Not sure if this is a bug or not. I am using a fairly recent master branch.

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# Setting up... import numpy as np a = np.zeros((10, 1), dtype=[('foo', 'f4'), ('bar', 'f4'), ('spam', 'f4')]) a['foo'] = np.random.random((10, 1)) a['bar'] = np.random.random((10, 1)) a['spam'] = np.random.random((10, 1)) a array([[(0.8748096823692322, 0.08278043568134308, 0.2463584989309311)], [(0.27129432559013367, 0.9645473957061768, 0.41787904500961304)], [(0.4902191460132599, 0.6772263646125793, 0.07460898905992508)], [(0.13542482256889343, 0.8646988868713379, 0.98673015832901)], [(0.6527929902076721, 0.7392181754112244, 0.5919206738471985)], [(0.11248272657394409, 0.5818713903427124, 0.9287213087081909)], [(0.47561103105545044, 0.48848700523376465, 0.7108170390129089)], [(0.47087424993515015, 0.6080209016799927, 0.6583810448646545)], [(0.08447299897670746, 0.39479559659957886, 0.13520188629627228)], [(0.7074970006942749, 0.8426893353462219, 0.19329732656478882)]], dtype=[('foo', '<f4'), ('bar', '<f4'), ('spam', '<f4')]) b = (a['bar'] > 0.4) b array([[False], [ True], [ True], [ True], [ True], [ True], [ True], [ True], [False], [ True]], dtype=bool) # ---- Boolean indexing of structured array with a (10,1) boolean array

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a[b]['foo'] array([ 0.27129433, 0.49021915, 0.13542482, 0.65279299, 0.11248273, 0.47561103, 0.47087425, 0.707497 ], dtype=float32) # ---- Boolean indexing of structured array with a (10,) boolean array

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a[b[:,0]]['foo'] array([[(0.27129432559013367, 0.9645473957061768, 0.41787904500961304)], [(0.4902191460132599, 0.6772263646125793, 0.07460898905992508)], [(0.13542482256889343, 0.8646988868713379, 0.98673015832901)], [(0.6527929902076721, 0.7392181754112244, 0.5919206738471985)], [(0.11248272657394409, 0.5818713903427124, 0.9287213087081909)], [(0.47561103105545044, 0.48848700523376465, 0.7108170390129089)], [(0.47087424993515015, 0.6080209016799927, 0.6583810448646545)], [(0.7074970006942749, 0.8426893353462219, 0.19329732656478882)]], dtype=[('foo', '<f4'), ('bar', '<f4'), ('spam', '<f4')])

So, if I index with a (10, 1) boolean array, I get back a (N,) shape result (regardless of whether I am accessing a field or not). But, if I index with a (10, ) boolean array, I get back a (N, 1) result. Note that other forms of indexing such as slicing and fancy indexing returns (N, 1) shaped results. Now, admittedly, this is actually consistent with boolean indexing of regular numpy arrays. I just wanted to make sure that this is intentional. This has caused some confusion for me recently when I (perhaps falsely) expected that the result from a boolean index of a structured array would result in a similarly structured array. The use-case was to modify an existing function by removing the unwanted "rows" with a simply boolean index statement instead of a slice. Cheers! Ben Root