What is a property object?

Question

I am not new to python, but I have a pretty basic question here.

I was playing around with python and found that there is the type property

>>> property
<type 'property'>

But I have only heard of properties in the function context.

>>> a = property()
<property object at 0x0246C090>

But what about property objects? What are they use? Property methods are not very intuitive or suggestive

>>> dir(a)
['__class__', '__delattr__', '__delete__', '__doc__', '__format__', '__get__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__set__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'deleter', 'fdel', 'fget', 'fset', 'getter', 'setter']

Thank you for the attention!

Answer 1

The property object is what you are actually thinking of as a property. Consider this example:

class Foo(object):
    def __init__(self):
        self._bar = 0

    @property
    def bar(self):
        return self._bar + 5

Foo.bar is a property object which has a __get__ method. When you write something like

x = Foo()
print(x.bar)

the lookup for x.bar finds that type(x).bar has a __get__ method, and so the attribute lookup becomes equivalent to

type(x).bar.__get__(x, type(x))

which produces the value x._bar + 5.

The use of property as a decorator somewhat obscures the fact that bar is a property object. An equivalent defintion is

class Foo(object):
     def __init__(self):
         self._bar = 0

     bar = property(lambda self: self._bar + 5)

which shows more explicitly that you are creating a property object with the given lambda expression as the getter for that property, and binding the object to the class attribute bar.

The property class (along with instance methods, class methods, and static methods) is a specific application of Python's general descriptor protocol, which defines the behavior of class attributes with __get__, __set__, and/or __del__ methods.

Answer 2

class MyClass:
    def __init__(self,*costs):
        self.costs = costs
    def item_cost(self):
        return sum(self.costs)

now you can do

MyClass(1,2,3,4).item_cost() #prints 10

but we can make it a property

class MyClass:
    def __init__(self,*costs):
        self.costs = costs
    @property
    def item_cost(self):
        return sum(self.costs)

and now we can access it as a simple variable

MyClass(1,2,3,4).item_cost

you could also create a setter for the value with

  ...
   @item_cost.setter
   def set_item_cost(self,value):
         pass #do something with value
  ...
 MyClass(1,2,3,4).item_cost = "yellow"

In general I find them to be sort of an anti-pattern... but some folks like em

(side note you could also make it a property using it as a regular function instead of a decorator MyClass.item_cost_prop = property(MyClass.item_cost) )

Answer 3

A property is an attribute object containing a getter and a setter method.