## Example: Use Data descriptors to create class attributes with
# their own functionality
class MyDescriptor:
  # when a 'MyDescriptor' object is created inside a class this
  # function is called first, it records the class name for later reference
  def __set_name__(self, obj, name):
      # here 'obj' is that class ('MyClass' in our case)
      # 'self' is our 'MyDescriptor' object
      # 'private_name' is our internal access name
      # using '"_" + name' for avoiding name collisions
      self.private_name = "_" + name

  # When a attribute is looked up (using '.' operator), this method is called
  def __get__(self, obj, objtype=None):
      # fetch 'private_name' from 'MyClass' instance using 'getattr()'
      value = getattr(obj, self.private_name)
      print(f"{self.private_name} was accessed")
      return value

  # When a attribute is altered (using '=' operator), this method is called
  def __set__(self, obj, value):
      # here we can decide what is valid value for 'my_var1'
      if self.private_name == "_my_var1":
          if value % 2 == 0:
              raise AttributeError("Not a valid number, require odd number")

      # decide what is valid value for 'my_var2'
      elif self.private_name == "_my_var2":
          if value % 2 != 0:
              raise AttributeError("Not a valid number, require even number")

      # set value to the variable 'private_name' of 'MyClass' instance
      setattr(obj, self.private_name, value)
      print(f"{self.private_name} was altered")

# define a class that will contain our descriptor objects
class MyClass:
  # initialize our descriptor object
  my_var1 = MyDescriptor()
  # create another attribute
  my_var2 = MyDescriptor()

  def __init__(self, var1, var2, var3, var4):
      # calls '__set__()' method of our descriptor to assign the value
      # here we are assigning 'var1' and 'var2' values to our descriptors
      # which are 'my_var1' and 'my_var2'
      self.my_var1 = var1
      self.my_var2 = var2
      # normal variables
      self.my_var3 = var3
      self.my_var4 = var4


## Create a instance of MyClass
my_instance = MyClass(11, 12, 30, 40)  # _my_var1 was altered \
                                     # _my_var2 was altered
# Notice: The print message we set in '__set__()' method is printed


## Check attribute names of our instance
print(my_instance.__dict__)  # {'_my_var1': 11, '_my_var2': 12, \
                           # 'my_var3': 30, 'my_var4': 40}


## Accessing the variables using the '.' operator
# calls the '__get__()' method of our descriptor to get its value
print(my_instance.my_var1)  # _my_var1 was accessed \
                          # 11
print(my_instance.my_var2)  # _my_var2 was accessed \
                          # 12


## Calling normal variables
print(my_instance.my_var3)  # 30
print(my_instance.my_var4)  # 40
# Notice: 'my_var3', 'my_var4' show normal behaviour without any print message


## Now trying to input invalid values according to out descriptors
my_instance.my_var1 = 42  # AttributeError: Not a valid value, require \
                        # odd number
my_instance.my_var2 = 41  # AttributeError: Not a valid value, require \
                        # even number
# Notice: 'AttributeError' is raised due the values didn't match,
# try a valid value