How to Write a Class in Python

Introduction to Classes

1. How to Write a Class in Python Picture 1

   Open Python IDE. You can learn how to do this in Install Python.

2. Use keyword class, followed by a space, the name of the class, and a colon.

   class Duck: 

3. Indent and add basic variables for class. To do this, press ↵ Enter or ⏎ Return. Indent and write out a basic variable followed by an equal sign, and then your variable surrounded in quotes.

   class Duck: says = "Quack" gender = "Male" name = "Richie" 

4. Access the variables by creating instances of the class.

   1. In python, the dot notation is used to access methods and/or variables defined in the class.
   2. An example is shown below.

   class Duck: says = "Quack" gender = "Male" name = "Richie" myDuck = Duck() # Create instance of Duck class what = myDuck.says # Will access says variable of class Duck and # assign it to the variable "what" print(what) # Will print "Quack" 

5. Add functions to the class (these are called methods of the class).

   1. This is where the functionality of classes and their ability to store values can be seen.

   class Duck: says = "Quack" gender = "Male" name = "Richie" def fly(): print('Duck flies') 

6. Call the method of the class; in this case, Duck.

   1. Methods use the dot notation as well:
   2. Just like a normal function, use parenthesis to call the method of myDuck

   class Duck: says = "Quack" gender = "Male" name = "Richie" def fly(): print('Duck flies') my_Duck = Duck() my_Duck.fly() # Will print "Duck flies" 

7. Change attributes of class.

   class Duck: says = "Quack" gender = "Male" name = "Richie" def fly(): print('Duck flies') my_Duck = Duck() my_Duck.gender = "Female" # Changes value of variable gender in my_Duck # Now, printing my_Duck.gender will output "Female" 

8. Initialize the Class. Classes run an initializing function every time the programmer creates an instance of that class.

   1. To create this function, add a few spaces between the first and second lines of the class and type def __init__(self): on the second line (make sure to indent).
   2. In the Duck example (self explained below):

   class Duck: def __init__(self): self.says = 'Quack' self.gender = "Male" self.name = "Richie" def fly(): print('Duck flies') my_Duck = Duck() # You can still get the variables the same way, but now # they are wrapped in a function - later they will be changed # by other functions in class Duck. 

   The self word is the instance of the Duck class that is being created. This word can be whatever the programmer wishes as long as it is the first argument of the __init__ function.

9. Add Default Arguments to __init__ function. A class that does not take arguments of any kind is clunky. First, type this into the python console after the class definition:

   class Duck: def __init__(self): self.says = 'Quack' self.gender = "Male" self.name = "Richie" def fly(): print('Duck flies') my_Duck = Duck() my_Duck.says = 'I don't want to quack' my_Duck.gender = "Female" my_Duck.name = 'Lizz' new_Duck = Duck() new_Duck.name = 'Dude' new_Duck.says = "IDK" 

   There is a much better way to do the same process - in one line. This will require a little manipulation of the Duck class:

   class Duck: def __init__(self, says='Quack', gender='Male', name='Richie'): self.says = says self.gender = gender self.name = name def fly(): print('Duck flies') 

   Let's delve into this example, beginning with the arguments:

   1. says='Quack', gender='Male', name='Richie' - these are default arguments - if the programmer inputs something else into the function, the argument will take that value instead. If the programmer doesn't input anything, the argument takes on the value assigned to it by the = operator.
   2. Finally, the variables are added to the instance of the class that is created when the programmer calls the class method.
10. Create Instances of Class with Default Variables. For this example, we will re-create the two previous Ducks - my_Duck and new_Duck.

    class Duck: def __init__(self, says='Quack', gender='Male', name='Richie'): self.says = says self.gender = gender self.name = name def fly(): print('Duck flies') my_Duck = Duck('I don't want to quack', 'Female','Lizz') new_Duck = Duck('IDK', name = 'Dude') # or new_Duck = Duck('IDK', 'Male', 'Dude') ''' Previous "chunky" code my_Duck = Duck() my_Duck.says = 'I don't want to quack' my_Duck.gender = "Female" my_Duck.name = 'Lizz' new_Duck = Duck() new_Duck.name = 'Dude' new_Duck.says = "IDK"'''