Object-Oriented Programming#

Class#

  • Each class has its defined attributes and methods

# Define a class
class Employee:

    # Constructor method
    def __init__(self, first, last, pay):
        self.first = first
        self.last = last
        self.email = first + '.' + last + '@email.com'
        self.pay = pay

    # instance method
    def fullname(self):
        return '{} {}'.format(self.first, self.last)

emp_1 = Employee('Corey', 'Schafer', 50000)
emp_2 = Employee('Test', 'Employee', 60000)

Types of Methods#

  • Class Method: a method bound to the class and with cls as the first default argument

  • Static Method: a self-contained method bound to the class

  • Instance Method: a method bound to the object instance of the class with self as the first default argument

class Employee:

    num_of_emps = 0
    raise_amt = 1.04

    def __init__(self, first, last, pay):
        self.first = first
        self.last = last
        self.email = first + '.' + last + '@email.com'
        self.pay = pay

        Employee.num_of_emps += 1

    def fullname(self):
        return '{} {}'.format(self.first, self.last)

    def apply_raise(self):
        self.pay = int(self.pay * self.raise_amt)

    @classmethod
    def set_raise_amt(cls, amount):
        cls.raise_amt = amount

    @classmethod
    def from_string(cls, emp_str):
        first, last, pay = emp_str.split('-')
        return cls(first, last, pay)

    @staticmethod
    def is_workday(day):
        if day.weekday() == 5 or day.weekday() == 6:
            return False
        return True


emp_1 = Employee('Corey', 'Schafer', 50000)
emp_2 = Employee('Test', 'Employee', 60000)

Employee.set_raise_amt(1.05)

print(Employee.raise_amt)
print(emp_1.raise_amt)
print(emp_2.raise_amt)

emp_str_1 = 'John-Doe-70000'
emp_str_2 = 'Steve-Smith-30000'
emp_str_3 = 'Jane-Doe-90000'

first, last, pay = emp_str_1.split('-')

#new_emp_1 = Employee(first, last, pay)
new_emp_1 = Employee.from_string(emp_str_1)

print(new_emp_1.email)
print(new_emp_1.pay)

import datetime
my_date = datetime.date(2016, 7, 11)

print(Employee.is_workday(my_date))

1.05
1.05
1.05
John.Doe@email.com
70000
True

Class Inheritance#

  • The key of OOP

  • Extend the original class constructor methods using super()

class Employee:

    raise_amt = 1.04

    def __init__(self, first, last, pay):
        self.first = first
        self.last = last
        self.email = first + '.' + last + '@email.com'
        self.pay = pay

    def fullname(self):
        return '{} {}'.format(self.first, self.last)

    def apply_raise(self):
        self.pay = int(self.pay * self.raise_amt)


class Developer(Employee):
    raise_amt = 1.10

    def __init__(self, first, last, pay, prog_lang):
        super().__init__(first, last, pay)
        self.prog_lang = prog_lang


class Manager(Employee):

    def __init__(self, first, last, pay, employees=None):
        super().__init__(first, last, pay)
        if employees is None:
            self.employees = []
        else:
            self.employees = employees

    def add_emp(self, emp):
        if emp not in self.employees:
            self.employees.append(emp)

    def remove_emp(self, emp):
        if emp in self.employees:
            self.employees.remove(emp)

    def print_emps(self):
        for emp in self.employees:
            print('-->', emp.fullname())


dev_1 = Developer('Corey', 'Schafer', 50000, 'Python')
dev_2 = Developer('Test', 'Employee', 60000, 'Java')

mgr_1 = Manager('Sue', 'Smith', 90000, [dev_1])

print(mgr_1.email)

mgr_1.add_emp(dev_2)
mgr_1.remove_emp(dev_2)

mgr_1.print_emps()

Sue.Smith@email.com
--> Corey Schafer

Special Methods#

  • Dunder methods (double-underscore)

  • To avoid overloading the expressions in coding

class Employee:

    raise_amt = 1.04

    def __init__(self, first, last, pay):
        self.first = first
        self.last = last
        self.email = first + '.' + last + '@email.com'
        self.pay = pay

    def fullname(self):
        return '{} {}'.format(self.first, self.last)

    def apply_raise(self):
        self.pay = int(self.pay * self.raise_amt)

    def __repr__(self):
        return "Employee('{}', '{}', {})".format(self.first, self.last, self.pay)

    def __str__(self):
        return '{} - {}'.format(self.fullname(), self.email)

    def __add__(self, other):
        return self.pay + other.pay

    def __len__(self):
        return len(self.fullname())


emp_1 = Employee('Corey', 'Schafer', 50000)
emp_2 = Employee('Test', 'Employee', 60000)

# print(emp_1 + emp_2)

print(len(emp_1))

13

Property Decorator#

  • @property: make a method function like attribute-accessing

  • @NAME.setter: make a method function like class-attribute assigning

  • @NAME.deleter: make a method function like class-attribute deleting

class Employee:

    def __init__(self, first, last):
        self.first = first
        self.last = last

    @property
    def email(self):
        return '{}.{}@email.com'.format(self.first, self.last)

    @property
    def fullname(self):
        return '{} {}'.format(self.first, self.last)
    
    @fullname.setter
    def fullname(self, name):
        first, last = name.split(' ')
        self.first = first
        self.last = last
    
    @fullname.deleter
    def fullname(self):
        print('Delete Name!')
        self.first = None
        self.last = None


emp_1 = Employee('John', 'Smith')
emp_1.fullname = "Corey Schafer"

print(emp_1.first)
print(emp_1.email)
print(emp_1.fullname)

del emp_1.fullname

Checking Functions#

  • isinstance(): Check an instance’s type

  • issubclass(): Check class inheritance

Name Mangling#

  • __NAME: Any identifier of this form within the class is textually replaced with _classname__NAME, where classname is the current class name, with leading underscore(s) stripped.

  • This is for the purpose of creating private variables to the class.

Reference#