[SciPy-user] usings numpy arrays in sets

[Robert Kern]

On Thu, Jun 25, 2009 at 08:04, Francesc Alted<faltet at pytables.org> wrote:
> A Thursday 25 June 2009 13:12:44 nicky van foreest escrigué:
>> Dear Francesc,
>>
>> Thanks. Your second suggestion works really nicely! I also thought
>> about the first solution, converting arrays to strings, but even emtpy
>> strings come with a memory penalty of 40 bytes (on my machine).
>
> Hmm, now that I think about this, I ask myself if it would not be useful to
> implement a functional `__hash__()` method for read-only arrays.  Perhaps
> there is a show stopper for that, but I can't see it.

Even if the memory is not writable from your array doesn't mean that
it isn't being modified from another.

In [1]: a = arange(10)

In [2]: b = a[:]

In [3]: b.flags.writeable = False

In [4]: b
Out[4]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

In [5]: a[3:7] = 10

In [6]: a
Out[6]: array([ 0,  1,  2, 10, 10, 10, 10,  7,  8,  9])

In [7]: b
Out[7]: array([ 0,  1,  2, 10, 10, 10, 10,  7,  8,  9])


-- 
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