Classes¶

Here is a class:

In [1]:
class Dog:
    species = "canis lupus"   # species is a class variable
    
    def __init__(self, name): # __init__ is the constructor
        self.name = name      # self.name is an object variable
    
    def sound(self):          # sound is a method (a method is a function of an object)
        return "Woof, I am " + self.name

Classes are entities used to pack data and functions related to a concept together. They allow to organise projects in smaller entities than the project itself which provides the following advantages:

  • classes are concepts and concepts help humans to understand what they do
  • classes encapsulate data and allows data hiding
  • debugging is easier since you can debug class by class
  • classes are safe because easier to debug and to test than a whole program (and using safe components leads to safe programs)
  • classes can be reused in other projects

It worth to take time to think about a class.

  • What is a Dog?
  • What will I need from that class?
  • It is generic enough to be reused in another project?
  • Can someone else understand it and extend it?

Objects¶

A class is a mold to make objects. To make an object you just have to call the class name with argument if the initialize methods needs some.

An object can use data and functions of the class:

In [2]:
max = Dog('Max')  # max est un objet de la classe Dog

Objets¶

Pour créer un objet, vous devez simplement appeler le nom de la classe avec un argument si les méthodes d'initialisation en ont besoin.

Un objet peut utiliser les données et les fonctions de la classe :

In [3]:
max = Dog('Maximus')

print('%s says "%s"' % (max.name, max.sound()))  # Object attributes are called with a dot
print("He is a", max.species)                    # between the object name and its attribute
print("BTW all dogs are", Dog.species)           # Same for class attributes

Maximus says "Woof, I am Maximus"
He is a canis lupus
BTW all dogs are canis lupus

Vocabulary¶

To be able to talk about classes and object, let's define the vocabulary introduced as comment in the class Dog:

The main trick is that self refers to the object. It is not mandatory to used the word self but it is a convention that I strongly advice to use. By definition, the object is the first argument of methods.

If something involves self then it is related to the object and we have:

  • instance variables (also called object variable)
  • methods which are functions of a class

If it does not invole self then it is related to the class itself and we have:

  • class variables
  • static methods (static since they are the same for all objects)

All these things are attributes of the class and objects. Python says everything which is after a dot . is an attribute.

To summarize:

type variable name function name
object instance variable method
class class variable static method

Static method¶

A static method can be made by two way:

  • a function defined inside the class without arguments
  • a function decorated by @staticmethod (better way, easier to see it is static)
In [4]:
class Dog:
    _classification = ['Animalia',
                      'Chordata',
                      'Synapsida',
                      'Mammalia',
                      'Carnivora',
                      'Caniformia',
                      'Canidae',
                      'Canis',
                      'Canis lupus',
                      'Canis lupus familiaris']
    
    @staticmethod
    def classification(level):   # no reference to self, the object
        return Dog._classification[-level:]
In [5]:
Dog.classification(3)
Out[5]:
['Canis', 'Canis lupus', 'Canis lupus familiaris']

Static methods can be called from objects too:

In [6]:
max = Dog()
max.classification(5)
Out[6]:
['Caniformia', 'Canidae', 'Canis', 'Canis lupus', 'Canis lupus familiaris']

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