API: Make MaskedArray.mask return a view, rather than the underlying mask

This prevents consumers from reshaping the mask in place, which breaks things

As a result, `x.mask is x.mask` returns `False`, but this was already true of `x.data is x.data`.

May also be related to gh-10270

4 files changed, 20 insertions, 11 deletions

diff --git a/numpy/lib/tests/test_function_base.py b/numpy/lib/tests/test_function_base.py
index 6d32c365a..0702c3856 100644
--- a/numpy/lib/tests/test_function_base.py
+++ b/numpy/lib/tests/test_function_base.py
@@ -945,7 +945,7 @@ class TestGradient(object):
         assert_equal(type(out), type(x))
         # And make sure that the output and input don't have aliased mask
         # arrays
-        assert_(x.mask is not out.mask)
+        assert_(x._mask is not out._mask)
         # Also check that edge_order=2 doesn't alter the original mask
         x2 = np.ma.arange(5)
         x2[2] = np.ma.masked
diff --git a/numpy/ma/core.py b/numpy/ma/core.py
index 90aff6ec8..69fae3512 100644
--- a/numpy/ma/core.py
+++ b/numpy/ma/core.py
@@ -3449,7 +3449,9 @@ class MaskedArray(ndarray):
 
         # We could try to force a reshape, but that wouldn't work in some
         # cases.
-        return self._mask
+        # Return a view so that the dtype and shape cannot be changed in place
+        # This still preserves nomask by identity
+        return self._mask.view()
 
     @mask.setter
     def mask(self, value):
diff --git a/numpy/ma/tests/test_core.py b/numpy/ma/tests/test_core.py
index 1f80ba26d..fb3f1a810 100644
--- a/numpy/ma/tests/test_core.py
+++ b/numpy/ma/tests/test_core.py
@@ -374,12 +374,12 @@ class TestMaskedArray(object):
 
         y2a = array(x1, mask=m, copy=1)
         assert_(y2a._data.__array_interface__ != x1.__array_interface__)
-        #assert_( y2a.mask is not m)
+        #assert_( y2a._mask is not m)
         assert_(y2a._mask.__array_interface__ != m.__array_interface__)
         assert_(y2a[2] is masked)
         y2a[2] = 9
         assert_(y2a[2] is not masked)
-        #assert_( y2a.mask is not m)
+        #assert_( y2a._mask is not m)
         assert_(y2a._mask.__array_interface__ != m.__array_interface__)
         assert_(allequal(y2a.mask, 0))
 
@@ -5203,3 +5203,10 @@ def test_fieldless_void():
     mx = np.ma.array(x, mask=x)
     assert_equal(mx.dtype, x.dtype)
     assert_equal(mx.shape, x.shape)
+
+
+def test_mask_shape_assignment_does_not_break_masked():
+    a = np.ma.masked
+    b = np.ma.array(1, mask=a.mask)
+    b.shape = (1,)
+    assert_equal(a.mask.shape, ())
diff --git a/numpy/ma/tests/test_old_ma.py b/numpy/ma/tests/test_old_ma.py
index 07bfb613f..1c523768d 100644
--- a/numpy/ma/tests/test_old_ma.py
+++ b/numpy/ma/tests/test_old_ma.py
@@ -270,7 +270,7 @@ class TestMa(object):
         y1 = array(x1, mask=m)
         assert_(y1._data is not x1)
         assert_(allequal(x1, y1._data))
-        assert_(y1.mask is m)
+        assert_(y1._mask is m)
 
         y1a = array(y1, copy=0)
         # For copy=False, one might expect that the array would just
@@ -280,19 +280,19 @@ class TestMa(object):
                 y1._mask.__array_interface__)
 
         y2 = array(x1, mask=m3, copy=0)
-        assert_(y2.mask is m3)
+        assert_(y2._mask is m3)
         assert_(y2[2] is masked)
         y2[2] = 9
         assert_(y2[2] is not masked)
-        assert_(y2.mask is m3)
+        assert_(y2._mask is m3)
         assert_(allequal(y2.mask, 0))
 
         y2a = array(x1, mask=m, copy=1)
-        assert_(y2a.mask is not m)
+        assert_(y2a._mask is not m)
         assert_(y2a[2] is masked)
         y2a[2] = 9
         assert_(y2a[2] is not masked)
-        assert_(y2a.mask is not m)
+        assert_(y2a._mask is not m)
         assert_(allequal(y2a.mask, 0))
 
         y3 = array(x1 * 1.0, mask=m)
@@ -318,14 +318,14 @@ class TestMa(object):
         assert_(x[3] is masked)
         assert_(x[4] is masked)
         x[[1, 4]] = [10, 40]
-        assert_(x.mask is m)
+        assert_(x._mask is m)
         assert_(x[3] is masked)
         assert_(x[4] is not masked)
         assert_(eq(x, [0, 10, 2, -1, 40]))
 
         x = array(d, mask=m2, copy=True)
         x.put([0, 1, 2], [-1, 100, 200])
-        assert_(x.mask is not m2)
+        assert_(x._mask is not m2)
         assert_(x[3] is masked)
         assert_(x[4] is masked)
         assert_(eq(x, [-1, 100, 200, 0, 0]))