggy ( loggy:String ):void
> {
>  // checking to make sure loggy's new value is kosher etc...
>  _loggy = loggy;
> }
> This syntax made the concept easy to understand and implement in an ActionScript class (even though it also uses the same name for both function).
> 
> I do NOT want to start any kind of language flame war.  I am really just trying to understand the reasons behind why this concept is implemented in Python this way, so that I can wrap my head around it and teach it effectively in my classes.
> 
> Irv
> 
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>