12.6 More Class Examples¶

Classes can be used to organize data structures, making them easier to maintain. In the Account example, various sources of information about an account are aggregated to a single Account object, and managed by attributes and methods. Without a custom type definition by the class statement, it would be more difficult to write a program that manages such information.

Another example is matrix that can be defined as follows:

class Matrix:
    def __init__(self, l=[]):
        import copy
        self.l=copy.deepcopy(l)

def get_row(self , i):
        return self.l[i][:]

def get_column(self , i):
        s=[]
        for row in self.l:
            s.append(row[i])
        return s

def __mul__(self , a):
        import copy
        if  type(a)==int:
            N=copy.deepcopy(self.l)
            for i in range(len(N)):
                for j in range(len(N[i])):
                    N[i][j]*=a
        return Matrix(N)

def __add__(self , other):
        import copy
        R=copy.deepcopy(self.l)
        S=copy.deepcopy(other.l)
        for i in range(len(R)):
            for j in range(len(R[i])):
                R[i][j]+=S[i][j]
        return Matrix(R)

def __str__(self):
        s=''
        for i in range(len(self.l)):
            for j in range(len(self.l[i])):
                s+=str(self.l[i][j])
                s+='\t'
            s+='\n'
        return s

m1=Matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
m2=Matrix([[1, 0, 0], [0, 1, 0], [0, 0, 1]])
print (m1)
print (m2)

m3=m2*-1
print (m3)

m4=m1+m3
print (m4)

The class Matrix above implements some matrix operations, including get_row, get_column, scaling and addition. It provides a centralized organization of data, and is therefore easier to use compared with a set of global functions that directly handle lists of lists.

The matrix addition and scaling operations are implemented using the special methods __add__ and __mul__, respectively. In __mul__, type checking is performed to decide whether the second operand is an integer or not. The identifier int here represents the int type. If the second operand to __mul__ is an integer, scaling is performed; otherwise the method performs nothing and returns None.