From 0ef21c038c1eb79602f5d59e5fa404db7eea8824 Mon Sep 17 00:00:00 2001
From: dcherian <deepak@cherian.net>
Date: Tue, 12 Jul 2022 09:14:34 -0600
Subject: [PATCH 1/3] Update map_blocks to use chunksizes property.

Raise nicer error if provided template has no dask arrays.

Closes #6763
---
 xarray/core/parallel.py   | 10 +++++-----
 xarray/tests/test_dask.py |  6 ++++++
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/xarray/core/parallel.py b/xarray/core/parallel.py
index fd1f3f9e999..9c778b2ee5a 100644
--- a/xarray/core/parallel.py
+++ b/xarray/core/parallel.py
@@ -380,12 +380,12 @@ def _wrapper(
     else:
         # template xarray object has been provided with proper sizes and chunk shapes
         indexes = dict(template._indexes)
-        if isinstance(template, DataArray):
-            output_chunks = dict(
-                zip(template.dims, template.chunks)  # type: ignore[arg-type]
+        output_chunks = template.chunksizes
+        if not output_chunks:
+            raise ValueError(
+                "Provided template has no dask arrays. "
+                " Please construct a template with appropriately chunked dask arrays."
             )
-        else:
-            output_chunks = dict(template.chunks)
 
     for dim in output_chunks:
         if dim in input_chunks and len(input_chunks[dim]) != len(output_chunks[dim]):
diff --git a/xarray/tests/test_dask.py b/xarray/tests/test_dask.py
index 0d6ee7e503a..82d1df6ea9b 100644
--- a/xarray/tests/test_dask.py
+++ b/xarray/tests/test_dask.py
@@ -1243,6 +1243,12 @@ def sumda(da1, da2):
         )
     xr.testing.assert_equal((da1 + da2).sum("x"), mapped)
 
+    # bad template: not chunked
+    with pytest.raises(ValueError, match="Provided template has no dask arrays"):
+        xr.map_blocks(
+            lambda a, b: (a + b).sum("x"), da1, args=[da2], template=da1.sum("x").compute()
+        )
+
 
 @pytest.mark.parametrize("obj", [make_da(), make_ds()])
 def test_map_blocks_add_attrs(obj):

From f3cf3b67083c7c6a7866dd388eece81adcf84c4a Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 12 Jul 2022 15:18:25 +0000
Subject: [PATCH 2/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 xarray/tests/test_dask.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/xarray/tests/test_dask.py b/xarray/tests/test_dask.py
index 82d1df6ea9b..51845b2159e 100644
--- a/xarray/tests/test_dask.py
+++ b/xarray/tests/test_dask.py
@@ -1246,7 +1246,10 @@ def sumda(da1, da2):
     # bad template: not chunked
     with pytest.raises(ValueError, match="Provided template has no dask arrays"):
         xr.map_blocks(
-            lambda a, b: (a + b).sum("x"), da1, args=[da2], template=da1.sum("x").compute()
+            lambda a, b: (a + b).sum("x"),
+            da1,
+            args=[da2],
+            template=da1.sum("x").compute(),
         )
 
 

From 72ec730fc88fa206e6297314d0ac7b7bd1579d7a Mon Sep 17 00:00:00 2001
From: dcherian <deepak@cherian.net>
Date: Tue, 12 Jul 2022 09:31:35 -0600
Subject: [PATCH 3/3] fix typing

---
 xarray/core/parallel.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/core/parallel.py b/xarray/core/parallel.py
index 9c778b2ee5a..2e3aff68a26 100644
--- a/xarray/core/parallel.py
+++ b/xarray/core/parallel.py
@@ -373,7 +373,7 @@ def _wrapper(
         new_indexes = template_indexes - set(input_indexes)
         indexes = {dim: input_indexes[dim] for dim in preserved_indexes}
         indexes.update({k: template._indexes[k] for k in new_indexes})
-        output_chunks = {
+        output_chunks: Mapping[Hashable, tuple[int, ...]] = {
             dim: input_chunks[dim] for dim in template.dims if dim in input_chunks
         }