BUG: Keep mask when calling methods on 0d arrays.

This also prevents `.squeeze()` from ever returning `masked`, which was incorrect

Fixes gh-9430.

2 files changed, 33 insertions, 10 deletions

diff --git a/numpy/ma/core.py b/numpy/ma/core.py
index 44096a25a..deddacfdc 100644
--- a/numpy/ma/core.py
+++ b/numpy/ma/core.py
@@ -2555,14 +2555,10 @@ def _arraymethod(funcname, onmask=True):
         result = result.view(type(self))
         result._update_from(self)
         mask = self._mask
-        if result.ndim:
-            if not onmask:
-                result.__setmask__(mask)
-            elif mask is not nomask:
-                result.__setmask__(getattr(mask, funcname)(*args, **params))
-        else:
-            if mask.ndim and (not mask.dtype.names and mask.all()):
-                return masked
+        if not onmask:
+            result.__setmask__(mask)
+        elif mask is not nomask:
+            result.__setmask__(getattr(mask, funcname)(*args, **params))
         return result
     methdoc = getattr(ndarray, funcname, None) or getattr(np, funcname, None)
     if methdoc is not None:
diff --git a/numpy/ma/tests/test_core.py b/numpy/ma/tests/test_core.py
index f755fd8b7..e3c35cf81 100644
--- a/numpy/ma/tests/test_core.py
+++ b/numpy/ma/tests/test_core.py
@@ -445,6 +445,12 @@ class TestMaskedArray(object):
         assert_not_equal(y._data.ctypes.data, x._data.ctypes.data)
         assert_not_equal(y._mask.ctypes.data, x._mask.ctypes.data)
 
+    def test_copy_0d(self):
+        # gh-9430
+        x = np.ma.array(43, mask=True)
+        xc = x.copy()
+        assert_equal(xc.mask, True)
+
     def test_copy_on_python_builtins(self):
         # Tests copy works on python builtins (issue#8019)
         assert_(isMaskedArray(np.ma.copy([1,2,3])))
@@ -3185,8 +3191,23 @@ class TestMaskedArrayMethods(object):
         data = masked_array([[1, 2, 3]], mask=[[1, 1, 1]])
         assert_equal(data.squeeze(), [1, 2, 3])
         assert_equal(data.squeeze()._mask, [1, 1, 1])
-        data = masked_array([[1]], mask=True)
-        assert_(data.squeeze() is masked)
+
+        # normal ndarrays return a view
+        arr = np.array([[1]])
+        arr_sq = arr.squeeze()
+        assert_equal(arr_sq, 1)
+        arr_sq[...] = 2
+        assert_equal(arr[0,0], 2)
+
+        # so maskedarrays should too
+        m_arr = masked_array([[1]], mask=True)
+        m_arr_sq = m_arr.squeeze()
+        assert_(m_arr_sq is not np.ma.masked)
+        assert_equal(m_arr_sq.mask, True)
+        m_arr_sq[...] = 2
+        # TODO: mask isn't copied to/from views yet in maskedarray, so we can
+        #       only check the data
+        assert_equal(m_arr.data[0,0], 2)
 
     def test_swapaxes(self):
         # Tests swapaxes on MaskedArrays.
@@ -3369,6 +3390,12 @@ class TestMaskedArrayMethods(object):
 
         assert_equal(MaskedArray.cumsum(marray.T, 0), control.cumsum(0))
 
+    def test_arraymethod_0d(self):
+        # gh-9430
+        x = np.ma.array(42, mask=True)
+        assert_equal(x.T.mask, x.mask)
+        assert_equal(x.T.data, x.data)
+
 
 class TestMaskedArrayMathMethods(object):