Subclassing array

[TG]

Hi.

i've already something about inheriting from array a few weeks ago and
had my answer. But again, there is something that I don't understand.
Here is my vector class, which works quite well :

class Vector(array):
def __new__(cls,len gth,data=None):
return super(Vector,cl s).__new__(cls, 'f')

def __init__(self,l ength,data=None ):
if data == None:
for _ in xrange(length):
self.append(0.0 )
else:
for i in xrange(length):
self.append(dat a[i])



Now, i want to inherit from this vector class :

class Stimulus(Vector ):
def __init__(self,w idth,height,lab el,data=None):
Vector.__init__ (self,width*hei ght,data)
self.width = width
self.height = height
self.label = label

This doesn't seem to work :[color=blue][color=green][color=darkred]
>>> s = Stimulus(10,10, "data")[/color][/color][/color]
TypeError: __new__() takes at most 3 arguments (4 given)

In order to make it work, it seems that I have to redefine __new__
again, like this.

def __new__(cls,wid th,height,label ,data=None):
return super(Stimulus, cls).__new__(cl s,width*height)

Why is that ?
When I call Vector.__init__ () in Stimulus, doesn't it also call __new__
? I don't understand the detail of callings to __new__ and __init__ in
python inheritance ...

[Alex Martelli]

Re: Subclassing array

TG <girodt@gmail.c om> wrote:
...[color=blue]
> When I call Vector.__init__ () in Stimulus, doesn't it also call __new__
> ? I don't understand the detail of callings to __new__ and __init__ in
> python inheritance ...[/color]

Calling a (new-style) class does __new__ first, THEN calls the class's
__init__ on the resulting instance -- and the arguments you're passing
when calling the class go to both __new__ and __init__.


Alex

[Sion Arrowsmith]

Re: Subclassing array

Alex Martelli <aleaxit@yahoo. com> wrote:[color=blue]
>TG <girodt@gmail.c om> wrote:[color=green]
>> When I call Vector.__init__ () in Stimulus, doesn't it also call __new__
>> ? I don't understand the detail of callings to __new__ and __init__ in
>> python inheritance ...[/color]
>Calling a (new-style) class does __new__ first, THEN calls the class's
>__init__ on the resulting instance -- and the arguments you're passing
>when calling the class go to both __new__ and __init__.[/color]

.... so you might want something like:

class Vector(array):
def __new__(cls,*ar gs):
return super(Vector,cl s).__new__(cls, 'f')

--
\S -- siona@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
___ | "Frankly I have no feelings towards penguins one way or the other"
\X/ | -- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

[Bruno Desthuilliers]

Re: Subclassing array

Sion Arrowsmith a écrit :[color=blue]
> Alex Martelli <aleaxit@yahoo. com> wrote:
>[color=green]
>>TG <girodt@gmail.c om> wrote:
>>[color=darkred]
>>>When I call Vector.__init__ () in Stimulus, doesn't it also call __new__
>>>? I don't understand the detail of callings to __new__ and __init__ in
>>>python inheritance ...[/color]
>>
>>Calling a (new-style) class does __new__ first, THEN calls the class's
>>__init__ on the resulting instance -- and the arguments you're passing
>>when calling the class go to both __new__ and __init__.[/color]
>
>
> ... so you might want something like:
>
> class Vector(array):
> def __new__(cls,*ar gs):
> return super(Vector,cl s).__new__(cls, 'f')
>[/color]
And if you want to support named arguments:

class Vector(array):
def __new__(cls,*ar gs, **kw):
return super(Vector,cl s).__new__(cls, 'f')

[TG]

Re: Subclassing array

That's great, thanks !

To put it short, when I create a Stimulus object, it first seek
__new__() method. But if I don't define it, it looks for the one
defined in Vector. This raises a problem because the parameters passed
to Stimulus(params ) aren't fitting with Vector parameters, raising an
exception.

That's why I have to use this *arg **kw syntax in order to allow my
subclasses having any arguments without causing troubles. Am I right ?

[bruno at modulix]

Re: Subclassing array

TG wrote:[color=blue]
> That's great, thanks !
>
> To put it short, when I create a Stimulus object, it first seek
> __new__() method. But if I don't define it, it looks for the one
> defined in Vector. This raises a problem because the parameters passed
> to Stimulus(params ) aren't fitting with Vector parameters, raising an
> exception.
>
> That's why I have to use this *arg **kw syntax in order to allow my
> subclasses having any arguments without causing troubles. Am I right ?
>[/color]

To put it short : yes !-)

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"