Properties

https://en.wikipedia.org/wiki/Property_%28programming%29#Python

Attributes are clear and concise

One of the strengths of Python is lack of clutter.

In [5]: class C:
        def __init__(self):
                self.x = 5
In [6]: c = C()
In [7]: c.x
Out[7]: 5
In [8]: c.x = 8
In [9]: c.x
Out[9]: 8

And we want to maintain this clarity.

Getter and Setters

But what if you need to add behavior later?

• do some calculation
• check data validity
• keep things in sync

In [5]: class C:
   ...:     def __init__(self):
   ...:         self.x = 5
   ...:     def get_x(self):
   ...:         return self.x
   ...:     def set_x(self, x):
   ...:         self.x = x
   ...:
In [6]: c = C()
In [7]: c.get_x()
Out[7]: 5
In [8]: c.set_x(8)
In [9]: c.get_x()
Out[9]: 8

This is verbose – Java?

Properties

class C:
    _x = None
    @property
    def x(self):
        return self._x
    @x.setter
    def x(self, value):
        self._x = value

In [28]: c = C()
In [30]: c.x = 5
In [31]: print(c.x)
5

Now the interface is like simple attribute access!

Decorators

What’s up with the “@” symbols?

Those are “decorations” it is a syntax for wrapping functions up with something special.

We will cover decorators in detail in another part of the program, but for now just copy the syntax.

@property
def x(self):

means: make a property called x with this as the “getter”.

@x.setter
def x(self, value):

means: make the “setter” of the ‘x’ property this new function

Read Only Attributes

You do not need to define a setter. If you don’t, you get a “read only” attribute:

In [11]: class D():
   ....:     def __init__(self, x=5):
   ....:         self._x = 5
   ....:     @property
   ....:     def getx(self):
   ....:     """I am read only"""
   ....:         return self._x
   ....:
In [12]: d = D()
In [13]: d.x
Out[13]: 5
In [14]: d.x = 6
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-14-c83386d97be3> in <module>()
----> 1 d.x = 6
AttributeError: can't set attribute

Deleters

If you want to do something special when a property is deleted, you can define a deleter as well:

In [11]: class D():
   ....:     def __init__(self, x=5):
   ....:         self._x = 5
   ....:     @property
   ....:     def x(self):
   ....:         return self._x
   ....:     @x.deleter
   ....:     def x(self):
   ....:         del self._x

If you leave this out, the property can’t be deleted, which is usually what you want.

[demo: properties_example.py]