Merge pull request #21837 from seberg/note-objects-null

DOC: Document expectation for object array initialization

doc/source/reference/c-api/array.rst

@@ -1323,6 +1323,29 @@ User-defined data types
 Special functions for NPY_OBJECT
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. warning::
+
+    When working with arrays or buffers filled with objects NumPy tries to
+    ensure such buffers are filled with ``None`` before any data may be read.
+    However, code paths may existed where an array is only initialized to
+    ``NULL``.
+    NumPy itself accepts ``NULL`` as an alias for ``None``, but may ``assert``
+    non-``NULL`` when compiled in debug mode.
+
+    Because NumPy is not yet consistent about initialization with None,
+    users **must** expect a value of ``NULL`` when working with buffers created
+    by NumPy.  Users **should** also ensure to pass fully initialized buffers
+    to NumPy, since NumPy may make this a strong requirement in the future.
+
+    There is currently an intention to ensure that NumPy always initalizes
+    object arrays before they may be read.  Any failure to do so will be
+    regarded as a bug.
+    In the future, users may be able to rely on non-NULL values when reading
+    from any array, although exceptions for writing to freshly created arrays
+    may remain (e.g. for output arrays in ufunc code).  As of NumPy 1.23
+    known code paths exists where proper filling is not done.
+
+
 .. c:function:: int PyArray_INCREF(PyArrayObject* op)
 
     Used for an array, *op*, that contains any Python objects. It