Deprecate bool(ds) (#6126)

doc/whats-new.rst

@@ -35,6 +35,11 @@ Deprecations
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 
+- Coercing a dataset to bool, e.g. ``bool(ds)``, is being deprecated and will raise an
+  error in a future version (not yet planned). For now, invoking ``Dataset.__bool__``
+  issues a ``PendingDeprecationWarning`` (:issue:`6124`, :pull:`6126`).
+  By `Michael Delgado <https://github.com/delgadom>`_.
+
 Bug fixes
 ~~~~~~~~~
 - Subclasses of ``byte`` and ``str`` (e.g. ``np.str_`` and ``np.bytes_``) will now serialise to disk rather than raising a ``ValueError: unsupported dtype for netCDF4 variable: object`` as they did previously (:pull:`5264`).

xarray/core/dataset.py

@@ -1450,6 +1450,14 @@ def __len__(self) -> int:
         return len(self.data_vars)
 
     def __bool__(self) -> bool:
+        warnings.warn(
+            "coercing a Dataset to a bool will be deprecated. "
+            "Using bool(ds.data_vars) to check for at least one "
+            "data variable or using Dataset.to_array to test "
+            "whether array values are true is encouraged.",
+            PendingDeprecationWarning,
+            stacklevel=2,
+        )
         return bool(self.data_vars)
 
     def __iter__(self) -> Iterator[Hashable]:

xarray/tests/test_dataset.py

@@ -544,7 +544,9 @@ def test_properties(self):
         assert "aasldfjalskdfj" not in ds.variables
         assert "dim1" in repr(ds.variables)
         assert len(ds) == 3
-        assert bool(ds)
+
+        with pytest.warns(PendingDeprecationWarning):
+            assert bool(ds)
 
         assert list(ds.data_vars) == ["var1", "var2", "var3"]
         assert list(ds.data_vars.keys()) == ["var1", "var2", "var3"]