CF encoding should preserve vlen dtype for empty arrays (#7862)

* CF encoding should preserve vlen dtype for empty arrays

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

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

* preserve vlen string dtype in netcdf4 and zarr backends

* check for h5py-variant ("vlen") in coding.strings.check_vlen_dtype

* add test to check preserving vlen dtype for empty vlen string arrays

* ignore call_overload error for np.dtype("O", metadata={"vlen": str})

* use filter.codec_id instead of private filter._meta as suggested in review

* update comment and add whats-new.rst entry

* fix whats-new.rst

* fix whats-new.rst (missing dot)

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Kai Mühlbauer <kai.muehlbauer@uni-bonn.de>
Co-authored-by: Kai Mühlbauer <kmuehlbauer@wradlib.org>
Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

doc/whats-new.rst

@@ -50,6 +50,8 @@ Bug fixes
   By `Mattia Almansi <https://github.com/malmans2>`_.
 - Don't call ``CachingFileManager.__del__`` on interpreter shutdown (:issue:`7814`, :pull:`7880`).
   By `Justus Magin <https://github.com/keewis>`_.
+- Preserve vlen dtype for empty string arrays (:issue:`7328`, :pull:`7862`).
+  By `Tom White <https://github.com/tomwhite>`_ and `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 - Ensure dtype of reindex result matches dtype of the original DataArray (:issue:`7299`, :pull:`7917`)
   By `Anderson Banihirwe <https://github.com/andersy005>`_.
 

xarray/backends/netCDF4_.py

@@ -65,10 +65,12 @@ def __init__(self, variable_name, datastore):
 
         dtype = array.dtype
         if dtype is str:
-            # use object dtype because that's the only way in numpy to
-            # represent variable length strings; it also prevents automatic
-            # string concatenation via conventions.decode_cf_variable
-            dtype = np.dtype("O")
+            # use object dtype (with additional vlen string metadata) because that's
+            # the only way in numpy to represent variable length strings and to
+            # check vlen string dtype in further steps
+            # it also prevents automatic string concatenation via
+            # conventions.decode_cf_variable
+            dtype = coding.strings.create_vlen_dtype(str)
         self.dtype = dtype
 
     def __setitem__(self, key, value):

xarray/backends/zarr.py

@@ -70,7 +70,14 @@ def __init__(self, variable_name, datastore):
         array = self.get_array()
         self.shape = array.shape
 
-        dtype = array.dtype
+        # preserve vlen string object dtype (GH 7328)
+        if array.filters is not None and any(
+            [filt.codec_id == "vlen-utf8" for filt in array.filters]
+        ):
+            dtype = coding.strings.create_vlen_dtype(str)
+        else:
+            dtype = array.dtype
+
         self.dtype = dtype
 
     def get_array(self):

xarray/coding/strings.py

@@ -29,7 +29,8 @@ def check_vlen_dtype(dtype):
     if dtype.kind != "O" or dtype.metadata is None:
         return None
     else:
-        return dtype.metadata.get("element_type")
+        # check xarray (element_type) as well as h5py (vlen)
+        return dtype.metadata.get("element_type", dtype.metadata.get("vlen"))
 
 
 def is_unicode_dtype(dtype):

xarray/conventions.py

@@ -108,6 +108,10 @@ def ensure_dtype_not_object(var: Variable, name: T_Name = None) -> Variable:
     if var.dtype.kind == "O":
         dims, data, attrs, encoding = _var_as_tuple(var)
 
+        # leave vlen dtypes unchanged
+        if strings.check_vlen_dtype(data.dtype) is not None:
+            return var
+
         if is_duck_dask_array(data):
             warnings.warn(
                 "variable {} has data in the form of a dask array with "

xarray/tests/test_backends.py

@@ -46,6 +46,7 @@
 )
 from xarray.backends.pydap_ import PydapDataStore
 from xarray.backends.scipy_ import ScipyBackendEntrypoint
+from xarray.coding.strings import check_vlen_dtype, create_vlen_dtype
 from xarray.coding.variables import SerializationWarning
 from xarray.conventions import encode_dataset_coordinates
 from xarray.core import indexing
@@ -859,6 +860,20 @@ def test_roundtrip_string_with_fill_value_nchar(self) -> None:
             with self.roundtrip(original) as actual:
                 assert_identical(expected, actual)
 
+    def test_roundtrip_empty_vlen_string_array(self) -> None:
+        # checks preserving vlen dtype for empty arrays GH7862
+        dtype = create_vlen_dtype(str)
+        original = Dataset({"a": np.array([], dtype=dtype)})
+        assert check_vlen_dtype(original["a"].dtype) == str
+        with self.roundtrip(original) as actual:
+            assert_identical(original, actual)
+            assert object == actual["a"].dtype
+            assert actual["a"].dtype == original["a"].dtype
+            # only check metadata for capable backends
+            # eg. NETCDF3 based backends do not roundtrip metadata
+            if actual["a"].dtype.metadata is not None:
+                assert check_vlen_dtype(actual["a"].dtype) == str
+
     @pytest.mark.parametrize(
         "decoded_fn, encoded_fn",
         [

xarray/tests/test_coding_strings.py

@@ -32,6 +32,10 @@ def test_vlen_dtype() -> None:
     assert strings.is_bytes_dtype(dtype)
     assert strings.check_vlen_dtype(dtype) is bytes
 
+    # check h5py variant ("vlen")
+    dtype = np.dtype("O", metadata={"vlen": str})  # type: ignore[call-overload]
+    assert strings.check_vlen_dtype(dtype) is str
+
     assert strings.check_vlen_dtype(np.dtype(object)) is None
 
 

xarray/tests/test_conventions.py

@@ -487,3 +487,18 @@ def test_decode_cf_error_includes_variable_name():
     ds = Dataset({"invalid": ([], 1e36, {"units": "days since 2000-01-01"})})
     with pytest.raises(ValueError, match="Failed to decode variable 'invalid'"):
         decode_cf(ds)
+
+
+def test_encode_cf_variable_with_vlen_dtype() -> None:
+    v = Variable(
+        ["x"], np.array(["a", "b"], dtype=coding.strings.create_vlen_dtype(str))
+    )
+    encoded_v = conventions.encode_cf_variable(v)
+    assert encoded_v.data.dtype.kind == "O"
+    assert coding.strings.check_vlen_dtype(encoded_v.data.dtype) == str
+
+    # empty array
+    v = Variable(["x"], np.array([], dtype=coding.strings.create_vlen_dtype(str)))
+    encoded_v = conventions.encode_cf_variable(v)
+    assert encoded_v.data.dtype.kind == "O"
+    assert coding.strings.check_vlen_dtype(encoded_v.data.dtype) == str