On Fri, 12 Mar 2010 11:26:19 am Alan Gauld wrote:
> "spir" <denis.s...@gmail.com> wrote
>
> > The issue is the object (self) is then a unicode one instead of my
> > own type.
>
> I think you need to modify self in __new__
The method signature for __new__ is usually written as:
def __new__(cls, args):
because when __new__ is called, no instance yet exists. __new__ is the
constructor method which creates an instance, so it gets the class as
the first instance.
__new__ can then do one of two things:
(1) return a new instance of your class; or
(2) return something else.
If it returns an instance of your class, Python then automatically calls
the initializer __init__ with that instance as an argument (plus any
other arguments passed to __new__).
>>> class MyClass(object):
... def __new__(cls):
... print "Calling __new__ on object %s" % cls
... return super(MyClass, cls).__new__(cls)
... def __init__(self):
... print "Calling __init__ on object %s" % self
...
>>> o = MyClass()
Calling __new__ on object <class '__main__.MyClass'>
Calling __init__ on object <__main__.MyClass object at 0xb7c6f44c>
>>> o
<__main__.MyClass object at 0xb7c6f44c>
For mutable types, you can modify self inside __init__, but that doesn't
work for immutable objects like unicode, str, int, etc. For them, you
have to do any changes inside __new__ BEFORE creating the instance.
In the second case, where __new__ returns something else, __init__ is
never called:
>>> class AnotherClass(MyClass):
... def __new__(cls):
... ignore = super(AnotherClass, cls).__new__(cls)
... return 42
...
>>> o = AnotherClass()
Calling __new__ on object <class '__main__.AnotherClass'>
>>> o
42
--
Steven D'Aprano
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