How can class variables be updated automatically in python?

Question

I am trying to automatically update class variables that are in a fix relation. E.g.

class vector:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
        self.r = (x**2+y**2+z**2)**0.5
        self.theta = tan((x**2+y**2)**0.5/z)
        self.phi = tan(y/x)

If I change the value x of an instance of this class, I want to update the radius and the angles automatically. And if I change an angle or the radius, I want to update the x,y,z components automatically. Is there any way to do this?

Edit: Okay I have the following solution now. Please correct me if there is a mistake or not conventional what I did.

from math import tan, cos, sin

class Vector:
    def __init__(self, x, y, z):
        self._x = x
        self._y = y
        self._z = z
        self._r = (self._x**2+self._y**2+self._z**2)**0.5
        self._theta = tan((self._x**2+self._y**2)**0.5/self._z)
        self._phi = tan(self._y/self._x)

    @property
    def x(self):
        return self._x
    @x.setter
    def x(self, new_x):
        self._x = new_x
        self._r = (self._x**2+self._y**2+self._z**2)**0.5
        self._theta = tan((self._x**2+self._y**2)**0.5/self._z)
        self._phi = tan(self._y/self._x)

    @property
    def y(self):
        return self._y

    @y.setter
    def y(self, new_y):
        self._y = new_y
        self._r = (self._x**2+self._y**2+self._z**2)**0.5
        self._theta = tan((self._x**2+self._y**2)**0.5/self._z)
        self._phi = tan(self._y/self._x)

    @property
    def z(self):
        return self._z

    @z.setter
    def z(self, new_z):
        self._z = new_z
        self._r = (self.x**2+self.y**2+self.z**2)**0.5
        self._theta = tan((self._x**2+self._y**2)**0.5/self._z)
        self._phi = tan(self._y/self._x)

    @property
    def r(self):
        return (self._x**2+self._y**2+self._z**2)**0.5

    @r.setter
    def r(self, new_r):
        self._r = new_r
        self._x = self._r*cos(self._theta)*cos(self._phi)
        self._y = self._r*cos(self._theta)*sin(self._phi)
        self._z = self._r*sin(self._theta)

    @property
    def theta(self):
        return tan((self._x**2+self._y**2)**0.5/self._z)

    @theta.setter
    def theta(self, new_theta):
        self._theta = new_theta
        self._x = self._r*cos(self._theta)*cos(self._phi)
        self._y = self._r*cos(self._theta)*sin(self._phi)
        self._z = self._r*sin(self._theta)

    @property
    def phi(self):
        return tan(self._y/self._x)

    @phi.setter
    def phi(self,new_phi):
        self._phi = new_phi
        self._x = self._r*cos(self._theta)*cos(self._phi)
        self._y = self._r*cos(self._theta)*sin(self._phi)
        self._z = self._r*sin(self._theta)

felipe · Accepted Answer · 2020-05-29 01:11:32Z

2

You are looking for the @property decorator, which sets an object's variable via a function statement.

from math import tan

class Vector:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

    @property
    def r(self):
        return (self.x**2+self.y**2+self.z**2)**0.5

    @property
    def theta(self):
        return tan((self.x**2+self.y**2)**0.5/self.z)

    @property
    def phi(self):
        return tan(self.y/self.x)

Therefore,

v = Vector(1, 2, 3)
v.phi # -2.185039863261519

answered May 29, 2020 at 1:11

felipe

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1 Comment

maximumphysics Over a year ago

That's what I was looking for, thanks. Is there a way to do the same in the other direction? So changing the angles and radius and obtaining the new x,y,z coordinates? I am new to decorator and haven't found anything usable yet.

Ashok Kumar · Accepted Answer · 2020-05-29 01:13:00Z

1

You could rewrite your class as below

import math

class Vector:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

    @property
    def radius(self):
        return (self.x**2+self.y**2+self.z**2)**0.5

    @property
    def theta(self):
        return math.tan((self.x**2+self.y**2)**0.5/self.z)

    @property
    def phi(self):
        return math.tan(self.y/self.x)

answered May 29, 2020 at 1:13

Ashok Kumar

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