Overriding the new method

**Peter Otten** · Jul 18 '05, 08:16 AM

Re: Overriding the __new__ method

Christoph Groth wrote:
[color=blue]
> Hello,
>
> The essay "Unifying types and classes in Python 2.2"
> http://www.python.org/2.2.1/descrintro.html says in section
> "Overriding the __new__ method":
>
> <quote>
> This class isn't very useful (it's not even the right way to go about
> unit conversions) but it shows how to extend the constructor of an
> immutable type. If instead of __new__ we had tried to override
> __init__, it wouldn't have worked:
>[color=green][color=darkred]
> >>> class inch(float):[/color][/color]
> ... "THIS DOESN'T WORK!!!"
> ... def __init__(self, arg=0.0):
> ... float.__init__( self, arg*0.0254)
> ...[color=green][color=darkred]
> >>> print inch(12)[/color][/color]
> 12.0[color=green][color=darkred]
> >>>[/color][/color]
> </quote>
>
> Well, I tried this with Python 2.2.1 and it _does_ work:[/color]

No, it doesn't.[color=blue]
>
> piglet:~$ python
> Python 2.2.1 (#1, Sep 7 2002, 14:34:30)
> [GCC 2.95.4 20011002 (Debian prerelease)] on linux2
> Type "help", "copyright" , "credits" or "license" for more information.[color=green][color=darkred]
>>>> class inch(float):[/color][/color]
> ... def __init__(self, arg=0.0):
> ... float.__init__( self, arg*0.0254)
> ...[color=green][color=darkred]
>>>> print inch(12)[/color][/color]
> 12.0[/color]

So one inch is one meter?

Python 2.3.3 (#1, Jan 3 2004, 13:57:08)
[GCC 3.2] on linux2
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> class inch(float):[/color][/color][/color]
.... def __new__(cls, v):
.... return float.__new__(c ls, v*0.0254)
....[color=blue][color=green][color=darkred]
>>> inch(12)[/color][/color][/color]
0.3047999999999 9996

See the difference?

Peter

**Christoph Groth** · Jul 18 '05, 08:16 AM

Re: Overriding the __new__ method

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**Gerrit** · Jul 18 '05, 08:16 AM

Re: Overriding the __new__ method

Christoph Groth wrote:[color=blue]
> According to my understanding, if float type's __init__ ignores its
> arguments then the value of `arg' above should be ignored. Instead,
> the constructed object has the value 12. Where is this 12 passed to
> the object being constructed?[/color]

To __new__.
[color=blue][color=green][color=darkred]
>>> class Test(object):[/color][/color][/color]
... def __new__(cls):
... print "calling new"
... return object.__new__( cls)
... def __init__(self):
... print "calling init"
...[color=blue][color=green][color=darkred]
>>> Test()[/color][/color][/color]
calling new
calling init
<__main__.Tes t object at 0xbf491f0c>

...they are both called, but it is constructed in __new__. __new__
returns the object, while __init__ returns nothing. __init__ is actually
called after initialization, because the initialization happens when
object.__new__ is called. Because it is an immutable type, it cannot be
changed after it has been initialized, so __init__ can't thange it.

Gerrit.

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