Rechunk where dict has missing axes (cubed-dev#546)

When rechunking with a dict that doesn't contain all axes, then the chunking
should be unchanged for those axes that are missing.

In particular, `a.rechunk({})` should be a no-op.

This is consistent with Dask (dask/dask#11261)
and Xarray (pydata/xarray#9286)

cubed/core/ops.py

@@ -756,6 +756,16 @@ def wrap(*a, block_id=None, **kw):
 
 
 def rechunk(x, chunks, target_store=None):
+    if isinstance(chunks, dict):
+        chunks = {validate_axis(c, x.ndim): v for c, v in chunks.items()}
+        for i in range(x.ndim):
+            if i not in chunks:
+                chunks[i] = x.chunks[i]
+            elif chunks[i] is None:
+                chunks[i] = x.chunks[i]
+    if isinstance(chunks, (tuple, list)):
+        chunks = tuple(lc if lc is not None else rc for lc, rc in zip(chunks, x.chunks))
+
     normalized_chunks = normalize_chunks(chunks, x.shape, dtype=x.dtype)
     if x.chunks == normalized_chunks:
         return x

cubed/tests/test_core.py

@@ -246,10 +246,19 @@ def test_multiple_ops(spec, executor):
     )
 
 
-@pytest.mark.parametrize("new_chunks", [(1, 2), {0: 1, 1: 2}])
-def test_rechunk(spec, executor, new_chunks):
+@pytest.mark.parametrize(
+    ("new_chunks", "expected_chunks"),
+    [
+        ((1, 2), ((1, 1, 1), (2, 1))),
+        ({0: 1, 1: 2}, ((1, 1, 1), (2, 1))),
+        ({1: 2}, ((2, 1), (2, 1))),  # dim 0 unchanged
+        ({}, ((2, 1), (1, 1, 1))),  # unchanged
+    ],
+)
+def test_rechunk(spec, executor, new_chunks, expected_chunks):
     a = xp.asarray([[1, 2, 3], [4, 5, 6], [7, 8, 9]], chunks=(2, 1), spec=spec)
     b = a.rechunk(new_chunks)
+    assert b.chunks == expected_chunks
     assert_array_equal(
         b.compute(executor=executor),
         np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]),