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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