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Groups > comp.lang.python > #104670

Descriptors vs Property

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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?


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)
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()

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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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