Fix zarr append dtype checks

xarray/backends/api.py

@@ -17,7 +17,7 @@
 
 import numpy as np
 
-from .. import backends, coding, conventions
+from .. import backends, conventions
 from ..core import indexing
 from ..core.combine import (
     _infer_concat_order_from_positions,
@@ -1277,28 +1277,40 @@ def _validate_region(ds, region):
         )
 
 
-def _validate_datatypes_for_zarr_append(dataset):
-    """DataArray.name and Dataset keys must be a string or None"""
+def _validate_datatypes_for_zarr_append(zstore, dataset):
+    """If variable exists in the store, confirm dtype of the data to append is compatible with
+    existing dtype.
+    """
+
+    existing_vars = zstore.get_variables()
 
-    def check_dtype(var):
+    def check_dtype(vname, var):
         if (
-            not np.issubdtype(var.dtype, np.number)
-            and not np.issubdtype(var.dtype, np.datetime64)
-            and not np.issubdtype(var.dtype, np.bool_)
-            and not coding.strings.is_unicode_dtype(var.dtype)
-            and not var.dtype == object
+            vname not in existing_vars
+            or np.issubdtype(var.dtype, np.number)
+            or np.issubdtype(var.dtype, np.datetime64)
+            or np.issubdtype(var.dtype, np.bool_)
+            or var.dtype == object
         ):
-            # and not re.match('^bytes[1-9]+$', var.dtype.name)):
+            # We can skip dtype equality checks under two conditions: (1) if the var to append is
+            # new to the dataset, because in this case there is no existing var to compare it to;
+            # or (2) if var to append's dtype is known to be easy-to-append, because in this case
+            # we can be confident appending won't cause problems. Examples of dtypes which are not
+            # easy-to-append include length-specified strings of type `|S*` or `<U*` (where * is a
+            # positive integer character length). For these dtypes, appending dissimilar lengths
+            # can result in truncation of appended data. Therefore, variables which already exist
+            # in the dataset, and with dtypes which are not known to be easy-to-append, necessitate
+            # exact dtype equality, as checked below.
+            pass
+        elif not var.dtype == existing_vars[vname].dtype:
             raise ValueError(
-                "Invalid dtype for data variable: {} "
-                "dtype must be a subtype of number, "
-                "datetime, bool, a fixed sized string, "
-                "a fixed size unicode string or an "
-                "object".format(var)
+                f"Mismatched dtypes for variable {vname} between Zarr store on disk "
+                f"and dataset to append. Store has dtype {existing_vars[vname].dtype} but "
+                f"dataset to append has dtype {var.dtype}."
             )
 
-    for k in dataset.data_vars.values():
-        check_dtype(k)
+    for vname, var in dataset.data_vars.items():
+        check_dtype(vname, var)
 
 
 def to_zarr(
@@ -1403,7 +1415,7 @@ def to_zarr(
     )
 
     if mode in ["a", "r+"]:
-        _validate_datatypes_for_zarr_append(dataset)
+        _validate_datatypes_for_zarr_append(zstore, dataset)
         if append_dim is not None:
             existing_dims = zstore.get_dimensions()
             if append_dim not in existing_dims:

xarray/tests/test_backends.py

@@ -82,7 +82,11 @@
     _NON_STANDARD_CALENDARS,
     _STANDARD_CALENDARS,
 )
-from .test_dataset import create_append_test_data, create_test_data
+from .test_dataset import (
+    create_append_string_length_mismatch_test_data,
+    create_append_test_data,
+    create_test_data,
+)
 
 try:
     import netCDF4 as nc4
@@ -2112,6 +2116,17 @@ def test_append_with_existing_encoding_raises(self):
                     encoding={"da": {"compressor": None}},
                 )
 
+    @pytest.mark.parametrize("dtype", ["U", "S"])
+    def test_append_string_length_mismatch_raises(self, dtype):
+        ds, ds_to_append = create_append_string_length_mismatch_test_data(dtype)
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target, mode="w")
+            with pytest.raises(ValueError, match="Mismatched dtypes for variable"):
+                ds_to_append.to_zarr(
+                    store_target,
+                    append_dim="time",
+                )
+
     def test_check_encoding_is_consistent_after_append(self):
 
         ds, ds_to_append, _ = create_append_test_data()

xarray/tests/test_dataset.py

@@ -76,6 +76,8 @@ def create_append_test_data(seed=None):
     time2 = pd.date_range("2000-02-01", periods=nt2)
     string_var = np.array(["ae", "bc", "df"], dtype=object)
     string_var_to_append = np.array(["asdf", "asdfg"], dtype=object)
+    string_var_fixed_length = np.array(["aa", "bb", "cc"], dtype="|S2")
+    string_var_fixed_length_to_append = np.array(["dd", "ee"], dtype="|S2")
     unicode_var = ["áó", "áó", "áó"]
     datetime_var = np.array(
         ["2019-01-01", "2019-01-02", "2019-01-03"], dtype="datetime64[s]"
@@ -94,6 +96,9 @@ def create_append_test_data(seed=None):
                 dims=["lat", "lon", "time"],
             ),
             "string_var": xr.DataArray(string_var, coords=[time1], dims=["time"]),
+            "string_var_fixed_length": xr.DataArray(
+                string_var_fixed_length, coords=[time1], dims=["time"]
+            ),
             "unicode_var": xr.DataArray(
                 unicode_var, coords=[time1], dims=["time"]
             ).astype(np.unicode_),
@@ -112,6 +117,9 @@ def create_append_test_data(seed=None):
             "string_var": xr.DataArray(
                 string_var_to_append, coords=[time2], dims=["time"]
             ),
+            "string_var_fixed_length": xr.DataArray(
+                string_var_fixed_length_to_append, coords=[time2], dims=["time"]
+            ),
             "unicode_var": xr.DataArray(
                 unicode_var[:nt2], coords=[time2], dims=["time"]
             ).astype(np.unicode_),
@@ -137,6 +145,33 @@ def create_append_test_data(seed=None):
     return ds, ds_to_append, ds_with_new_var
 
 
+def create_append_string_length_mismatch_test_data(dtype):
+    def make_datasets(data, data_to_append):
+        ds = xr.Dataset(
+            {"temperature": (["time"], data)},
+            coords={"time": [0, 1, 2]},
+        )
+        ds_to_append = xr.Dataset(
+            {"temperature": (["time"], data_to_append)}, coords={"time": [0, 1, 2]}
+        )
+        assert all(objp.data.flags.writeable for objp in ds.variables.values())
+        assert all(
+            objp.data.flags.writeable for objp in ds_to_append.variables.values()
+        )
+        return ds, ds_to_append
+
+    u2_strings = ["ab", "cd", "ef"]
+    u5_strings = ["abc", "def", "ghijk"]
+
+    s2_strings = np.array(["aa", "bb", "cc"], dtype="|S2")
+    s3_strings = np.array(["aaa", "bbb", "ccc"], dtype="|S3")
+
+    if dtype == "U":
+        return make_datasets(u2_strings, u5_strings)
+    elif dtype == "S":
+        return make_datasets(s2_strings, s3_strings)
+
+
 def create_test_multiindex():
     mindex = pd.MultiIndex.from_product(
         [["a", "b"], [1, 2]], names=("level_1", "level_2")