Python - How to edit a class object value automatically?

Question

How can I change a class object with some rules when it assigned? for example:

class Foo:
    x=0
    if x < 0:
        x = -1
    else:
        x = 1


p = Foo()
p.x = 1234
print(p.X)

So when I print p.x I expect 1 to be printed. but 1234 is printed. What should I do?

Welcome to stackoverflow!! this might helpes you https://stackoverflow.com/questions/2627002/whats-the-pythonic-way-to-use-getters-and-setters — mukesh kudi, Oct 03 '18 at 12:10
Do you want to set the class property `Foo.x` or the object property `p.x`? — return42, Oct 03 '18 at 12:19

score 1 · Answer 1 · answered Oct 03 '18 at 12:10

The first x = 0 you declare is a class attribute. It is the same for all objects of the class.

When you write p.x, you create an instance attribute, a value of x that belongs to that specific object. That hides the Foo.x in that particular object (that's how the python attribute lookup works).

However, that value remains the default for other objects of the class, eg, if you create a new object

foo2 = Foo()
print(f2.x) # prints 1

Aran-Fey · Accepted Answer · 2018-10-03T12:39:31.937

0

You have to make x a property, with a setter function:

class Foo:
    def __init__(self):
        self.x = 1

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

    @x.setter
    def x(self, value):
        if value < 0:
            value = -1
        else:
            value = 1

        self._x = value

This makes it so the function decorated with @x.setter is executed whenever an assignment to the x attribute is performed:

>>> foo = Foo()
>>> foo.x = 1234
>>> foo.x
1

edited Oct 03 '18 at 12:39

answered Oct 03 '18 at 12:14

Aran-Fey

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I got it. Thanks :) – myregm Oct 03 '18 at 12:19
You should initialize `self.x` in `Foo.__init__` else trying to read `x` before setting it will raise an attribute error. – bruno desthuilliers Oct 03 '18 at 12:37

Python - How to edit a class object value automatically?

2 Answers2