python-classes-advanced.md

Advanced Python Concepts

Learning Objects

• learn advanced python class concepts
• use default parameters with python classes
• use static class variables
• TODO: add @staticmethod

Default Parameters

Python allows us to provide default values for parameters in any function we provide. Let's write a Point class that has x and y variables. If no x and y values are provided when a Point is initialized x and y should both default to zero.

class Point():
  def __init__(self, x=0, y=0):
    self.x = x
    self.y = y

Add a __str__ method that prints points in the standard tuple format (x, y):

    def __str__(self):
        return '({},{})'.format(self.x, self.y)

Add a method called distance that calculates and returns the distance between the current point and the origin. Use the mathematical distance formula where the distance between a point and the origin is defined as the square root of (x_x + y_y).

Use Python's math module by adding import math at the top:

import math

class Point():
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def distance(self):
        return math.sqrt(self.x**2+self.y**2)

OR leverage the fact that the square root of a number is equivalent to raising that number to the 1/2

  def distance(self):
    return (self.x ** 2 + self.y ** 2) ** .5

Test it:

p0 = Point()
p2 = Point(3, 4)

print(p0.distance())
0.0

print(p2.distance())
5.0

Class Variables

Let's say we wanted to make our distance method a little more robust. Let's change our distance method so instead of just returning the distance between a point and the origin, it can give us the distance between the point it's invoked on, and any other point of our choosing.

Change the distance method to accept a reference to a second Point as an optional parameter. The second point parameter should have a default value of None. The distance formula between two points is the square root of the difference between the two x coordinates (dx) squared, plus the difference between the two y coordinates (dy): sqrt(dx^2 + dy^2)

class Point():
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    # def distance(self):
    #     return math.sqrt(self.x**2+self.y**2)

    def distance(self, p2):
        dx = self.x - p2.x
        dy = self.y - p2.y
        return math.sqrt(dx**2 + dy**2)

# p0 = Point()
# p2 = Point(3, 4)
p3 = Point(6,13)
p4 = Point(1,1)

# print(p0.distance())
# print(p2.distance())
print(p3.distance(p4))

Great! Now, what if we wanted to make this method do both? i.e. we want to have the option of passing in a second point, but we also want the method to give us the distance from the origin if we dont pass in a second point. We can do this by defaulting the p2 parameter to None, then writing a conditional that sets p2 to the origin if a second point isn't passed in.

But first, let's create a class variable called ORIGIN which is a Point object at (0,0) and is accessible by ALL instances of Point. Class variables are available even without creating any instances of a class. In this case, we're creating a class variable that holds an instance of the class itself, so we have to attach it to the class after we've defined the class.

class Point():
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    # def distance(self):
    #     return math.sqrt(self.x**2+self.y**2)

    def distance(self, p2):
        dx = self.x - p2.x
        dy = self.y - p2.y
        return math.sqrt(dx**2 + dy**2)

# attach ORIGIN after the Point class is defined
Point.ORIGIN = Point()

# p0 = Point()
# p2 = Point(3, 4)
p3 = Point(6,13)
p4 = Point(1,1)

# print(p0.distance())
# print(p2.distance())
print(p3.distance(p4))

# we can access ORIGIN through the Point class.
print(Point.ORIGIN)
(0,0)

p4 = Point(3,4)
p5 = Point(3,19)

# Distance defaults to calculating how far away a Point is from ORIGIN
p4.distance()
5.0

# Distance will calculate the distance from one point to another if a
# a second Point is provided as a parameter.
p4.distance(p5)
15.0

Static class methods

TODO: add this