Better error message when no backend engine is found. (pydata#5300)

* Better error message when no backend engine is found.

I consider this progress towards fixing GH5291 but not a complete fix
yet.

* Better error message for tutorial datasets

* flake8

xarray/backends/plugins.py

@@ -106,7 +106,24 @@ def guess_engine(store_spec):
         except Exception:
             warnings.warn(f"{engine!r} fails while guessing", RuntimeWarning)
 
-    raise ValueError("cannot guess the engine, try passing one explicitly")
+    installed = [k for k in engines if k != "store"]
+    if installed:
+        raise ValueError(
+            "did not find a match in any of xarray's currently installed IO "
+            f"backends {installed}. Consider explicitly selecting one of the "
+            "installed backends via the ``engine`` parameter to "
+            "xarray.open_dataset(), or installing additional IO dependencies:\n"
+            "http://xarray.pydata.org/en/stable/getting-started-guide/installing.html\n"
+            "http://xarray.pydata.org/en/stable/user-guide/io.html"
+        )
+    else:
+        raise ValueError(
+            "xarray is unable to open this file because it has no currently "
+            "installed IO backends. Xarray's read/write support requires "
+            "installing optional dependencies:\n"
+            "http://xarray.pydata.org/en/stable/getting-started-guide/installing.html\n"
+            "http://xarray.pydata.org/en/stable/user-guide/io.html"
+        )
 
 
 def get_backend(engine):

xarray/tests/test_backends.py

@@ -2771,7 +2771,9 @@ def test_open_badbytes(self):
         with pytest.raises(ValueError, match=r"HDF5 as bytes"):
             with open_dataset(b"\211HDF\r\n\032\n", engine="h5netcdf"):
                 pass
-        with pytest.raises(ValueError, match=r"cannot guess the engine"):
+        with pytest.raises(
+            ValueError, match=r"match in any of xarray's currently installed IO"
+        ):
             with open_dataset(b"garbage"):
                 pass
         with pytest.raises(ValueError, match=r"can only read bytes"):
@@ -2823,7 +2825,10 @@ def test_open_fileobj(self):
             # `raises_regex`?). Ref https://github.com/pydata/xarray/pull/5191
             with open(tmp_file, "rb") as f:
                 f.seek(8)
-                with pytest.raises(ValueError, match="cannot guess the engine"):
+                with pytest.raises(
+                    ValueError,
+                    match="match in any of xarray's currently installed IO",
+                ):
                     with pytest.warns(
                         RuntimeWarning,
                         match=re.escape("'h5netcdf' fails while guessing"),

xarray/tests/test_plugins.py

@@ -157,3 +157,23 @@ def test_build_engines_sorted():
 
     assert set(indices) < {0, -1}
     assert list(backend_entrypoints) == sorted(backend_entrypoints)
+
+
+@mock.patch(
+    "xarray.backends.plugins.list_engines",
+    mock.MagicMock(return_value={"dummy": DummyBackendEntrypointArgs()}),
+)
+def test_no_matching_engine_found():
+    with pytest.raises(
+        ValueError, match="match in any of xarray's currently installed IO"
+    ):
+        plugins.guess_engine("not-valid")
+
+
+@mock.patch(
+    "xarray.backends.plugins.list_engines",
+    mock.MagicMock(return_value={}),
+)
+def test_no_engines_installed():
+    with pytest.raises(ValueError, match="no currently installed IO backends."):
+        plugins.guess_engine("not-valid")

xarray/tutorial.py

@@ -36,13 +36,48 @@ def _construct_cache_dir(path):
     "RGB.byte": "https://github.com/mapbox/rasterio/raw/1.2.1/tests/data/RGB.byte.tif",
     "shade": "https://github.com/mapbox/rasterio/raw/1.2.1/tests/data/shade.tif",
 }
+file_formats = {
+    "air_temperature": 3,
+    "rasm": 3,
+    "ROMS_example": 4,
+    "tiny": 3,
+    "eraint_uvz": 3,
+}
+
+
+def _check_netcdf_engine_installed(name):
+    version = file_formats.get(name)
+    if version == 3:
+        try:
+            import scipy  # noqa
+        except ImportError:
+            try:
+                import netCDF4  # noqa
+            except ImportError:
+                raise ImportError(
+                    f"opening tutorial dataset {name} requires either scipy or "
+                    "netCDF4 to be installed."
+                )
+    if version == 4:
+        try:
+            import h5netcdf  # noqa
+        except ImportError:
+            try:
+                import netCDF4  # noqa
+            except ImportError:
+                raise ImportError(
+                    f"opening tutorial dataset {name} requires either h5netcdf "
+                    "or netCDF4 to be installed."
+                )
 
 
 # idea borrowed from Seaborn
 def open_dataset(
     name,
     cache=True,
     cache_dir=None,
+    *,
+    engine=None,
     **kws,
 ):
     """
@@ -94,13 +129,18 @@ def open_dataset(
         if not path.suffix:
             # process the name
             default_extension = ".nc"
+            if engine is None:
+                _check_netcdf_engine_installed(name)
             path = path.with_suffix(default_extension)
+        elif path.suffix == ".grib":
+            if engine is None:
+                engine = "cfgrib"
 
         url = f"{base_url}/raw/{version}/{path.name}"
 
     # retrieve the file
     filepath = pooch.retrieve(url=url, known_hash=None, path=cache_dir)
-    ds = _open_dataset(filepath, **kws)
+    ds = _open_dataset(filepath, engine=engine, **kws)
     if not cache:
         ds = ds.load()
         pathlib.Path(filepath).unlink()