Classes are collections of methods and attributes. Briefly, attributes are variables of the object (for example, each instance of the Employee class has its own name, age, salary, and benefits; all of them are attributes).

Methods are simply functions that modify attributes (for example, to set the employee name, to set his/her age, and also to read this information from a database or from a CSV list). To create a class, use the class keyword.

In the following example, we will create a class for an incrementer. The purpose of this object is to keep track of the value of an integer and eventually increase it by 1:

class Incrementer(object): 
 def __init__(self): 
 print ("Hello world, I'm the constructor") 
 self._i = 0

Everything within the def indentation is a class method. In this case, the method named __init__ sets the i internal variable to zero (it looks exactly like a function described in the previous chapter). Look carefully at the method's definition. Its argument is self (this is the object itself), and every internal variable's access is made through self:

1.  __init__ is not just a method; it's the constructor (it's called when the object is created). In fact, when we build an Increment object, this method is automatically called, as follows:

i = Incrementer() 
# prints "Hello world, I'm the constructor"

2. Now, let's create the increment() method, which increments the i internal counter and returns the status. Within the class definition, including the method:

def increment(self): 
    self._i += 1 
    return self._i

3. Then, run the following code:

i = Incrementer() 
print (i.increment()) 
print (i.increment()) 
print (i.increment())

4. The preceding code results in the following output:

Hello world, I'm the constructor 
1 
2 
3

Finally, let's see how we can create methods that accept parameters. We will now create the set_counter method, which sets the _i internal variable:

1. Within the class definition, add the following code:

def set_counter(self, counter): 
 self._i = counter

2. Then, run the following code:

i = Incrementer() 
i.set_counter(10) 
print (i.increment()) 
print (i._i)

3. The preceding code gives this output:

Hello world, I'm the constructor 
11 
11