when to use __new__, when to use __init__

[Ethan Furman]

On 10/14/2013 03:07 PM, Peter Cacioppi wrote:
> I've dome some reading on the difference between __new__ and __init__, and never really groked it. I just followed the advice that you should almost always use __init__.

Object creation in Python is a two step process:

   - create the object  (aka __new__, and make sure you return the new object! ;)

   - configure the object  (aka __init__)

If the object is immutable, everything has to be done in __new__.

If the object is mutable, then you should split your code along the creation/configuration guidelines of __new__ and 
__init__, even though you could do it all in __new__.  Why?  To make subclassing easier.

As an example, consider the new Enum[1] data type: my personal preference is to not specify the numbers, and to have 
docstrings on the Enum members.  In order to achieve this I have to override __new__ as that is when the class 
structures are created, but I set the docstring in __init__:

==================================================================================
class AutoEnum(Enum):
     """
     Automatically numbers enum members starting from 1.
     Includes support for a custom docstring per member.
     """
     __last_number__ = 0
     def __new__(cls, *args):
         """Ignores arguments (will be handled in __init__."""
         value = cls.__last_number__ + 1
         cls.__last_number__ = value
         obj = object.__new__(cls)
         obj._value_ = value
         return obj
     def __init__(self, *args):
         """Can handle 0 or 1 argument; more requires a custom __init__.
         0  = auto-number w/o docstring
         1  = auto-number w/ docstring
         2+ = needs custom __init__ (don't call this __init__)
         """
         if len(args) == 1 and isinstance(args[0], (str, unicode)):
             self.__doc__ = args[0]
         elif args:
             raise TypeError('%s not dealt with -- need custom __init__' % (args,))
==================================================================================

Now, if I need some other Enum class with auto-numbering, but different arguments I can easily subclass AutoEnum:

=================================================================================
class Rounds(AutoEnum):
     def __init__(self, x_length, y_length):
         self.x_length = x_length
         self.y_length = y_length
     SMALL_CICRLE = 100, 100
     LARGE_ELLIPSE = 5000, 3000
=================================================================================

[1] enum34 is available on PyPI if you aren't able to move to Python3.4.

--
~Ethan~