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

1 files changed, 10 insertions, 0 deletions

diff --git a/doc/release/1.17.0-notes.rst b/doc/release/1.17.0-notes.rst
index 71ad17673..5307da3ba 100644
--- a/doc/release/1.17.0-notes.rst
+++ b/doc/release/1.17.0-notes.rst
@@ -66,6 +66,16 @@ zero::
    >>> np.zeros(10)//1
    array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])
 
+``MaskedArray.mask`` now returns a view of the mask, not the mask itself
+------------------------------------------------------------------------
+Returning the mask itself was unsafe, as it could be reshaped in place which
+would violate expectations of the masked array code. It's behavior is now
+consistent with the ``.data`` attribute, which also returns a view.
+
+The underlying mask can still be accessed with ``._mask`` if it is needed.
+Tests that contain ``assert x.mask is not y.mask`` or similar will need to be
+updated.
+
 Do not lookup ``__buffer__`` attribute in `numpy.frombuffer`
 ------------------------------------------------------------
 Looking up ``__buffer__`` attribute in `numpy.frombuffer` was undocumented and