Use ensure_ndarray in a few more places (#506)

* Use `ensure_ndarray` to check `chunk`'s type

Instead of checking to see if `chunk` is an `ndarray`, use
`ensure_ndarray` to get an `ndarray` viewing the underlying data in
`chunk`. This way we know attributes like `dtype` are available and can
be checked easily. Also makes this a bit more friendly with other
array-like types.

* Use `ensure_ndarray` on `out`

Since we are interested in getting an `ndarray` representing the buffer
within `out` for writing into, go ahead and use `ensure_ndarray` to
coerce the underlying buffer into an `ndarray`. This way we can avoid a
needless check and just write into any array-like value for `out` that
is provided.

* Handle `out` when it doesn't supply a buffer

Appears that `out` can also be a Zarr `Array` or any other array-like
that does not expose a buffer, but does allow writing into. In these
cases `ensure_ndarray` will fail as there is not an underlying buffer
that can be used with an `ndarray`. To also handle this case, catch the
`TypeError` that `ensure_ndarray` will raise in this case and use that
to indicate whether `out` is now an `ndarray` or not. This allows us to
continue to write into arbitrary buffers, but also correctly handle
objects that do not expose buffers.

* Adds release notes entry

docs/release.rst

@@ -40,6 +40,9 @@ Upcoming Release
 * Use ``math.ceil`` for scalars.
   By :user:`John Kirkham <jakirkham>`; :issue:`500`.
 
+* Use ``ensure_ndarray`` in a few more places.
+  By :user:`John Kirkham <jakirkham>`; :issue:`506`.
+
 * Refactor out ``_tofile``/``_fromfile`` from ``DirectoryStore``.
   By :user:`John Kirkham <jakirkham>`; :issue:`503`.
 

zarr/core.py

@@ -1572,6 +1572,12 @@ def _chunk_getitem(self, chunk_coords, chunk_selection, out, out_selection,
 
         assert len(chunk_coords) == len(self._cdata_shape)
 
+        out_is_ndarray = True
+        try:
+            out = ensure_ndarray(out)
+        except TypeError:
+            out_is_ndarray = False
+
         # obtain key for chunk
         ckey = self._chunk_key(chunk_coords)
 
@@ -1590,7 +1596,7 @@ def _chunk_getitem(self, chunk_coords, chunk_selection, out, out_selection,
 
         else:
 
-            if (isinstance(out, np.ndarray) and
+            if (out_is_ndarray and
                     not fields and
                     is_contiguous_selection(out_selection) and
                     is_total_slice(chunk_selection, self._chunks) and
@@ -1769,7 +1775,7 @@ def _encode_chunk(self, chunk):
                 chunk = f.encode(chunk)
 
         # check object encoding
-        if isinstance(chunk, np.ndarray) and chunk.dtype == object:
+        if ensure_ndarray(chunk).dtype == object:
             raise RuntimeError('cannot write object array without object codec')
 
         # compress