Feature/align in dot (#3699)

* add tests

* implement align

* whats new

* fix changes to whats new

* review: fix typos

doc/whats-new.rst

@@ -21,12 +21,15 @@ v0.15.0 (unreleased)
 
 Breaking changes
 ~~~~~~~~~~~~~~~~
-
 - Remove ``compat`` and ``encoding`` kwargs from ``DataArray``, which
   have been deprecated since 0.12. (:pull:`3650`). 
   Instead, specify the encoding when writing to disk or set 
   the ``encoding`` attribute directly.
   By `Maximilian Roos <https://github.com/max-sixty>`_
+- :py:func:`xarray.dot`, :py:meth:`DataArray.dot`, and the ``@`` operator now
+  use ``align="inner"`` (except when ``xarray.set_options(arithmetic_join="exact")``;
+  :issue:`3694`) by `Mathias Hauser <https://github.com/mathause>`_.
+
 
 New Features
 ~~~~~~~~~~~~

xarray/core/computation.py

@@ -26,6 +26,7 @@
 from . import duck_array_ops, utils
 from .alignment import deep_align
 from .merge import merge_coordinates_without_align
+from .options import OPTIONS
 from .pycompat import dask_array_type
 from .utils import is_dict_like
 from .variable import Variable
@@ -1175,6 +1176,11 @@ def dot(*arrays, dims=None, **kwargs):
     subscripts = ",".join(subscripts_list)
     subscripts += "->..." + "".join([dim_map[d] for d in output_core_dims[0]])
 
+    join = OPTIONS["arithmetic_join"]
+    # using "inner" emulates `(a * b).sum()` for all joins (except "exact")
+    if join != "exact":
+        join = "inner"
+
     # subscripts should be passed to np.einsum as arg, not as kwargs. We need
     # to construct a partial function for apply_ufunc to work.
     func = functools.partial(duck_array_ops.einsum, subscripts, **kwargs)
@@ -1183,6 +1189,7 @@ def dot(*arrays, dims=None, **kwargs):
         *arrays,
         input_core_dims=input_core_dims,
         output_core_dims=output_core_dims,
+        join=join,
         dask="allowed",
     )
     return result.transpose(*[d for d in all_dims if d in result.dims])

xarray/tests/test_computation.py

@@ -1043,6 +1043,60 @@ def test_dot(use_dask):
     pickle.loads(pickle.dumps(xr.dot(da_a)))
 
 
+@pytest.mark.parametrize("use_dask", [True, False])
+def test_dot_align_coords(use_dask):
+    # GH 3694
+
+    if use_dask:
+        if not has_dask:
+            pytest.skip("test for dask.")
+
+    a = np.arange(30 * 4).reshape(30, 4)
+    b = np.arange(30 * 4 * 5).reshape(30, 4, 5)
+
+    # use partially overlapping coords
+    coords_a = {"a": np.arange(30), "b": np.arange(4)}
+    coords_b = {"a": np.arange(5, 35), "b": np.arange(1, 5)}
+
+    da_a = xr.DataArray(a, dims=["a", "b"], coords=coords_a)
+    da_b = xr.DataArray(b, dims=["a", "b", "c"], coords=coords_b)
+
+    if use_dask:
+        da_a = da_a.chunk({"a": 3})
+        da_b = da_b.chunk({"a": 3})
+
+    # join="inner" is the default
+    actual = xr.dot(da_a, da_b)
+    # `dot` sums over the common dimensions of the arguments
+    expected = (da_a * da_b).sum(["a", "b"])
+    xr.testing.assert_allclose(expected, actual)
+
+    actual = xr.dot(da_a, da_b, dims=...)
+    expected = (da_a * da_b).sum()
+    xr.testing.assert_allclose(expected, actual)
+
+    with xr.set_options(arithmetic_join="exact"):
+        with raises_regex(ValueError, "indexes along dimension"):
+            xr.dot(da_a, da_b)
+
+    # NOTE: dot always uses `join="inner"` because `(a * b).sum()` yields the same for all
+    # join method (except "exact")
+    with xr.set_options(arithmetic_join="left"):
+        actual = xr.dot(da_a, da_b)
+        expected = (da_a * da_b).sum(["a", "b"])
+        xr.testing.assert_allclose(expected, actual)
+
+    with xr.set_options(arithmetic_join="right"):
+        actual = xr.dot(da_a, da_b)
+        expected = (da_a * da_b).sum(["a", "b"])
+        xr.testing.assert_allclose(expected, actual)
+
+    with xr.set_options(arithmetic_join="outer"):
+        actual = xr.dot(da_a, da_b)
+        expected = (da_a * da_b).sum(["a", "b"])
+        xr.testing.assert_allclose(expected, actual)
+
+
 def test_where():
     cond = xr.DataArray([True, False], dims="x")
     actual = xr.where(cond, 1, 0)

xarray/tests/test_dataarray.py

@@ -3973,6 +3973,43 @@ def test_dot(self):
         with pytest.raises(TypeError):
             da.dot(dm.values)
 
+    def test_dot_align_coords(self):
+        # GH 3694
+
+        x = np.linspace(-3, 3, 6)
+        y = np.linspace(-3, 3, 5)
+        z_a = range(4)
+        da_vals = np.arange(6 * 5 * 4).reshape((6, 5, 4))
+        da = DataArray(da_vals, coords=[x, y, z_a], dims=["x", "y", "z"])
+
+        z_m = range(2, 6)
+        dm_vals = range(4)
+        dm = DataArray(dm_vals, coords=[z_m], dims=["z"])
+
+        with xr.set_options(arithmetic_join="exact"):
+            with raises_regex(ValueError, "indexes along dimension"):
+                da.dot(dm)
+
+        da_aligned, dm_aligned = xr.align(da, dm, join="inner")
+
+        # nd dot 1d
+        actual = da.dot(dm)
+        expected_vals = np.tensordot(da_aligned.values, dm_aligned.values, [2, 0])
+        expected = DataArray(expected_vals, coords=[x, da_aligned.y], dims=["x", "y"])
+        assert_equal(expected, actual)
+
+        # multiple shared dims
+        dm_vals = np.arange(20 * 5 * 4).reshape((20, 5, 4))
+        j = np.linspace(-3, 3, 20)
+        dm = DataArray(dm_vals, coords=[j, y, z_m], dims=["j", "y", "z"])
+        da_aligned, dm_aligned = xr.align(da, dm, join="inner")
+        actual = da.dot(dm)
+        expected_vals = np.tensordot(
+            da_aligned.values, dm_aligned.values, axes=([1, 2], [1, 2])
+        )
+        expected = DataArray(expected_vals, coords=[x, j], dims=["x", "j"])
+        assert_equal(expected, actual)
+
     def test_matmul(self):
 
         # copied from above (could make a fixture)
@@ -3986,6 +4023,24 @@ def test_matmul(self):
         expected = da.dot(da)
         assert_identical(result, expected)
 
+    def test_matmul_align_coords(self):
+        # GH 3694
+
+        x_a = np.arange(6)
+        x_b = np.arange(2, 8)
+        da_vals = np.arange(6)
+        da_a = DataArray(da_vals, coords=[x_a], dims=["x"])
+        da_b = DataArray(da_vals, coords=[x_b], dims=["x"])
+
+        # only test arithmetic_join="inner" (=default)
+        result = da_a @ da_b
+        expected = da_a.dot(da_b)
+        assert_identical(result, expected)
+
+        with xr.set_options(arithmetic_join="exact"):
+            with raises_regex(ValueError, "indexes along dimension"):
+                da_a @ da_b
+
     def test_binary_op_propagate_indexes(self):
         # regression test for GH2227
         self.dv["x"] = np.arange(self.dv.sizes["x"])