[NumPy-Tickets] [NumPy] #1798: Type promotion rules regression

[NumPy Trac]

#1798: Type promotion rules regression
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 Reporter:  mwiebe      |       Owner:  somebody
     Type:  defect      |      Status:  new     
 Priority:  highest     |   Milestone:  1.6.0   
Component:  numpy.core  |     Version:  devel   
 Keywords:              |  
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 [scalar float64]*[array int8] should produce [array float64], but when the
 value of the scalar float64 is small, it may produce [array float16] or
 [array float32]. Here is the current behavior:

 {{{
 >>> 3.1*np.ones(2,dtype=np.int8)
 array([ 3.09960938,  3.09960938], dtype=float16)
 >>> 1e10*np.ones(2,dtype=np.int8)
 array([  1.00000000e+10,   1.00000000e+10], dtype=float32)
 >>> 1e40*np.ones(2,dtype=np.int8)
 array([  1.00000000e+40,   1.00000000e+40])
 }}}

 I propose a fix in two parts.

 1. Fix this in the PyArray_ResultType/numpy.result_type function of the
 type promotion API. In particular, the algorithm should track the promoted
 array and scalar types separately, then if the kind of the promoted scalar
 type is greater than the kind of the promoted array type, continue the
 promotion of the promoted array type with the scalars without using
 minscalartype.

 2. Change the ufunc to detect binary operators during creation, and set a
 flag for it. Then, during evaluation, use PyArray_ResultType to do the
 type promotion for binary operators instead of the existing slow linear
 search. The ufunc doesn't have a flags field, but it does have a field
 called 'check_return' which is unused, and could be repurposed as a flags
 field without changing the size of the python type.

-- 
Ticket URL: <http://projects.scipy.org/numpy/ticket/1798>
NumPy <http://projects.scipy.org/numpy>
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