When is a property needed instead of just exposing the variable in Python?

Question

This is the example of using properties given in the Python documentation:

Class C(object):
    def __init__(self):
        self._x = None

    def getx(self):
        return self._x
    def setx(self, value):
        self._x = value
    def delx(self):
        del self._x
    x = property(getx, setx, delx, "I'm the 'x' property.")

As far as I can tell, this behaves just like a regular property-less instance variable:

Class D(object):
    def __init__(self):
        self.x = None

Of course this doesn't have the ability to add additional logic to the getter or setter, but, since there is no difference in how other code interacts with C and D, you could always add a property later if you needed it without breaking any other code.

I know that this creates a problem if you use mutable types that are declared at the class level, i.e.:

class E(object):
    x = [1,2,3]

Apart from that case, though, is it bad practice to leave out the properties until it they are needed or is it acceptable?

Thanks!

What do you mean by "leave out the properties until they are needed"? — Blender, May 13 '13 at 01:46
Use class D and then change it to the implementation in class C if you needed to actually do something else in the getter or setter — Computerish, May 13 '13 at 01:47
Definitely. If you don't need getters and setters, don't use them. They'll just complicate your class's structure. Also, I would use the `@property` decorator: http://stackoverflow.com/questions/6304040/real-world-example-about-how-to-use-property-feature-in-python — Blender, May 13 '13 at 01:49
I would say the converse - It's bad practise to to clutter your code with properties if they are not necessary — John La Rooy, May 13 '13 at 01:53
possible duplicate of [When and how to use the builtin function property() in python](http://stackoverflow.com/questions/1554546/when-and-how-to-use-the-builtin-function-property-in-python) — Mirzhan Irkegulov, Feb 20 '14 at 04:07

David Marx · Answer 1 · 2013-05-13T02:02:34.627

You use property to evaluate the value an object attribute that is a funciton of other attributes. Consider the following class:

class shape:
    def __init__(self,l,w):
        self.length = l
        self.width  = w
    @property
    def area(self):
        return self.length*self.width

If we change the value of either length or width, the value of area will be updated immediately when we ask for it.

[In:]  s = shape(2,3)

[In:]  print s.length, s.width, s.area
[Out:] 2 3 6

[In:]  s.length = 5

[In:]  print s.length, s.width, s.area
[Out:] 5 3 15

To help clarify how this works, consider the following case: if we try to define area in __init__():

class shape2:
    def __init__(self,l,w):
        self.length = l
        self.width  = w
        self.area = self.length * self.width

area will only be evaluated when the class is instantiated. Changing the values of length or width won't modify the value of area:

[In:]  s = shape2(2,3)

[In:]  print s.length, s.width, s.area
[Out:] 2 3 6

[In:]  s.length = 5

[In:]  print s.length, s.width, s.area
[Out:] 5 3 6

Thanks! I understand the use for properties, I just wanted know check if there was any need for them in simple cases. — Computerish, May 13 '13 at 02:04

score 1 · Accepted Answer · answered May 13 '13 at 01:56

What I think personally is that Python exposes variables as part of its philosophy, so you do not need to make properties out of everything. You can use properties to add logic to what was once a simple variable, or in order to keep the (client) code clean.

Classes like C in are useless outside tutorials/documentation.

(This is my own opinion. I don't know what is "considered" bad practice, as I am not an experienced developer or something)

When is a property needed instead of just exposing the variable in Python?

2 Answers2