BUG: Preserve identity of dtypes in make_mask_descr

Partially addresses #8666

2 files changed, 30 insertions, 7 deletions

diff --git a/numpy/ma/core.py b/numpy/ma/core.py
index ea4a1d85f..ad67c582a 100644
--- a/numpy/ma/core.py
+++ b/numpy/ma/core.py
@@ -1297,14 +1297,19 @@ def _recursive_make_descr(datatype, newtype=bool_):
                 # Prepend the title to the name
                 name = (field[-1], name)
             descr.append((name, _recursive_make_descr(field[0], newtype)))
-        return descr
     # Is this some kind of composite a la (np.float,2)
     elif datatype.subdtype:
-        mdescr = list(datatype.subdtype)
-        mdescr[0] = _recursive_make_descr(datatype.subdtype[0], newtype)
-        return tuple(mdescr)
+        descr = list(datatype.subdtype)
+        descr[0] = _recursive_make_descr(datatype.subdtype[0], newtype)
+        descr = tuple(descr)
     else:
-        return newtype
+        descr = newtype
+
+    new_dtype = np.dtype(descr)
+    if new_dtype == datatype:
+        new_dtype = datatype
+
+    return new_dtype
 
 
 def make_mask_descr(ndtype):
@@ -1340,7 +1345,7 @@ def make_mask_descr(ndtype):
     # Make sure we do have a dtype
     if not isinstance(ndtype, np.dtype):
         ndtype = np.dtype(ndtype)
-    return np.dtype(_recursive_make_descr(ndtype, np.bool))
+    return _recursive_make_descr(ndtype, np.bool)
 
 
 def getmask(a):
diff --git a/numpy/ma/tests/test_core.py b/numpy/ma/tests/test_core.py
index ca1ef16c4..93898c4d0 100644
--- a/numpy/ma/tests/test_core.py
+++ b/numpy/ma/tests/test_core.py
@@ -4000,32 +4000,50 @@ class TestMaskedArrayFunctions(TestCase):
         self.assertTrue(c.flags['C'])
 
     def test_make_mask_descr(self):
-        # Test make_mask_descr
         # Flexible
         ntype = [('a', np.float), ('b', np.float)]
         test = make_mask_descr(ntype)
         assert_equal(test, [('a', np.bool), ('b', np.bool)])
+        assert_(test is make_mask_descr(test))
+
         # Standard w/ shape
         ntype = (np.float, 2)
         test = make_mask_descr(ntype)
         assert_equal(test, (np.bool, 2))
+        assert_(test is make_mask_descr(test))
+
         # Standard standard
         ntype = np.float
         test = make_mask_descr(ntype)
         assert_equal(test, np.dtype(np.bool))
+        assert_(test is make_mask_descr(test))
+
         # Nested
         ntype = [('a', np.float), ('b', [('ba', np.float), ('bb', np.float)])]
         test = make_mask_descr(ntype)
         control = np.dtype([('a', 'b1'), ('b', [('ba', 'b1'), ('bb', 'b1')])])
         assert_equal(test, control)
+        assert_(test is make_mask_descr(test))
+
         # Named+ shape
         ntype = [('a', (np.float, 2))]
         test = make_mask_descr(ntype)
         assert_equal(test, np.dtype([('a', (np.bool, 2))]))
+        assert_(test is make_mask_descr(test))
+
         # 2 names
         ntype = [(('A', 'a'), float)]
         test = make_mask_descr(ntype)
         assert_equal(test, np.dtype([(('A', 'a'), bool)]))
+        assert_(test is make_mask_descr(test))
+
+        # nested boolean types should preserve identity
+        base_type = np.dtype([('a', int, 3)])
+        base_mtype = make_mask_descr(base_type)
+        sub_type = np.dtype([('a', int), ('b', base_mtype)])
+        test = make_mask_descr(sub_type)
+        assert_equal(test, np.dtype([('a', bool), ('b', [('a', bool, 3)])]))
+        assert_(test.fields['b'][0] is base_mtype)
 
     def test_make_mask(self):
         # Test make_mask