[Tutor] Use of "getter" and "setter" attributes

[Sydney Shall]

On 08/09/2014 06:01, Danny Yoo wrote:
>>>>> @property
>>>>> def _avatar(self):
>>>>> return self._avatar
>>> Hi Joel,
>>>
>>> The above code looks strange to me.  The method and the field name
>>> should not use the same name.
>> ah! good catch Danny.  I didn't write it, I was commenting on the OP code.
>>
>> But (and maybe this was discussed earlier in the thread), what value
>> is using the property decorator instead of just saving the data to
>> attributes?
>
> Let's first point to documentation that says what "property" is:
>
>     https://docs.python.org/2/library/functions.html#property
>
> If you read it a bit, one of the key terms that should come up is
> "managed attribute".
>
> What do they mean by this?
>
> A managed attribute is an attribute that the class's definition
> actively controls.
>
> Let's use a concrete example: say that we'd like to make sure a
> Person's name is always capitalized.  We might try to enforce this
> capitalization property in the constructor.
>
> ###############################
> class Person(object):
>      def __init__(self, name):
>          self.name = name.title()
>      def greet(self):
>          print("Hi, I'm %s" % self.name)
>
> p = Person("joel goldstick")
> p.greet()
> ###############################
>
> However, this does not stop clients from assigning directly to the name,
>
> #####################
> p.name = "joel goldstick"
> #####################
>
> and therefore breaking a desire to keep the name capitalized.  So this
> might be a problem.
>
> So what we'd like is the following: to make sure that there's some
> kind of program logic that kicks in whenever we assign to Person.name.
>
> In some programming languages, we do this by marking the attribute
> name in some way that makes it clear not to access it directly, and we
> provide "setter" and "getter" methods, the code that can "manage" this
> attribute.
>
> #############################
> class Person(object):
>      def __init__(self, name):
>          self._name = name.title()
>
>      def getName(self):
>          return self._name
>
>      def setName(self, name):
>          self._name = name.title()
>
>      def greet(self):
>          print("Hi, I'm %s" % self._name)
>
> ## client code:
> p = Person("joel goldstick")
> print(p.getName())
> p.greet()
>
> p.setName("juan christian")
> print(p.getName())
> p.greet()
> #############################
>
>
> Python allows us to get "setter" and "getter"-like behavior while
> still allowing client code to use the attribute with what looks like
> direct attribute access:
>
> #################################
> class Person(object):
>      def __init__(self, name):
>          self._name = name.title()
>
>      @property
>      def name(self):
>          return self._name
>
>      @name.setter
>      def name(self, newName):
>          self._name = newName.title()
>
>      def greet(self):
>          print("Hi, I'm %s" % self._name)
>
> ## client code:
> p = Person("joel goldstick")
> print(p.name)
> p.greet()
>
> p.name= "juan christian"
> print(p.name)
> p.greet()
> #################################
>
> where now the client code looks simpler, but the class definition
> still gets to manage the attribute's value.
>
>
> Hopefully that helps to make the suggestions in this thread a little
> more understandable in context.  Python's properties allow us to make
> the client code look direct but still allow for attribute management.
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I was taught to [generally] always use getter and setter, and I have 
found this clear and useful. But I am a beginner, so my question is 
whether there there are any other reasons to use the methods outlined 
above by the always helpful Danny Yoo, in addition to the simplification 
to which he draws attention?

-- 
Sydney Shall
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