Descriptors vs Property

[Veek. M]

Ian Kelly wrote:

> On Fri, Mar 11, 2016 at 10:59 PM, Veek. M <vek.m1234 at 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.
thanks okay i'll read that and get back
> 
>> 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.
 
But type() just gives me:
TypeError: type() takes 1 or 3 arguments
on py2,3