Fix as_compatible_data for read-only np.ma.MaskedArray (#7788)

* Fix as_compatible_data for non-writeable float MaskedArray

* Add test for #2377

---------

Co-authored-by: dcherian <deepak@cherian.net>

Author: maxhollmann

doc/whats-new.rst

@@ -27,6 +27,8 @@ New Features
 - Add support for lshift and rshift binary operators (``<<``, ``>>``) on
   :py:class:`xr.DataArray` of type :py:class:`int` (:issue:`7727` , :pull:`7741`).
   By `Alan Brammer <https://github.com/abrammer>`_.
+- Fix `as_compatible_data` for masked float arrays, now always creates a copy when mask is present (:issue:`2377`, :pull:`7788`).
+  By `Max Hollmann <https://github.com/maxhollmann>`_.
 
 
 Breaking changes

xarray/core/variable.py

@@ -274,8 +274,7 @@ def as_compatible_data(data, fastpath=False):
         mask = np.ma.getmaskarray(data)
         if mask.any():
             dtype, fill_value = dtypes.maybe_promote(data.dtype)
-            data = np.asarray(data, dtype=dtype)
-            data[mask] = fill_value
+            data = duck_array_ops.where_method(data, ~mask, fill_value)
         else:
             data = np.asarray(data)
 

xarray/tests/test_variable.py

@@ -2560,6 +2560,19 @@ def test_masked_array(self):
         assert_array_equal(expected, actual)
         assert np.dtype(float) == actual.dtype
 
+        original = np.ma.MaskedArray([1.0, 2.0], mask=[True, False])
+        original.flags.writeable = False
+        expected = [np.nan, 2.0]
+        actual = as_compatible_data(original)
+        assert_array_equal(expected, actual)
+        assert np.dtype(float) == actual.dtype
+
+        # GH2377
+        actual = Variable(dims=tuple(), data=np.ma.masked)
+        expected = Variable(dims=tuple(), data=np.nan)
+        assert_array_equal(expected, actual)
+        assert actual.dtype == expected.dtype
+
     @pytest.mark.filterwarnings("ignore:Converting non-nanosecond")
     def test_datetime(self):
         expected = np.datetime64("2000-01-01")