Classes

Classes

Classes are the core of Python.

Instance & Class variables

The difference is:

• Instance variables: for data which is unique to every object

• Class variables: for data shared between different instances of a class

Let’s take a look at an example:

class Cal(object):
    # pi is a class variable
    pi = 3.142

    def __init__(self, radius):
        # self.radius is an instance variable
        self.radius = radius

    def area(self):
        return self.pi * (self.radius ** 2)

a = Cal(32)
a.area()
# Output: 3217.408
a.pi
# Output: 3.142
a.pi = 43
a.pi
# Output: 43

b = Cal(44)
b.area()
# Output: 6082.912
b.pi
# Output: 3.142
b.pi = 50
b.pi
# Output: 50

There are not many issues while using immutable class variables. This is the major reason due to which beginners do not try to learn more about this subject because everything works! If you also believe that instance and class variables can not cause any problem if used incorrectly then check the next example.

class SuperClass(object):
    superpowers = []

    def __init__(self, name):
        self.name = name

    def add_superpower(self, power):
        self.superpowers.append(power)

foo = SuperClass('foo')
bar = SuperClass('bar')
foo.name
# Output: 'foo'

bar.name
# Output: 'bar'

foo.add_superpower('fly')
bar.superpowers
# Output: ['fly']

foo.superpowers
# Output: ['fly']

That is the beauty of the wrong usage of mutable class variables. To make your code safe against this kind of surprise attacks then make sure that you do not use mutable class variables. You may use them only if you know what you are doing.

New style classes

• Old base classes do not inherit from anything

• New style base classes inherit from object

Example:

class OldClass():
    def __init__(self):
        print('I am an old class')

class NewClass(object):
    def __init__(self):
        print('I am a jazzy new class')

old = OldClass()
# Output: I am an old class

new = NewClass()
# Output: I am a jazzy new class

Inheritance from object allows new style classes to utilize some magic:

1. Optimizations like __slots__.

2. super() and descriptors and the likes

Bottom line? Always try to use new-style classes!

Note: Python 3 only has new-style classes. It does not matter whether you subclass from objector not. However it is recommended that you still subclass from object.

Magic Methods (dunder)

Magic methods, commonly called dunder (double underscore) methods.

__init__

It is a class initializer.

Whenever an instance of a class is created its __init__ method is called.

Example:

class GetTest(object):
    def __init__(self):
        print('Greetings!!')
    def another_method(self):
        print('I am another method which is not'
              ' automatically called')

a = GetTest()
# Output: Greetings!!

a.another_method()
# Output: I am another method which is not automatically
# called

__init__ is called immediately after an instance is created.

You can pass arguments to the class during it’s initialization:

class GetTest(object):
    def __init__(self, name):
        print('Greetings!! {0}'.format(name))
    def another_method(self):
        print('I am another method which is not'
              ' automatically called')

a = GetTest('yasoob')
# Output: Greetings!! yasoob

# Try creating an instance without the name arguments
b = GetTest()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() takes exactly 2 arguments (1 given)

__getitem__

Implementing getitem in a class allows its instances to use the [] (indexer) operator.

Example:

class GetTest(object):
    def __init__(self):
        self.info = {
            'name':'Yasoob',
            'country':'Pakistan',
            'number':12345812
        }

    def __getitem__(self,i):
        return self.info[i]

foo = GetTest()

foo['name']
# Output: 'Yasoob'

foo['number']
# Output: 12345812

Without the __getitem__ method we would get this error:

>>> foo['name']

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'GetTest' object has no attribute '__getitem__'