Classes and Objects in Python

1. A Python dictionary works like this

   >>> d = {'a': 1, 'b': 2, 'c': 88}
   >>> d
   {'a': 1, 'c': 88, 'b': 2}
   >>> d['a']
   1
   >>> d['c']
   88
   >>>
   

2. A Python module fruit.py works like this:

   def shout(fruit):
       print('I LOVE ' + fruit.upper() + '!!!')
   
   favourite = 'pomegranites'
   

   Then

   >>> import fruit
   
   >>> fruit.shout('bananas')
   I LOVE BANANAS!!!
   
   >>> fruit.shout('apples')
   I LOVE APPLES!!!
   
   >>> fruit.favourite
   'pomegranites'
   
   >>> fruit.shout(fruit.favourite)
   I LOVE POMEGRANITES!!!
   

3. Like a module, a Python class contains definitions and variables

   class Fruit:
       def __init__(self, fav):
           self.favourite = fav
   
       def shout(self, fruit):
           print("I LOVE " + fruit.upper() + "!!!")
   
       def shoutd(self, fruit=None):
           if not fruit:
               fruit = self.favourite
           print("I LOVE " + fruit.upper() + "!!!")
   

   Then

   >>> import fruit_class
   >>> f = fruit_class.Fruit('mango')
   >>> f.shout('apple')
   I LOVE APPLE!!!
   >>> f.shout()
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: shout() takes exactly 2 arguments (1 given)
   >>> f.shoutd('pear')
   I LOVE PEAR!!!
   >>> f.shoutd()
   I LOVE MANGO!!!
   

   Note how self refers to the object itself and how you need to pass self to each method of the class.

4. Classes can inherit behaviour from each other

   class Generalist:
   
       def __init__(self, param):
           print("Generalist:", param)
   
       def method(self, param):
           print("Generalist.method:", param)
   
       def othermethod(self, param):
           print("Generalist.othermethod:", param)
   
   class Specialist(Generalist):
   
       def __init__(self, param):
           super(Specialist, self).__init__(param)
           print("Specialist:", param)
   
       def method(self, param):
           Generalist.method(self, param)
           print("Specialist.method:", param)
   

   then

   >>> g = Generalist('jack')
   Generalist: jack
   >>> s = Specialist('jill')
   Generalist: jill
   Specialist: jill
   >>> g.method('a')
   Generalist.method: a
   >>> g.othermethod('a')
   Generalist.othermethod: a
   >>> s.method('a')
   Generalist.method: a
   Specialist.method: a
   >>> s.othermethod('a')
   Generalist.othermethod: a
   >>>
   

5. Here is a more useful Python class. It's for a queue

   class Queue:
       def __init__(self):
           self.items = []
   
       def isEmpty(self):
           return self.items == []
   
       def enqueue(self, item):
           self.items.insert(0,item)
   
       def dequeue(self):
           return self.items.pop()
   
       def size(self):
           return len(self.items)
   

   The Queue class can be used like this:

   q = Queue()
   q.enqueue('cat')
   q.enqueue('dog')
   q.enqueue(3)
   q.enqueue(4)
   print(q.dequeue())
   print(q.dequeue())
   print(q.dequeue())
   del q
   

6. This is object-oriented programming which encompasses some or all of the notions of encapsulation, data hiding, inheritance and classes. Note that Python does not enforce data hiding.