[REPRINT]Properties vs. Getters and Setters

http://www.python-course.eu/python3_properties.php

Our new class means breaking the interface. The attribute x is not available anymore. That's why in Java e.g. people are recommended to use only private attributes with getters and setters, so that they can change the implementation without having to change the interface.

But Python offers a solution to this problem. The solution is called properties!

The class with a property looks like this:

class P:

    def __init__(self,x):
        self.x = x

    @property
    def x(self):
        return self.__x

    @x.setter
    def x(self, x):
        if x < 0:
            self.__x = 0
        elif x > 1000:
            self.__x = 1000
        else:
            self.__x = x

A method which is used for getting a value is decorated with "@property", i.e. we put this line directly in front of the header. The method which has to function as the setter is decorated with "@x.setter". If the function had been called "f", we would have to decorate it with "@f.setter".

Two things are noteworthy: We just put the code line "self.x = x" in the init method and the property method x is used to check the limits of the values. The second interesting thing is that we wrote "two" methods with the same name and a different number of parameters "def x(self)" and "def x(self,x)". We have learned in a previous chapter of our course that this is not possible. It works here due to the decorating:

>>> from p import P
>>> p1 = P(1001)
>>> p1.x
1000
>>> p1.x = -12
>>> p1.x
0
>>>

Alternatively, we could have used a different syntax without decorators to define the property. As you can see, the code is definitely less elegant and we have to make sure that we use the getter function in the init method again:

class P:

    def __init__(self,x):
        self.set_x(x)

    def get_x(self):
        return self.__x

    def set_x(self, x):
        if x < 0:
            self.__x = 0
        elif x > 1000:
            self.__x = 1000
        else:
            self.__x = x

    x = property(get_x, set_x)

There is still another problem in the most recent version. We have now two ways to access or change the value of x: Either by using "p1.x = 42" or by "p1.set_x(42)". This way we are violating one of the fundamentals of Python: "There should be one-- and preferably only one --obvious way to do it."