[Numpy-discussion] Scalar casting rules use-case reprise

Dag Sverre Seljebotn d.s.seljebotn@astro.uio...
Fri Jan 4 08:01:15 CST 2013


On 01/04/2013 02:46 PM, Nathaniel Smith wrote:
> On Fri, Jan 4, 2013 at 11:09 AM, Matthew Brett <matthew.brett@gmail.com> wrote:
>> Hi,
>>
>> Reading the discussion on the scalar casting rule change I realized I
>> was hazy on the use-cases that led to the rule that scalars cast
>> differently from arrays.
>>
>> My impression was that the primary use-case was for lower-precision
>> floats. That is, when you have a large float32 arr, you do not want to
>> double your memory use with:
>>
>>>>> large_float32 + 1.0 # please no float64 here
>>
>> Probably also:
>>
>>>>> large_int8 + 1 # please no int32 / int64 here.
>>
>> That makes sense.  On the other hand these are more ambiguous:
>>
>>>>> large_float32 + np.float64(1) # really - you don't want float64?
>>
>>>>> large_int8 + np.int32(1) # ditto
>>
>> I wonder whether the main use-case was to deal with the automatic
>> types of Python floats and scalars?  That is, I wonder whether it
>> would be worth considering (in the distant long term), doing fancy
>> guess-what-you-mean stuff with Python scalars, on the basis that they
>> are of unspecified dtype, and make 0 dimensional scalars follow the
>> array casting rules.  As in:
>>
>>>>> large_float32 + 1.0
>> # no upcast - we don't know what float type you meant for the scalar
>>>>> large_float32 + np.float64(1)
>> # upcast - you clearly meant the scalar to be float64
>
> Hmm, but consider this, which is exactly the operation in your example:
>
> In [9]: a = np.arange(3, dtype=np.float32)
>
> In [10]: a / np.mean(a) # normalize
> Out[10]: array([ 0.,  1.,  2.], dtype=float32)
>
> In [11]: type(np.mean(a))
> Out[11]: numpy.float64
>
> Obviously the most common situation where it's useful to have the rule
> to ignore scalar width is for avoiding "width contamination" from
> Python float and int literals. But you can easily end up with numpy
> scalars from indexing, high-precision operations like np.mean, etc.,
> where you don't "really mean" you want high-precision. And at least
> it's easy to understand the rule: same-kind scalars don't affect
> precision.
>
> ...Though arguably the bug here is that np.mean actually returns a
> value with higher precision. Interestingly, we seem to have some
> special cases so that if you want to normalize each row of a matrix,
> then again the dtype is preserved, but for a totally different
> reasons. In
>
> a = np.arange(4, dtype=np.float32).reshape((2, 2))
> a / np.mean(a, axis=0, keepdims=True)
>
> the result has float32 type, even though this is an array/array
> operation, not an array/scalar operation. The reason is:
>
> In [32]: np.mean(a).dtype
> Out[32]: dtype('float64')
>
> But:
>
> In [33]: np.mean(a, axis=0).dtype
> Out[33]: dtype('float32')
>
> In this respect np.var and np.std behave like np.mean, but np.sum
> always preserves the input dtype. (Which is curious because np.sum is
> just like np.mean in terms of potential loss of precision, right? The
> problem in np.mean is the accumulating error over many addition
> operations, not the divide-by-n at the end.)
>
> It is very disturbing that even after this discussion none of us here
> seem to actually have a precise understanding of how the numpy type
> selection system actually works :-(. We really need a formal
> description...

I think this is a usability wart -- if you don't understand, then 
newcomers certainly don't.

Very naive question:

If one is re-doing this anyway, how important are the primitive 
(non-record) NumPy scalars at all? How much would break if one simply 
always uses Python's int and double, declare that scalars never 
interacts with the dtype?

  a) any computation returning scalars can return float()/int()

  b) float() are silently truncated to float32

  c) integral values that don't fit either wrap around/truncates/raises 
error

  d) the only things that determines dtype is the dtypes of arrays, 
never scalars

Too naive?

I guess the opposite idea is what Travis mentioned in his 
passing-the-torch post, about making scalars and 0-d-arrays the same.

Dag Sverre



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