Classes

In Python, classes are how data types are defined. This means that defining a Class in Python is synonymous with creating a custom data type.

The purpose and supposed benefits of object orientation and classes is beyond the scope of this reference; suffice to say that if you would like to look into this more, or get a refresher on some OOB premises, https://python-textbok.readthedocs.io/en/1.0/Classes.html may be helpful.

And for a full list of double underlined built-in class methods, see https://docs.python.org/3/reference/datamodel.html#basic-customization.

Basic Class Definition

With that said, here is a class which just contains some data:

class Pet:
   def __init__(self, age, weight, gender):
      self.age = age
      self.weight = weight
      self.gender = gender

To access data in the class, you have to create an instance of said class.

pet1 = Pet(5, 50)
print(pet1.weight)
print(pet1.age)

50
5

The self portion of the class definition is not a keyword, but rather a self referencing identifier which points to the object itself after it is created.

Inheritance

If we wanted to include the Pet class's parameters in a new class:

class Cat(Pet):

   def animalType(self):
      return "Cat"

An instance of this class will inherit the variables listed in Pet, and once created, will also inherit the methods found in the parent (if there were any.)

joe = Cat(5, 30, "Male")
print(joe.animalType())

Cat

There are multiple built-in methods innate to Python classes which can be reimplemented in user-defined classes. These can be used to add extra features to the class, and interface with other built-in functions.

class Dog(Pet):
   def __init__(self, age, weight, gender, name, breed="Unknown")
      # Calling parent class's init method inside new class
      super().__init__(age, weight)
      self.name = name
      self.breed = breed
   
   def __repr__(self):
      return "Dog: %s, %s, age %s" % (self.name, self.gender, self.age)
   
   def __dir__(self):
      return map(str, [self.age, self.weight, self.name, self.gender, self.breed])

The built-ins repr() and dir() can now be used to get a string representation or a directory listing of the class.

bob = Dog(10, 70, "Bob", "Male")
print(repr(bob))
print(dir(bob))

Dog: Male, Bob, age 10
['10', '70', 'Bob', 'Male', 'Unknown']

Be careful when using inheritance— most of the time, it serves only to make things more complicated. The super() function can be used to access parent-class specific data while within the class definition, and the parent class's init can be called inside the derived class, but both of these things only serve to make using the actual objects a more confusing and tiring venture.

Another Example

class Beeper:
   def __init__(self):
      self.counter = 0
      self.volume = 0
   
   def beep(self):
      if self.volume > 5:
         print("Beep.")
      if self.volume > 10:
         print("BEEP!")
      else:
         print("beep")
      
      self.counter += 1

The values of counter and volume will change the outcome of beep().

alarm = Beeper()
alarm.beep()
alarm.volume = 5
alarm.beep()
print("Alarm went off", alarm.counter, "times.")

beep
Beep.
beep
Alarm went off 2 times.

Object as an Iterable

class InfiniteSet:
   def __init__(self):
      self.num = 0
   
   # Called to determine what is being iterated through:
   def __iter__(self):
      return self
   
   # Called to determine what the data should look like in sequence:    
   def __next__(self):
      self.num += 1
      return self.num

for i in InfiniteSet():
   if i <= 500000:
      print(i, end='r')
   else:
      print()
      break

The above will display every number from 0 to 500000 on one line, and then stop at 500000 and continue on to the next line.

500000

These are only the basics— classes and object orientation are extremely complicated topics, and Python uses them profusely, and in somewhat different ways than languages such as C++ tend to do. For more information about how data is handled in Python, see https://docs.python.org/3/reference/datamodel.html#objects-values-and-types