Objects in Python

Everything in Python is an 'object'.

Defining custom types of objects is easy:

In [1]:
class Employee:
    empCount = 0
    
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary
        Employee.empCount += 1
        
    def fire(self):
        self.salary = 0
        Employee.empCount -= 1

  • empCount is a class variable, and can be accessed inside and outside of the class as Employee.empCount

  • Functions within the class (type) definition are called 'methods'.

  • The first method __init__ is a special method (called the class 'constructor' or initialization method.

  • You define class methods like normal functions, except that the first argument to each method is the explicit self parameter.

    • Objects are created by 'calling' the type like a function.
    • Arguments in this call are passed to the constructor
In [2]:
# This creates the first employee Joe
joe = Employee("Joe",100000)
print('Employee name is ',joe.name)
print(joe.name, 'salary is $', joe.salary)

Employee name is  Joe
Joe salary is $ 100000
In [3]:
# This will create the second employee Marc
marc = Employee("Marc",120000)
print('Employee name is ',marc.name)
print(marc.name, 'salary is $', marc.salary)

Employee name is  Marc
Marc salary is $ 120000
In [4]:
print("Total employee number = ",Employee.empCount)

Total employee number =  2

We can add, remove and modify attributes at any time:

In [5]:
joe.age = 28
joe.salary = 110000

print('Employee name is ',joe.name)
print(joe.name, 'salary is $', joe.salary)
print(joe.name, 'age is', joe.age)

Employee name is  Joe
Joe salary is $ 110000
Joe age is 28

Let's fire Joe.

In [6]:
joe.fire()
In [7]:
joe.salary
Out[7]:
0
In [8]:
print("Total employee number = ",Employee.empCount)

Total employee number =  1

iclicker question

What is the output of the following code snippet?

In [9]:

  • a) Error message, because the function Change can't be called in the __init__ function

  • b) 'Old'

  • c) 'New'

In [ ]: