From 644f5bdddf74282df909d7eaca477eaed8b52154 Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Mon, 6 Feb 2023 20:33:01 -0500
Subject: [PATCH 1/3] test for error message

---
 xarray/tests/test_computation.py | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/xarray/tests/test_computation.py b/xarray/tests/test_computation.py
index bfe51c5fcd8..c1be98670cb 100644
--- a/xarray/tests/test_computation.py
+++ b/xarray/tests/test_computation.py
@@ -1220,6 +1220,25 @@ def func(da):
     assert_identical(expected.chunk(), actual)
 
 
+@requires_dask
+def test_apply_dask_new_output_sizes_not_supplied_same_dim_names() -> None:
+    # test for missing output_sizes kwarg sneaking through
+    # see GH discussion 7503
+
+    data = np.random.randn(4, 4, 3, 2)
+    da = xr.DataArray(data=data, dims=("x", "y", "i", "j")).chunk(x=1, y=1)
+
+    with pytest.raises(ValueError, match="output_sizes"):
+        xr.apply_ufunc(
+            np.linalg.pinv,
+            da,
+            input_core_dims=[["i", "j"]],
+            output_core_dims=[["i", "j"]],
+            exclude_dims=set(("i", "j")),
+            dask="parallelized",
+        )
+
+
 def pandas_median(x):
     return pd.Series(x).median()
 

From 017e04bdb1c3093b5e17a3cb9888e1faff0242e9 Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Mon, 6 Feb 2023 20:33:11 -0500
Subject: [PATCH 2/3] fix

---
 xarray/core/computation.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xarray/core/computation.py b/xarray/core/computation.py
index d0ba083cf0c..8fe333e735f 100644
--- a/xarray/core/computation.py
+++ b/xarray/core/computation.py
@@ -723,7 +723,9 @@ def apply_variable_ufunc(
                 dask_gufunc_kwargs["output_sizes"] = output_sizes_renamed
 
             for key in signature.all_output_core_dims:
-                if key not in signature.all_input_core_dims and key not in output_sizes:
+                if (
+                    key not in signature.all_input_core_dims or key in exclude_dims
+                ) and key not in output_sizes:
                     raise ValueError(
                         f"dimension '{key}' in 'output_core_dims' needs corresponding (dim, size) in 'output_sizes'"
                     )

From bb981732f281bffe7643438e98ac0e97dfa6054c Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Mon, 6 Feb 2023 20:41:17 -0500
Subject: [PATCH 3/3] whatsnew

---
 doc/whats-new.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 3d1669589f2..daa785dc6af 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -58,6 +58,9 @@ Bug fixes
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_ and `Scott Chamberlin <https://github.com/scottcha>`_.
 - Handle ``keep_attrs`` option in binary operators of :py:meth:`Dataset` (:issue:`7390`, :pull:`7391`).
   By `Aron Gergely <https://github.com/arongergely>`_.
+- Improve error message when using dask in :py:func:`apply_ufunc` with ``output_sizes`` not supplied. (:pull:`7509`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
+
 
 Documentation
 ~~~~~~~~~~~~~