Classes and Special Methods in Python

These are my study notes on Classes and special methods. As always, if something is wrong then you can always correct me, it would help me and everybody else.

Class ‘Employee’ example

This is the example class that we’re going to use, a class Employee which we will inherit from to create more classes like Manager and Supervisor:

class Employee(object):
    """Class simulating an Employee with basic attributes."""

    total_employees = 0

    def __init__(self, name, rate, position):
        self.name = name
        self.rate = rate
        self.owed = 0
        self.position = position
        Employee.total_employees += 1

Let’s break this up a little bit, we’re creating the class Employee that holds a variable total_employee and every time we initialize our class we will add a +1 to our total employee number. This initialization of this class will require a name, rate and position. Remember that the keyword self refers to the class itself, these variables are shared in the whole class. So far, so good.

Save this code as class_example.py and import it in python:

>>> from class_example import Employee
>>> employee1 = Employee('Anne', 20, 'manager')
>>> employee1.name
'Anne'
>>> employee1.rate
20
>>> employee1.position
'manager'
>>>

Now we can instantiate our class in employee1 and inspect the employee’s name, rate and position.

Initializing a class with __init__

This method let us initialize an instance of our class Employee with x = Class(Args). This way, every time we want time instantiate an object then we have to pass parameters that will act as attributes to out class object. you can see en the last line that every time we initialize our class it will bump the number of total_employees, let’s check this out:

>>> from class_example import Employee
>>> emp1 = Employee('Anne', 20, 'manager')
>>> Employee.total_employees
1
>>> emp2 = Employee('John', 25, 'supervisor')
>>> Employee.total_employees
2
>>> emp3 = Employee('Fran', 30, 'developer')
>>> Employee.total_employees
3

See? Every time a new employee is created, the number of employees goes up.

Changing the String representation with __repr__

If we check the string representation of our class then we will have something like this:

>>> emp1
<class_example.Employee object at 0x7f3809baec50>

To give a more accurate description of our object, we can change the default representation to a string that describe the initialized class more appropriately re-defining the __repr__ method:

class Employee(object):
    """Class simulating an Employee with basic attributes."""

    total_employees = 0
    -- SNIP --
    def __repr__(self):
        return 'Employee object with basic attributes'

Now we can check again to see how our string representation gets displayed:

>>> emp1
Employee object with basic attributes
>>>

Adding more functionality to our class

Let’s add hours worked to our employee and how much do we owe to our employee:

class Employee(object):
    -- SNIP --

    def add_hours_worked(self, total_hours):
        self.owed = total_hours * self.rate
        print('{} work hours where added to {}'.format(total_hours, self.name))

    def pay(self):
        print('Employee {} was paid with {} USD'.format(self.name, self.owed))
        self.owed = 0

with Employee.add_hours_worked(total_hours) method we calculate how much we have to pay to our employee based on the total_hours he has worked. And with the method pay() we let our user know that we have payed the owed amount to our employee and now we owe nothing to him until he has completed more working hours.

Inheritance

Now that we have defined a very generic Employee, we’re going to define other classes such as Developer and Manager that inherits the same properties and methods of the class Employee but change a few ones like the string representation and rate, also let’s add a bonus to his payment.

We’re rewriting add_working_hours() to add a bonus system, if this developer has worked more than 150 hours in a month then a bonus will be added, but we’re not rewriting the whole method,

class Developer(Employee):
    """Employee in charge of development and bug-crushing activities."""

    def __init__(self, name, rate, position):
        super().__init__(name, rate, position)
        self.bonus = 300

    def __repr__(self):
        return self.__doc__

    def add_working_hours(self, total_hours):
        self.owed = total_hours * self.rate
        if total_hours > 150:
            self.owed += self.bonus
        print('{} work hours were added to {}'.format(total_hours, self.name))

This method let us inherits all variables, initializing values and even methods, but what have we changed here exactly? Well, we added a bonus of 300 USD as initialization attribute, also we changed the string representation to the special method __doc__, this means that whenever we inspect the our object, it will return the docstring as our representation:

>>> dev_emp = Developer('Anne', 20, 'developer')
>>> dev_emp
Employee in charge of development and bug-crushing activities.

We also modified add_hours_worked() and added a simple control flow

...
        if total_hours > 150:
            self.owed += self.bonus
...

As you can see here, if our developer has reached more than 150 hours, he can have a bonification of 300USD to his payment, we can see that in action:

>>> dev_emp = Developer('Anne', 20, 'developer')
>>> dev_emp2 = Developer('Johnny', 20, 'developer')
>>> dev_emp.add_hours_worked(160)
160 work hours where added to Anne
>>> dev_emp2.add_hours_worked(130)
130 work hours where added to Johnny
>>> dev_emp.pay()
Employee Anne was paid with 3500 USD
>>> dev_emp2.pay()
Employee Johnny was paid with 2600 USD

The developer ‘Anne’ and ‘Johnny’ earn 20 USD per hour of work, as ‘Anne’ reaches 160 hours of work in her month then she can have a bonification (160 * 20 + 300 = 3500), ‘Johnny’ instead, worked only 130 hours this month, he cannot have his bonification (130 * 20 = 2600).

But have you realized that we payed our Developers and we didn’t set a pay() method to our Developer class? This is because we inherits the methods found in Employee class so we don’t need to rewrite them.

Changing our values directly

We can change our class values directly, for example, if we want to change our employee’s names we can do that easily with:

>>> dev_emp = Developer('Anne', 20, 'developer')
>>> dev_emp.name = 'Not Anne'
>>> dev_emp.name
'Not Anne'

Other Special Methods

Class.__getattrib__(attribute) to return the attribute of a class, beware though, this is the same as Class.attribute which is shorter:

>>> dev_emp.__getattribute__('rate')
20
>>> dev_emp.rate
20

Class.__dict__ to return a dictionary of the Class’ attributes:

>>> dev_emp.__dict__
{'name': 'not anne', 'rate': 20, 'owed': 0, 'position': 'developer', 'bonus': 300}

Class.__module__ to return the module that holds that Class:

>>> dev_emp.__module__
'class_example'

Set attributes to a Class

Setting an attribute to our class is easy: Class.new_attribute = value.
Let’s set the attribute active so we can know if our employee Developer is currently active:

>>> from class_example import *
>>> dev_emp = Developer('Anne', 20, 'developer')
>>> dev_emp.active = True
>>> dev_emp.active
True

These are my study notes for today, any questions feel free to ask.
Here’s the full code used in the examples:

class Employee(object):
    """Class simulating an Employee with basic attributes."""
    total_employees = 0

    def __init__(self, name, rate, position):

        self.name = name
        self.rate = rate
        self.owed = 0
        self.position = position
        Employee.total_employees += 1

    def add_working_hours(self, total_hours):
        self.owed = total_hours * self.rate
        print('{} work hours where added to {}'.format(total_hours, self.name))

    def pay(self):
        print('Employee {} was paid with {}USD'.format(self.name, self.owed))

    def __repr__(self):
        return self.__doc__


class Developer(Employee):
    """Employee in charge of development and bug-crushing activities."""

    def __init__(self, name, rate, position):
        super().__init__(name, rate, position)
        self.bonus = 300

    def __repr__(self):
        return self.__doc__

    def add_working_hours(self, total_hours):
        self.owed = total_hours * self.rate
        if total_hours > 150:
            self.owed += self.bonus
        print('{} work hours were added to {}'.format(total_hours, self.name))