friendlier error messages for missing chunk managers

doc/whats-new.rst

@@ -76,6 +76,8 @@ Bug fixes
   <https://github.com/josephnowak>`_.
 - Fix binning by multiple variables where some bins have no observations. (:issue:`9630`).
   By `Deepak Cherian <https://github.com/dcherian>`_.
+- Improve the error message raised when using chunked-array methods if no chunk manager is available (:pull:`9676`)
+  By `Justus Magin <https://github.com/keewis>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/namedarray/parallelcompat.py

@@ -96,6 +96,10 @@ def guess_chunkmanager(
     """
 
     chunkmanagers = list_chunkmanagers()
+    if len(chunkmanagers) == 0:
+        raise ValueError(
+            "no chunk managers available. Try installing `dask` or a package that provides a chunk manager."
+        )
 
     if manager is None:
         if len(chunkmanagers) == 1:

xarray/tests/test_parallelcompat.py

@@ -17,7 +17,7 @@
     list_chunkmanagers,
     load_chunkmanagers,
 )
-from xarray.tests import has_dask, requires_dask
+from xarray.tests import requires_dask
 
 
 class DummyChunkedArray(np.ndarray):
@@ -158,7 +158,9 @@ def test_get_chunkmanger_via_set_options(self, register_dummy_chunkmanager) -> N
             chunkmanager = guess_chunkmanager(None)
             assert isinstance(chunkmanager, DummyChunkManager)
 
-    def test_fail_on_nonexistent_chunkmanager(self) -> None:
+    def test_fail_on_nonexistent_chunkmanager(
+        self, register_dummy_chunkmanager
+    ) -> None:
         with pytest.raises(ValueError, match="unrecognized chunk manager foo"):
             guess_chunkmanager("foo")
 
@@ -167,9 +169,11 @@ def test_get_dask_if_installed(self) -> None:
         chunkmanager = guess_chunkmanager(None)
         assert isinstance(chunkmanager, DaskManager)
 
-    @pytest.mark.skipif(has_dask, reason="requires dask not to be installed")
-    def test_dont_get_dask_if_not_installed(self) -> None:
-        with pytest.raises(ValueError, match="unrecognized chunk manager dask"):
+    def test_no_chunk_manager_available(self, monkeypatch) -> None:
+        monkeypatch.setattr(
+            "xarray.namedarray.parallelcompat.list_chunkmanagers", lambda: {}
+        )
+        with pytest.raises(ValueError, match="no chunk managers available"):
             guess_chunkmanager("dask")
 
     @requires_dask