[Numpy-discussion] Bytes vs. Unicode in Python3

[Bruce Southey]

On 12/04/2009 10:12 AM, David Cournapeau wrote:
> On Fri, Dec 4, 2009 at 9:23 PM, Francesc Alted<faltet at pytables.org>  wrote:
>    
>> A Thursday 03 December 2009 14:56:16 Dag Sverre Seljebotn escrigué:
>>      
>>> Pauli Virtanen wrote:
>>>        
>>>> Thu, 03 Dec 2009 14:03:13 +0100, Dag Sverre Seljebotn wrote:
>>>> [clip]
>>>>
>>>>          
>>>>> Great! Are you storing the format string in the dtype types as well? (So
>>>>> that no release is needed and acquisitions are cheap...)
>>>>>            
>>>> I regenerate it on each buffer acquisition. It's simple low-level C code,
>>>> and I suspect it will always be fast enough. Of course, we could *cache*
>>>> the result in the dtype. (If dtypes are immutable, which I don't remember
>>>> right now.)
>>>>          
>>> We discussed this at SciPy 09 -- basically, they are not necesarrily
>>> immutable in implementation, but anywhere they are not that is a bug and
>>> no code should depend on their mutability, so we are free to assume so.
>>>        
>> Mmh, the only case that I'm aware about dtype *mutability* is changing the
>> names of compound types:
>>
>> In [19]: t = np.dtype("i4,f4")
>>
>> In [20]: t
>> Out[20]: dtype([('f0', '<i4'), ('f1', '<f4')])
>>
>> In [21]: hash(t)
>> Out[21]: -9041335829180134223
>>
>> In [22]: t.names = ('one', 'other')
>>
>> In [23]: t
>> Out[23]: dtype([('one', '<i4'), ('other', '<f4')])
>>
>> In [24]: hash(t)
>> Out[24]: 8637734220020415106
>>
>> Perhaps this should be marked as a bug?  I'm not sure about that, because the
>> above seems quite useful.
>>      
> Hm, that's strange - I get the same hash in both cases, but I thought
> I took into account names when I implemented the hashing protocol for
> dtype. Which version of numpy on which os are you seeing this ?
>
> David
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>    
Hi,
On the same linux 64-bit Fedora 11, I get the same hash with Python2.4 
and numpy 1.3 but different hashes for Python2.6 and numpy 1.4.

Bruce

Python 2.6 (r26:66714, Jun  8 2009, 16:07:29)
[GCC 4.4.0 20090506 (Red Hat 4.4.0-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import numpy as np
 >>> np.__version__
'1.4.0.dev7750'
 >>> t = np.dtype("i4,f4")
 >>> t
dtype([('f0', '<i4'), ('f1', '<f4')])
 >>> hash(t)
-9041335829180134223
 >>> t.names = ('one', 'other')
 >>> t
dtype([('one', '<i4'), ('other', '<f4')])
 >>> hash(t)
8637734220020415106


Python 2.4.5 (#1, Oct  6 2008, 09:54:35)
[GCC 4.3.2 20080917 (Red Hat 4.3.2-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import numpy as np
 >>> np.__version__
'1.3.0.dev6653'
 >>> t = np.dtype("i4,f4")
 >>> hash(t)
140053539914640
 >>> t.names = ('one', 'other')
 >>> hash(t)
140053539914640