property.py

class A(object):
  
  def __init__(self, key, value):
    self.key = key
    self.value = value
    
  @property
  def double_value(self):
    return self.value * 2
    
    
alpha = A('hello', 2)
print alpha.key # prints 'hello'
print alpha.value # prints 2
print alpha.double_value # prints 4