Object Representation

Describing Object Representation

• Internally, instances are implemented using a dictionary
• This dictionary is accessible using the __dict__ attribute
• This dictionary contains the data unique to each instance

>>> a = Account('Todd', 10)
>>> a.__dict__
{'balance': 10, 'name': 'Todd'}

• Modifications to an instance are always reflected in __dict__
• Instances are also linked back to their class using __class__
• Classes are linked back to their base class using __bases__

Details about Special Methods

• Whenever an attribute is set using obj.name=value, the special method obj.__setattr__('name', value) is invoked

• The same goes for the following:

  • obj.name=val invokes obj.__setattr__('name', val)
  • del obj.name invokes obj.__delattr__('name')
  • obj.name invokes obj.__getattribute__('name')

Restriction of Setting Instance Attributes

• A class can restrict the set of legal instance attribute names by defining __slots__
• When __slots__ is defined, instance can't assign new attributes unless the attribute is specified in __slots__
• This restriction prevents someone from adding new attributes to instances
• The use of __slots__ can hurt the performance of classes using inheritance

References

• Python Essential Reference