Creation and Deletion

Describing Object Creation

• The class method __new__() has the following properties:

  • Invoked to create an instance
  • Creates the self variable

• The class method __init__() has the following properties:

  • Invoked to initialize the attributes of an object
  • Invoked immediately after an object has been created
  • Initializes the instance variables
• Essentially, __init__() is immediately called after __new__()
• The following classes use these special methods:

>>> class Employee(object):
...     def __init__(self, name, salary):
...         self.name = name
...         self.salary = salary
...
...     def __new__(cls, name, salary):
...         if 0 < salary < 10000:
...             instance = object.__new__(cls)
...             return instance
...         else:
...             return None

>>> class Employee2(object):
...     def __new__(cls, name, salary):
...         if 0 < salary < 10000:
...             instance = object.__new__(cls)
...             return instance
...         else:
...             return None

>>> Employee('Joe', 5000).name
'Joe'
>>> Employee2('Joe', 5000).name
AttributeError: 'Employee' object has no attribute 'name'

Describing Object Deletion

• The class method __del__() has the following properties

  • Invoked for an object during program termination
  • Invoked when an object is garbage collected
  • Only defined in situations where resource management is critical
• Essentially, __del__() is automatically called during deletion

• The following are examples of situations when we want to use __del__():

  • Releasing a lock
  • Shutting down a connection
• As stated previously, this is because these situations handle crtical resource management issues

Summarizing Special Methods

References

• Python Essential Reference
• Example of new and init Special Methods
• Use-Cases for del Special Method