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Description
Create a view on a structured array (e.g. selecting a list of columns) causes problems with comparisons when the array is masked.
Example
Create a view on a masked structured array:
import numpy as np
data = np.ma.array([
('Mon', 'Sunny', 30, 20),
('Tue', 'Cloudy', 28, 17),
('Wed', 'Rain', 25, 17),
('Thu', 'Sunny', 29, 18),
('Fri', 'Sunny', 30, 20),
], dtype=[('day','|S3'),('weather','|S10'),('high','i4'),('low','i4')])
data2 = data[['day','high']]
print (data2)[('Mon', 30) ('Tue', 28) ('Wed', 25) ('Thu', 29) ('Fri', 30)]
Try to do a comparison on the array:
print (data2[1:] == data2[:-1])Traceback (most recent call last):
File "test_numpy14.py", line 12, in <module>
print (data2[1:] == data2[:-1])
File "XXX/lib/python2.7/site-packages/numpy/ma/core.py", line 4033, in __eq__
return self._comparison(other, operator.eq)
File "XXX/lib/python2.7/site-packages/numpy/ma/core.py", line 3993, in _comparison
sdata = sbroadcast.filled(odata)
File "XXX/lib/python2.7/site-packages/numpy/ma/core.py", line 3709, in filled
fill_value = _check_fill_value(fill_value, self.dtype)
File "XXX/lib/python2.7/site-packages/numpy/ma/core.py", line 459, in _check_fill_value
raise ValueError(err_msg % (fill_value, fdtype))
ValueError: Unable to transform [('Mon', 30) ('Tue', 28) ('Wed', 25) ('Thu', 29)] to dtype [('day', '|S3'), ('', '|V10'), ('high', '<i4'), ('', '|V4')]
A similar problem happens when calling numpy.ma.unique on the view. The problem persists even after copying the data (e.g. data2 = data2.copy())
I am using numpy 1.14.0 with Python 2.7.6. The problem does not appear in numpy 1.13.3, or when using a regular (non-masked) structured array.
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