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~
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