[Numpy-discussion] numpy type mismatch

[Charles R Harris]

On Fri, Jun 10, 2011 at 3:43 PM, Benjamin Root <ben.root at ou.edu> wrote:

>
>
> On Fri, Jun 10, 2011 at 3:24 PM, Charles R Harris <
> charlesr.harris at gmail.com> wrote:
>
>>
>>
>> On Fri, Jun 10, 2011 at 2:17 PM, Benjamin Root <ben.root at ou.edu> wrote:
>>
>>>
>>>
>>> On Fri, Jun 10, 2011 at 3:02 PM, Charles R Harris <
>>> charlesr.harris at gmail.com> wrote:
>>>
>>>>
>>>>
>>>> On Fri, Jun 10, 2011 at 1:50 PM, Benjamin Root <ben.root at ou.edu> wrote:
>>>>
>>>>> Came across an odd error while using numpy master.  Note, my system is
>>>>> 32-bits.
>>>>>
>>>>> >>> import numpy as np
>>>>> >>> type(np.sum([1, 2, 3], dtype=np.int32)) == np.int32
>>>>> False
>>>>> >>> type(np.sum([1, 2, 3], dtype=np.int64)) == np.int64
>>>>> True
>>>>> >>> type(np.sum([1, 2, 3], dtype=np.float32)) == np.float32
>>>>> True
>>>>> >>> type(np.sum([1, 2, 3], dtype=np.float64)) == np.float64
>>>>> True
>>>>>
>>>>> So, only the summation performed with a np.int32 accumulator results in
>>>>> a type that doesn't match the expected type.  Now, for even more
>>>>> strangeness:
>>>>>
>>>>> >>> type(np.sum([1, 2, 3], dtype=np.int32))
>>>>> <type 'numpy.int32'>
>>>>> >>> hex(id(type(np.sum([1, 2, 3], dtype=np.int32))))
>>>>> '0x9599a0'
>>>>> >>> hex(id(np.int32))
>>>>> '0x959a80'
>>>>>
>>>>> So, the type from the sum() reports itself as a numpy int, but its
>>>>> memory address is different from the memory address for np.int32.
>>>>>
>>>>>
>>>> One of them is probably a long, print out the typecode, dtype.char.
>>>>
>>>> Chuck
>>>>
>>>>
>>>>
>>> Good intuition, but odd result...
>>>
>>> >>> import numpy as np
>>> >>> a = np.sum([1, 2, 3], dtype=np.int32)
>>> >>> b = np.int32(6)
>>> >>> type(a)
>>> <type 'numpy.int32'>
>>> >>> type(b)
>>> <type 'numpy.int32'>
>>> >>> a.dtype.char
>>> 'i'
>>> >>> b.dtype.char
>>> 'l'
>>>
>>> So, the standard np.int32 is getting listed as a long somehow?  To
>>> further investigate:
>>>
>>>
>> Yes, long shifts around from int32 to int64 depending on the OS. For
>> instance, in 64 bit Windows it's 32 bits while in 64 bit Linux it's 64 bits.
>> On 32 bit systems it is 32 bits.
>>
>> Chuck
>>
>>
> Right, that makes sense.  But, the question is why does sum() put out a
> result dtype that is not identical to the dtype that I requested, or even
> the dtype of the input array?  Could this be an indication of a bug
> somewhere?  Even if the bug is harmless (it was only noticed within the test
> suite of larry), is this unexpected?
>
>
I expect sum is using a ufunc and it acts differently on account of the
cleanup of the ufunc casting rules. And yes, a long *is* int32 on your
machine. On mine

In [4]: dtype('q') # long long
Out[4]: dtype('int64')

In [5]: dtype('l') # long
Out[5]: dtype('int64')

The mapping from C types to numpy width types isn't 1-1. Personally, I think
we should drop long ;) But it used to be the standard Python type in the C
API. Mark has also pointed out the problems/confusion this ambiguity causes
and someday we should probably think it out and fix it. But I don't think it
is the most pressing problem.

Chuck
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