Class

Class Definition

class Person(object):
    '''This is a simple example class'''
    count = 0

    def __init__(self, name, age):
        self.name = name
        self.age = age
        Person.count += 1

    def displayPerson(self):
        print('name: ' + self.name + ' age: ' + str(self.age))

__init__ and self, what do they do?

__init__ is like a constructor in Python.

When a class defines an __init__, class instatiation automatically invokes __init__() for the newly-created class instance.

self represents the instance of the object itself

For example, when I call Person('Mike', 22), Python creates an object, and passes it as the first paremeter to the __init__ method. Any additional parameters like ('Mike', 22) will also get passed as arguments.

Class Object: Attribute references and instantiation.

• Attribute references

    Person.count
    Person.displayPerson()

• Instantiation

    mike = Person('Mike', 22)

Instance Object: Attribute references.

    mike.count
    mike.age
    mike.displayPerson()

Class and Instance Variables

Class variables are for attributes and methods shared by all instances.

Instance variables are for data unique to each instance.

class Person(object):
    '''This is a simple example class'''
    count = 0

    def __init__(self, name, age):
        self.name = name
        self.age = age
        Person.count += 1

    def displayPerson(self):
        print('name: ' + self.name + ' age: ' + str(self.age))

mike = Person('Mike', 22)
lily = Person('Lily', 25)

print(mike.count)
print(lily.count)

print(mike.age)
print(lily.age)

The output is

2    # the count shared by all people
2    # the count shared by all people
22   # the age unique to mike
25   # the age unique to lily

3 ways of inheritance

class Base(object):
    def __init__(self):
        print('base class')

class A(Base):
    def __init__(self):
        Base.__init__(self)

class B(Base):
    def __init__(self):
        super(B, self).__init__()

class C(Base):
    def __init__(self):
        super().__init__()

Any differences?

1. In class A, naming the parent class explicitly takes advtange in multiple inheritance cases.
2. In class B, super(B, self).__init()__ is for Python2
3. In class C, super().__init__() is new in Python3, you can invoke super() without arguments.