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Python: Class vs. Instance variables - Understand the basics of classes and objects
By Guest on 30th October 2022 03:56:26 AM | Syntax: PYTHON | Views: 207


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  1.  
  2. 9.3.5. Class and Instance Variables
  3. -----------------------------------
  4.  
  5. Generally speaking, instance variables are for data unique to each
  6. instance and class variables are for attributes and methods shared by
  7. all instances of the class:
  8.  
  9.    class Dog:
  10.  
  11.        kind = 'canine'         # class variable shared by all instances
  12.  
  13.        def __init__(self, name):
  14.            self.name = name    # instance variable unique to each instance
  15.  
  16.    >>> d = Dog('Fido')
  17.    >>> e = Dog('Buddy')
  18.    >>> d.kind                  # shared by all dogs
  19.    'canine'
  20.    >>> e.kind                  # shared by all dogs
  21.    'canine'
  22.    >>> d.name                  # unique to d
  23.    'Fido'
  24.    >>> e.name                  # unique to e
  25.    'Buddy'
  26.  
  27. As discussed in A Word About Names and Objects, shared data can have
  28. possibly surprising effects with involving *mutable* objects such as
  29. lists and dictionaries. For example, the *tricks* list in the
  30. following code should not be used as a class variable because just a
  31. single list would be shared by all *Dog* instances:
  32.  
  33.    class Dog:
  34.  
  35.        tricks = []             # mistaken use of a class variable
  36.  
  37.        def __init__(self, name):
  38.            self.name = name
  39.  
  40.        def add_trick(self, trick):
  41.            self.tricks.append(trick)
  42.  
  43.    >>> d = Dog('Fido')
  44.    >>> e = Dog('Buddy')
  45.    >>> d.add_trick('roll over')
  46.    >>> e.add_trick('play dead')
  47.    >>> d.tricks                # unexpectedly shared by all dogs
  48.    ['roll over', 'play dead']
  49.  
  50. Correct design of the class should use an instance variable instead:
  51.  
  52.    class Dog:
  53.  
  54.        def __init__(self, name):
  55.            self.name = name
  56.            self.tricks = []    # creates a new empty list for each dog
  57.  
  58.        def add_trick(self, trick):
  59.            self.tricks.append(trick)
  60.  
  61.    >>> d = Dog('Fido')
  62.    >>> e = Dog('Buddy')
  63.    >>> d.add_trick('roll over')
  64.    >>> e.add_trick('play dead')
  65.    >>> d.tricks
  66.    ['roll over']
  67.    >>> e.tricks
  68.    ['play dead']
















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