more friendly error message in case no chunk manager is available

doc/whats-new.rst

@@ -65,6 +65,8 @@ Bug fixes
   By `Stephan Hoyer <https://github.com/shoyer>`_.
 - Fix regression in the interoperability of :py:meth:`DataArray.polyfit` and :py:meth:`xr.polyval` for date-time coordinates. (:pull:`9691`).
   By `Pascal Bourgault <https://github.com/aulemahal>`_.
+- 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>`_.
 - Fix CF decoding of ``grid_mapping`` to allow all possible formats, add tests (:issue:`9761`, :pull:`9765`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 - Add ``User-Agent`` to request-headers when retrieving tutorial data (:issue:`9774`, :pull:`9782`)

xarray/namedarray/parallelcompat.py

@@ -95,6 +95,10 @@ def guess_chunkmanager(
     """
 
     chunkmanagers = list_chunkmanagers()
+    if len(chunkmanagers) == 0:
+        raise ImportError(
+            "no chunk managers available. Try installing `dask` or another 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):
@@ -158,18 +158,20 @@
             chunkmanager = guess_chunkmanager(None)
             assert isinstance(chunkmanager, DummyChunkManager)
 
-    def test_fail_on_nonexistent_chunkmanager(self) -> None:
-        with pytest.raises(ValueError, match="unrecognized chunk manager foo"):
+    def test_fail_on_nonexistent_chunkmanager(
+        self, register_dummy_chunkmanager
+    ) -> None:
+        with pytest.raises(ImportError, match="unrecognized chunk manager foo"):
             guess_chunkmanager("foo")
 
     @requires_dask
     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", dict)
+        with pytest.raises(ImportError, match="no chunk managers available"):
             guess_chunkmanager("dask")
 
     @requires_dask