friendlier error messages for missing chunk managers (#9676)

* raise an error message while guessing if there's no chunkmanager available

* don't skip the no chunkmanager test if dask is not installed

* whats-new

* ensure at least one chunk manager is available

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* remove additional blank line from a bad merge

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* improve the wording

Co-authored-by: Tom Nicholas <[email protected]>

* switch to ImportError

* raise a helpful `ImportError` for known chunk managers

* make sure the new `ImportError` is actually raised

* check that the more specific error message is preferred

* prefer the more specific error

* also use `ImportError` as indicator for `chunks=None`

* move and improve the whats-new entry

* captialize global variable KNOWN_CHUNKMANAGERS

* chunkmanagers -> available_chunkmanagers

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* also use the string repr for printing `manager`

* reword

* more repr

* reflow

* adapt the test to the new error message

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tom Nicholas <[email protected]>
Co-authored-by: Deepak Cherian <[email protected]>

doc/whats-new.rst

@@ -21,6 +21,8 @@ v.2024.12.0 (unreleased)
 
 New Features
 ~~~~~~~~~~~~
+- Improve the error message raised when using chunked-array methods if no chunk manager is available or if the requested chunk manager is missing (:pull:`9676`)
+  By `Justus Magin <https://github.com/keewis>`_. (:pull:`9676`)
 - Better support wrapping additional array types (e.g. ``cupy`` or ``jax``) by calling generalized
   duck array operations throughout more xarray methods. (:issue:`7848`, :pull:`9798`).
   By `Sam Levang <https://github.com/slevang>`_.

xarray/backends/zarr.py

@@ -1471,7 +1471,7 @@ def open_zarr(
             )  # attempt to import that parallel backend
 
             chunks = {}
-        except ValueError:
+        except (ValueError, ImportError):
             chunks = None
 
     if kwargs:

xarray/namedarray/parallelcompat.py

@@ -46,6 +46,12 @@ def compute(
 
 T_ChunkedArray = TypeVar("T_ChunkedArray", bound=ChunkedArrayMixinProtocol)
 
+KNOWN_CHUNKMANAGERS = {
+    "dask": "dask",
+    "cubed": "cubed-xarray",
+    "arkouda": "arkouda-xarray",
+}
+
 
 @functools.lru_cache(maxsize=1)
 def list_chunkmanagers() -> dict[str, ChunkManagerEntrypoint[Any]]:
@@ -95,29 +101,42 @@ def guess_chunkmanager(
     Else use whatever is installed, defaulting to dask if there are multiple options.
     """
 
-    chunkmanagers = list_chunkmanagers()
+    available_chunkmanagers = list_chunkmanagers()
 
     if manager is None:
-        if len(chunkmanagers) == 1:
+        if len(available_chunkmanagers) == 1:
             # use the only option available
-            manager = next(iter(chunkmanagers.keys()))
+            manager = next(iter(available_chunkmanagers.keys()))
         else:
             # use the one in options (default dask)
             manager = OPTIONS["chunk_manager"]
 
     if isinstance(manager, str):
-        if manager not in chunkmanagers:
+        if manager not in available_chunkmanagers and manager in KNOWN_CHUNKMANAGERS:
+            raise ImportError(
+                f"chunk manager {manager!r} is not available."
+                f" Please make sure {KNOWN_CHUNKMANAGERS[manager]!r} is installed"
+                " and importable."
+            )
+        elif len(available_chunkmanagers) == 0:
+            raise ImportError(
+                "no chunk managers available. Try installing `dask` or another package"
+                " that provides a chunk manager."
+            )
+        elif manager not in available_chunkmanagers:
             raise ValueError(
-                f"unrecognized chunk manager {manager} - must be one of: {list(chunkmanagers)}"
+                f"unrecognized chunk manager {manager!r} - must be one of the installed"
+                f" chunk managers: {list(available_chunkmanagers)}"
             )
 
-        return chunkmanagers[manager]
+        return available_chunkmanagers[manager]
     elif isinstance(manager, ChunkManagerEntrypoint):
         # already a valid ChunkManager so just pass through
         return manager
     else:
         raise TypeError(
-            f"manager must be a string or instance of ChunkManagerEntrypoint, but received type {type(manager)}"
+            "manager must be a string or instance of ChunkManagerEntrypoint,"
+            f" but received type {type(manager)}"
         )
 
 

xarray/tests/test_parallelcompat.py

@@ -11,13 +11,14 @@
 from xarray.namedarray._typing import _Chunks
 from xarray.namedarray.daskmanager import DaskManager
 from xarray.namedarray.parallelcompat import (
+    KNOWN_CHUNKMANAGERS,
     ChunkManagerEntrypoint,
     get_chunked_array_type,
     guess_chunkmanager,
     list_chunkmanagers,
     load_chunkmanagers,
 )
-from xarray.tests import has_dask, requires_dask
+from xarray.tests import requires_dask
 
 
 class DummyChunkedArray(np.ndarray):
@@ -158,18 +159,36 @@ 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:
-        with pytest.raises(ValueError, match="unrecognized chunk manager foo"):
+    def test_fail_on_known_but_missing_chunkmanager(
+        self, register_dummy_chunkmanager, monkeypatch
+    ) -> None:
+        monkeypatch.setitem(KNOWN_CHUNKMANAGERS, "test", "test-package")
+        with pytest.raises(
+            ImportError, match="chunk manager 'test' is not available.+test-package"
+        ):
+            guess_chunkmanager("test")
+
+    def test_fail_on_nonexistent_chunkmanager(
+        self, register_dummy_chunkmanager
+    ) -> None:
+        with pytest.raises(ValueError, 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("foo")
+
+    def test_no_chunk_manager_available_but_known_manager_requested(
+        self, monkeypatch
+    ) -> None:
+        monkeypatch.setattr("xarray.namedarray.parallelcompat.list_chunkmanagers", dict)
+        with pytest.raises(ImportError, match="chunk manager 'dask' is not available"):
             guess_chunkmanager("dask")
 
     @requires_dask