Add test to ensure that number of bytes of sparse array is correctly

reported

xarray/tests/test_dataarray.py

@@ -3259,6 +3259,21 @@ def test_from_series_sparse(self) -> None:
         actual_sparse.data = actual_sparse.data.todense()
         assert_identical(actual_sparse, actual_dense)
 
+    @requires_sparse
+    def test_sparse_nbytes(self) -> None:
+        # https://github.com/pydata/xarray/issues/4842#issue-793245791
+        df = pd.DataFrame()
+        df["x"] = np.repeat(np.arange(10), 10)
+        df["y"] = np.repeat(np.arange(10), 10)
+        df["time"] = np.tile(pd.date_range("2000-01-01", "2000-03-10", freq="W"), 10)
+        df["rate"] = 10.0
+        df = df.set_index(["time", "y", "x"])
+
+        sparse_ds = xr.Dataset.from_dataframe(df, sparse=True)
+        rate = sparse_ds["rate"]
+        assert rate.nbytes < 8000
+        assert rate.size * rate.dtype.itemsize == 8000
+
     @requires_sparse
     def test_from_multiindex_series_sparse(self) -> None:
         # regression test for GH4019