Inherit from array

[TG]

Hi there.

I'm trying to create a simple class called Vector which inherit from
array.

class Vector(array):
def __init__(self,l ength):
"""initiali ze a vector of random floats of size length. floats
are in interval [0;1]"""
array.__init__( self,'f')
for _ in xrange(length):
self.apprend(ra ndom())

but then :[color=blue][color=green][color=darkred]
>>> v = Vector(10)[/color][/color][/color]
TypeError: array() argument 1 must be char, not int

Well, I guess it means array's __init__ method is not called with
proper arguments ... It seems there is a problem with __init__
overloading, like when I call Vector(x), it directly calls __init__
method from array rather than the one defined in Vector class. Anyone
got an idea on this ?

[bruno at modulix]

Re: Inherit from array

TG wrote:[color=blue]
> Hi there.
>
> I'm trying to create a simple class called Vector which inherit from
> array.[/color]

Which array ?

bruno@bousin ~ $ python
Python 2.4.2 (#1, Feb 9 2006, 02:40:32)
[GCC 3.4.5 (Gentoo 3.4.5, ssp-3.4.5-1.0, pie-8.7.9)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> array[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'array' is not defined


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

[TG]

Re: Inherit from array

from array import array
class Vector(array):
def __init__(self,s ize):
print "pouet"
array.__init__( 'f')
print "pouet"

v = Vector('c')
print repr(v)

will output :

pouet
pouet
array('c')

[Philippe Martin]

Re: Inherit from array

I think he did

from array import *


Philippe




bruno at modulix wrote:
[color=blue]
> TG wrote:[color=green]
>> Hi there.
>>
>> I'm trying to create a simple class called Vector which inherit from
>> array.[/color]
>
> Which array ?
>
> bruno@bousin ~ $ python
> Python 2.4.2 (#1, Feb 9 2006, 02:40:32)
> [GCC 3.4.5 (Gentoo 3.4.5, ssp-3.4.5-1.0, pie-8.7.9)] on linux2
> Type "help", "copyright" , "credits" or "license" for more information.[color=green][color=darkred]
>>>> array[/color][/color]
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> NameError: name 'array' is not defined
>
>
> --
> bruno desthuilliers
> python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
> p in 'onurb@xiludom. gro'.split('@')])"[/color]

[TG]

Re: Inherit from array

Obviously, there is something I didn't catch in python's inheritance.

from array import array
class Vector(array):
def __init__(self,s ize):
print self.typecode
array.__init__( self,'f')
[color=blue][color=green][color=darkred]
>>> v = Vector('c')[/color][/color][/color]
c

Here, it says the typecode is 'c' - I thought such an information was
initalized during the array.__init__( self,'f') but obviously I was
wrong.

Maybe the typecode is defined before, during the call to __new__ method
.... But here i'm getting lost.

[bruno at modulix]

Re: Inherit from array

TG wrote:[color=blue]
> Obviously, there is something I didn't catch in python's inheritance.[/color]

Nope. Obviously, array.array doesn't respect the usual rules.
[color=blue]
> from array import array
> class Vector(array):
> def __init__(self,s ize):
> print self.typecode
> array.__init__( self,'f')
>
>[color=green][color=darkred]
>>>>v = Vector('c')[/color][/color]
>
> c
>
> Here, it says the typecode is 'c' - I thought such an information was
> initalized during the array.__init__( self,'f') but obviously I was
> wrong.
>
> Maybe the typecode is defined before, during the call to __new__ method[/color]

I think this must be something along this line.
[color=blue]
> ... But here i'm getting lost.
>[/color]
Let's see :

from array import array

class Vector(array):
def __new__(cls, size):
v = super(Vector, cls).__new__(cl s, 'f')
#print "v is %s" % v
return v
def __init__(self, size):
self.size = size

v = Vector(42)
print v


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

[bruno at modulix]

Re: Inherit from array

Philippe Martin wrote:[color=blue]
>
> bruno at modulix wrote:[color=green]
>>TG wrote:
>>[color=darkred]
>>>Hi there.
>>>
>>>I'm trying to create a simple class called Vector which inherit from
>>>array.[/color]
>>
>>Which array ?[/color]
>
> I think he did
>
> from array import *
>
>[/color]

oops ! Sorry, I forgot this was in the standard lib (well, I never used
this module, so....)

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

[Sion Arrowsmith]

Re: Inherit from array

TG <girodt@gmail.c om> wrote [something like]:[color=blue]
>from array import array
>class Vector(array):
> def __init__(self,s ize):
> array.__init__( 'f')
>
>v = Vector('c')
>print repr(v)
>
>will output :
>
>array('c')[/color]

Is this a case of new-sytle classes being confusing? Because
I'm certainly confused. I guess what's needed is:

class Vector(array):
def __new__(cls, size):
self = array.__new__(a rray, 'f')
...
return self

But how does one determine what classes need to have __init__
overridden and which __new__ when subclassing?

--
\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 at modulix]

Re: Inherit from array

Sion Arrowsmith wrote:[color=blue]
> TG <girodt@gmail.c om> wrote [something like]:[color=green]
>>from array import array[/color]
>[color=green]
>>class Vector(array):
>> def __init__(self,s ize):
>> array.__init__( 'f')
>>
>>v = Vector('c')
>>print repr(v)
>>
>>will output :
>>
>>array('c')[/color]
>
>
> Is this a case of new-sytle classes being confusing?[/color]

Nope. FWIW, array is coded in C, and seems not to follow all standard
conventions...
[color=blue]
> Because
> I'm certainly confused. I guess what's needed is:
>
> class Vector(array):
> def __new__(cls, size):
> self = array.__new__(a rray, 'f')
> ...
> return self[/color]

Yes.
[color=blue]
> But how does one determine what classes need to have __init__
> overridden and which __new__ when subclassing?[/color]

It's the first exemple I see of a mutable type needing this.

NB :

http://www.python.org/doc/2.4.2/ref/customization.html

"""
__new__() is intended mainly to allow subclasses of immutable types
(like int, str, or tuple) to customize instance creation.
"""

Usually, overriding __init__ is the way to go.

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

[TG]

Re: Inherit from array

Hmm ... I'm definitely not a python wizard, but it seems to be quite a
special case that breaks the rules ... unpythonic, isn't it ?

Has anyone seen a PEP on this subject ?

Just in case a troll reads this message : i'm not saying python sucks
or has huge design flaws here ...

[bruno at modulix]

Re: Inherit from array

TG wrote:[color=blue]
> Hmm ... I'm definitely not a python wizard, but it seems to be quite a
> special case that breaks the rules ...[/color]

Yes and no. The primary use case for __new__ was to allow subclassing of
immutable types. array.array is not immutable, but it's still a special
case, in that it enforce type-based restrictions.

I guess that it does so by creating different types based on the
typecode (this is low-level, C stuff), so this can only happen in the
constructor (the object is already created when the initializer is called).
[color=blue]
> unpythonic, isn't it ?[/color]

Not really -> practicallity beats purity !-)

But this should definitively be documented. I don't have time to do so
right now - anybody willing to take care of this ?


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

[Scott David Daniels]

Re: Inherit from array

bruno at modulix wrote:[color=blue]
> TG wrote:[color=green]
>> Hmm ... I'm definitely not a python wizard, but it seems to be quite a
>> special case that breaks the rules ...[/color]
>
> Yes and no. The primary use case for __new__ was to allow subclassing of
> immutable types. array.array is not immutable, but it's still a special
> case, in that it enforce type-based restrictions.[/color]
The alternative is to either:
1) Allow arrays that have no type.
or 2) Allow arrays that change type.
Neither is a tasty alternative. __new__ sets the stuff that
must be invariant (and for an array, that is the element type),
and __init__ does any further initialization (initialization
should be re-runnable). In the case of array, filling the array
with data seems a great use of __init__.


--
-Scott David Daniels
scott.daniels@a cm.org