[Numpy-discussion] question about memory ownership

[Robert Kern]

On Sat, Jan 17, 2009 at 22:35, Darren Dale <dsdale24 at gmail.com> wrote:
>
> On Sat, Jan 17, 2009 at 11:23 PM, Robert Kern <robert.kern at gmail.com> wrote:
>>
>> On Sat, Jan 17, 2009 at 22:06, Darren Dale <dsdale24 at gmail.com> wrote:
>> > Hello,
>> >
>> > Would someone be so kind as to explain how to create an ndarray subclass
>> > that owns its own memory? I think RealisticInfoArray at
>> >
>> > http://docs.scipy.org/doc/numpy/user/basics.subclassing.html#basics-subclassing
>> > does not own its own memory, do you have to call ndarray.__new__
>> > directly,
>> > or is there another way?
>>
>> Does it matter? Unless if something actually digs down to grab the
>> base object, the RealisticInfoArray object is the only thing keeping
>> it alive. When it goes away, the underlying array goes away.
>>
>> But take a look at the implementation of memmap for a subclass that
>> calls ndarray.__new__ directly.
>
>
> I'm still working on my physical quantities package, and I want to prevent
> something like the following:
>
> q=[1,2,3,4]*kg*m**2/s**2
> qq=q[:2]
> qq.units=BTU
>
> That would give me qq in units of british thermal units, but it would also
> modify the magnitude of q[:2]. It seems the most straight forward solution
> is to simply disallow in-place modification of units when the array doesn't
> own its own memory.

Ah. I'd take a functional approach to unit conversion, then. You can't
modify units in-place. Memory can't be much of a concern since the
conversion code will have temporaries.

But you could check that .base is an instance of your quantity class.
If it's a regular array, go ahead and modify in-place.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco