13.2 Standard Class Methods

Standard Class Methods

__str__ returns the class object formatted as a string (in a readable format). The return type is a string.

__init__ constructs an instance of the class object given some values. The return type is the class itself.


Remember that, in classes, methods defined in the body of the class have the keyword self as their first argument. This is a special argument that allows us to reference variables stored in the object as well as other methods in the class. It is also used to create variables (aka "fields" or "attributes") that store the unique contents of the class. Our usage of self will be absolutely necessary for our implementation of these special methods


class Vector:
	def __init__(self, vals):
		This is the constructor
		self - a reference to the object we are creating
		vals - a list of integers that are the contents of our vector
		self.vals = vals  # we're using the self keyword to create a
		# field, property, or attribute of the class

	def __str__(self):
		String Function

		Converts the object to a string in readable format for
		return str(self.vals)  # return the contents of the vector

vec = Vector([2, 3, 2])
print(str(vec))  # would print [2, 3, 2]

View code on GitHub.



Have your teacher walk you through this one

Build a class called Matrix that takes a list of lists (containing integers) and store it as a field. Add an assertion using assert to ensure that the list of lists is rectangular (i.e. assert len(list_0) = len(list_i) for i in range(n)). You should also implement a __str__ method so that we can print the contents of the matrix using print without having to access its field

Polar Coordinates

Have your teacher walk you through this one as well

Write a class called PolarCoordinates that will take a value called radius and a value called angle. When we print this class, we want the coordinates in Cartesian coordinates, or we want you to print two values: x and y. (If you don't know the conversion formula, x = radius * cos(angle), y = radius * sin(angle). Use Python's built-in math library for the cosine and sine operators. More on Polar vs Cartesian coordinates can be found here.

Matrix Frobenius Norm

Write a modified version of the Matrix class(that was defined in one of the example problems in this section) so that the __str__ method instead returns a string containing a single number: the matrix’s Frobenius norm. The formula for the Frobenius norm will be the square root of the sum of all the elements squared in the matrix. Also, the unmodified Matrix class code will be given.


Write a class called Line which will take the arguments slope and intercept in its constructor. When we print the class, the __str__ method should return a string with the line expressed in the form y=mx+b where m and b are the slope and intercept respectively.

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