Properties

Motivation

• Pythonicity: attributes should be public

  • Python is a language of consenting adults

  • Private would be a double underscore (__) prefix ⟶ does not play well with inheritance

  class Person:
      'class with attributes left unprotected'
      def __init__(self, firstname, lastname):
          self.firstname = firstname
          self.lastname = lastname
  

• Can add a property later if necessary

  person = Person('Joerg', 'Faschingbauer')
  person.lastname = 'Haubentaucher'    # <--- write access possible but unwanted
  

Readonly Attributes

• @property

• Built-in decorator (Decorators)

• ⟶ read-only attribute

  class Person:
      def __init__(self, firstname, lastname):
          self._firstname = firstname
          self._lastname = lastname
  
      @property
      def firstname(self):
          return self._firstname
  
      @property
      def lastname(self):
          return self._lastname
  

• Attributes are protected

  person = Person('Joerg', 'Faschingbauer')
  person.lastname = 'Haubentaucher'
  

  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  Cell In[4], line 2
        1 person = Person('Joerg', 'Faschingbauer')
  ----> 2 person.lastname = 'Haubentaucher'
  
  AttributeError: property 'lastname' of 'Person' object has no setter
  

And Write Access?

• @attrname.setter

• Decorator around setter method attrname

  class Person:
      def __init__(self, firstname, lastname):
          self._firstname = firstname
          self._lastname = lastname
  
      @property
      def firstname(self):
          return self._firstname
  
      @property
      def lastname(self):
          return self._lastname
  
      @lastname.setter
      def lastname(self, n):
          if n == 'Haubentaucher':
              self._lastname = n
          else:
              raise AttributeError('no!')
  

• Fine grained access control on attribute

  person = Person('Joerg', 'Faschingbauer')
  person.lastname = 'Haubentaucher'
  

  person.lastname = 'Meier'
  

  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  Cell In[7], line 1
  ----> 1 person.lastname = 'Meier'
  
  Cell In[5], line 19, in Person.lastname(self, n)
       17     self._lastname = n
       18 else:
  ---> 19     raise AttributeError('no!')
  
  AttributeError: no!
  

Calculated Attributes

class Person:
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname

    @property
    def fullname(self):
        return f'{self.firstname} {self.lastname}'

person = Person('Joerg', 'Faschingbauer')
person.fullname

'Joerg Faschingbauer'