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Re: Descriptors vs Property

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On Fri, Mar 11, 2016 at 10:59 PM, Veek. M <vek.m1234@gmail.com> wrote:
> A property uses the @property decorator and has @foo.setter
> @foo.deleter.
>
> A descriptor follows the descriptor protocol and implements the __get__
> __set__ __delete__ methods.
>
> But they both do essentially the same thing, allow us to do:
> foo = 10
> del foo
> x = foo
>
> So why do we have two ways of doing this?

Properties *are* descriptors. Properties just provide a more natural
syntax for a very common case.

> Also,
> #####################
> class TypedProperty(object):
>     def __init__(self,name,type,default=None):
>         self.name = "_" + name
>         self.type = type
>         self.default = default if default else type()
>
>     def __get__(self,instance,cls):
>         return getattr(instance,self.name,self.default)
>
>     def __set__(self,instance,value):
>         if not isinstance(value,self.type):
>             raise TypeError("Must be a %s" % self.type)
>         setattr(instance,self.name,value)
>
>     def __delete__(self,instance):
>         raise AttributeError("Can't delete attribute")
>
> class Foo(object):
>     name = TypedProperty("name",str)
>     num  = TypedProperty("num",int,42)
>
> In this example, the class TypedProperty defines a descriptor where type
> checking is
> performed when the attribute is assigned and an error is produced if an
> attempt is made
> to delete the attribute. For example:
>
> f = Foo()
> a = f.name           # Implicitly calls Foo.name.__get__(f,Foo)
> f.name = "Guido"     # Calls Foo.name.__set__(f,"Guido")
> del f.name           # Calls Foo.name.__delete__(f)
> ##################################
>
> I didn't follow this. Foo is a composition of TypedProperty.
> You've got a 'Foo' type with two attributes 'name' and 'num'.
> When you do f.name you are actually doing:
> f.name.__get__(self, instance, cls)

More accurately, you're doing
f.__class__.__dict__['name'].__get__(self, instance, cls). But yes,
this is how the descriptor protocol works.

> What the heck??
>
> I didn't follow this example at all.. What is he doing in there?
> Also, what's this bit:
> self.default = default if default else type()

If the default parameter has a truthy value, it gets set to
self.default. Otherwise, the type parameter is called with no
arguments, and the resulting instance is used as self.default instead.

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Descriptors vs Property "Veek. M" <vek.m1234@gmail.com> - 2016-03-12 11:29 +0530
  Re: Descriptors vs Property Chris Angelico <rosuav@gmail.com> - 2016-03-12 17:05 +1100
  Re: Descriptors vs Property Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-11 23:12 -0700
    Re: Descriptors vs Property "Veek. M" <vek.m1234@gmail.com> - 2016-03-12 11:54 +0530
      Re: Descriptors vs Property Chris Angelico <rosuav@gmail.com> - 2016-03-12 17:35 +1100
      Re: Descriptors vs Property Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-11 23:37 -0700
  Re: Descriptors vs Property "Veek. M" <vek.m1234@gmail.com> - 2016-03-12 11:50 +0530
    Re: Descriptors vs Property Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-12 18:13 +0100
      Re: Descriptors vs Property "Veek. M" <vek.m1234@gmail.com> - 2016-03-13 12:48 +0530
        Re: Descriptors vs Property Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-13 10:25 +0100
          Re: Descriptors vs Property "Veek. M" <vek.m1234@gmail.com> - 2016-03-13 15:26 +0530
            Re: Descriptors vs Property Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-13 11:35 +0100
              Re: Descriptors vs Property "Veek. M" <vek.m1234@gmail.com> - 2016-03-13 18:50 +0530
                Re: Descriptors vs Property Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-13 19:15 +0100
                Re: Descriptors vs Property Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-14 15:34 +0000
                Re: Descriptors vs Property Rustom Mody <rustompmody@gmail.com> - 2016-03-14 10:16 -0700
                Re: Descriptors vs Property Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-16 01:53 +0100
  Re: Descriptors vs Property Ethan Furman <ethan@stoneleaf.us> - 2016-03-16 08:15 -0700

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