Can __new__ prevent __init__ from being called?

[Steven Bethard]

Colin J. Williams wrote:
> This prompts a similar query.  __new__ appears to be intended for 
> immutable objects but it seems to be called as part of constructor 
> process for all instances.

That's because Python has no builtin way of determining whether or not a 
given type is immutable.  If you wanted to, you could define both 
__new__ and __init__, the first to set immutable parts and the second to 
set mutable parts, e.g.:

py> class PartlyMutableTuple(tuple):
...     def __new__(cls, *args, **kwargs):
...         return super(PartlyMutableTuple, cls).__new__(cls, args)
...     def __init__(self, *args, **kwargs):
...         self.__dict__.update(kwargs)
...
py> t = PartlyMutableTuple(1, 2, 3, a=4, b=5)
py> t
(1, 2, 3)
py> t.a, t.b
(4, 5)
py> t.a, t.b = t.b, t.a
py> t.a, t.b
(5, 4)
py> t[0] = 2
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
TypeError: object does not support item assignment

I don't think I'd advise this strategy, but by always calling both 
__new__ and __init__, Python makes it possible...

Steve