From 68d5c73446e6ea925b47e2947dd6e1995203d413 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 2 Sep 2025 14:03:01 -0400
Subject: [PATCH 01/16] Support NETCDF4_CLASSIC in the h5engine backend

---
 xarray/backends/h5netcdf_.py  | 12 +++++++++---
 xarray/tests/test_backends.py | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 01275d8db5b..339a88dc7ce 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -26,6 +26,7 @@
     FileManager,
 )
 from xarray.backends.locks import HDF5_LOCK, combine_locks, ensure_lock, get_write_lock
+from xarray.backends.netcdf3 import encode_nc3_attr_value, encode_nc3_variable
 from xarray.backends.netCDF4_ import (
     BaseNetCDF4Array,
     _build_and_get_enum,
@@ -197,7 +198,7 @@ def open(
                     f"{magic_number!r} is not the signature of a valid netCDF4 file"
                 )
 
-        if format not in [None, "NETCDF4"]:
+        if format not in [None, "NETCDF4", "NETCDF4_CLASSIC"]:
             raise ValueError("invalid format for h5netcdf backend")
 
         kwargs = {
@@ -318,10 +319,15 @@ def set_dimension(self, name, length, is_unlimited=False):
             self.ds.dimensions[name] = length
 
     def set_attribute(self, key, value):
+        if self.format != "NETCDF4":
+            value = encode_nc3_attr_value(value)
         self.ds.attrs[key] = value
 
     def encode_variable(self, variable, name=None):
-        return _encode_nc4_variable(variable, name=name)
+        if self.format == "NETCDF4":
+            return _encode_nc4_variable(variable, name=name)
+
+        return encode_nc3_variable(variable, name=name)
 
     def prepare_variable(
         self, name, variable, check_encoding=False, unlimited_dims=None
@@ -330,7 +336,7 @@ def prepare_variable(
 
         _ensure_no_forward_slash_in_name(name)
         attrs = variable.attrs.copy()
-        dtype = _get_datatype(variable, raise_on_invalid_encoding=check_encoding)
+        dtype = _get_datatype(variable, self.format, raise_on_invalid_encoding=check_encoding)
 
         fillvalue = attrs.pop("_FillValue", None)
 
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index c54e3c8d135..6268c1c52ee 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4576,6 +4576,20 @@ def create_store(self):
                 yield store
 
 
+@requires_h5netcdf
+class TestNetCDF4ClassicViaH5NetCDFData(NetCDF3Only, CFEncodedBase):
+    engine: T_NetcdfEngine = "h5netcdf"
+    file_format: T_NetcdfTypes = "NETCDF4_CLASSIC"
+
+    @contextlib.contextmanager
+    def create_store(self):
+        with create_tmp_file() as tmp_file:
+            with backends.H5NetCDFStore.open(
+                tmp_file, mode="w", format="NETCDF4_CLASSIC"
+            ) as store:
+                yield store
+
+
 @requires_scipy_or_netCDF4
 class TestGenericNetCDFData(NetCDF3Only, CFEncodedBase):
     # verify that we can read and write netCDF3 files as long as we have scipy

From 120af0732150dc996da6bec67d6fa73d43009794 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 2 Sep 2025 17:24:28 -0400
Subject: [PATCH 02/16] convert bytes attributes to numpy.bytes_ in
 NETCDF4_CLASSIC format with h5netcdf engine

---
 xarray/backends/h5netcdf_.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 339a88dc7ce..f6e3676b892 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -321,6 +321,8 @@ def set_dimension(self, name, length, is_unlimited=False):
     def set_attribute(self, key, value):
         if self.format != "NETCDF4":
             value = encode_nc3_attr_value(value)
+            if isinstance(value, bytes):
+                value = np.bytes_(value)
         self.ds.attrs[key] = value
 
     def encode_variable(self, variable, name=None):

From f8f44f0f73433f1e36ef179b0975c046849927ca Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 2 Sep 2025 18:01:18 -0400
Subject: [PATCH 03/16] added test to confirm string attributes are stored as
 numpy char arrays with NETCDF4_CLASSIC

---
 xarray/tests/test_backends.py | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 6268c1c52ee..b9837c03aac 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4575,9 +4575,20 @@ def create_store(self):
             ) as store:
                 yield store
 
+    @requires_h5netcdf
+    def test_string_attributes_stored_as_char(self, tmp_path):
+        import h5netcdf
+
+        original = Dataset(attrs={"foo": "bar"})
+        store_path = tmp_path / "tmp.nc"
+        original.to_netcdf(store_path, engine=self.engine, format=self.file_format)
+        with h5netcdf.File(store_path, "r") as ds:
+            # Check that the attribute is stored as a char array
+            assert ds._h5file.attrs["foo"].dtype == np.dtype("S3")
+
 
 @requires_h5netcdf
-class TestNetCDF4ClassicViaH5NetCDFData(NetCDF3Only, CFEncodedBase):
+class TestNetCDF4ClassicViaH5NetCDFData(TestNetCDF4ClassicViaNetCDF4Data):
     engine: T_NetcdfEngine = "h5netcdf"
     file_format: T_NetcdfTypes = "NETCDF4_CLASSIC"
 
@@ -4590,6 +4601,8 @@ def create_store(self):
                 yield store
 
 
+# TODO: add cross-engine tests for NETCDF4_CLASSIC
+
 @requires_scipy_or_netCDF4
 class TestGenericNetCDFData(NetCDF3Only, CFEncodedBase):
     # verify that we can read and write netCDF3 files as long as we have scipy

From 522d37da8eea1da391a8be48a5372dae6d308cff Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 2 Sep 2025 23:28:33 -0400
Subject: [PATCH 04/16] Added change to whats-new

---
 doc/whats-new.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 29d25c0b734..47dc27d9a51 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -23,6 +23,9 @@ New Features
 - ``compute=False`` is now supported by :py:meth:`DataTree.to_netcdf` and
   :py:meth:`DataTree.to_zarr`.
   By `Stephan Hoyer <https://github.com/shoyer>`_.
+- The ``h5netcdf`` engine can now write ``NETCDF4_CLASSIC`` files
+  (:issue:`10676`, :pull:``).
+  By `David Huard <https://github.com/huard>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

From 783c407f385efb525ee7e324d49b6b4eeb038103 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 2 Sep 2025 23:31:47 -0400
Subject: [PATCH 05/16] run pre-commit

---
 xarray/backends/h5netcdf_.py  | 4 +++-
 xarray/tests/test_backends.py | 1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index f6e3676b892..8490d105fcc 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -338,7 +338,9 @@ def prepare_variable(
 
         _ensure_no_forward_slash_in_name(name)
         attrs = variable.attrs.copy()
-        dtype = _get_datatype(variable, self.format, raise_on_invalid_encoding=check_encoding)
+        dtype = _get_datatype(
+            variable, self.format, raise_on_invalid_encoding=check_encoding
+        )
 
         fillvalue = attrs.pop("_FillValue", None)
 
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index b9837c03aac..da843430dc0 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4603,6 +4603,7 @@ def create_store(self):
 
 # TODO: add cross-engine tests for NETCDF4_CLASSIC
 
+
 @requires_scipy_or_netCDF4
 class TestGenericNetCDFData(NetCDF3Only, CFEncodedBase):
     # verify that we can read and write netCDF3 files as long as we have scipy

From 2f1c7817bc605bff1f15851ffc395280e715d2df Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Wed, 3 Sep 2025 00:36:12 -0400
Subject: [PATCH 06/16] Added test comparing CDL representation of test data
 written with netcdf4 and h5netcdf

---
 xarray/tests/test_backends.py | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index da843430dc0..b6c1120c375 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4600,8 +4600,26 @@ def create_store(self):
             ) as store:
                 yield store
 
+    @requires_netCDF4
+    def test_cdl_representation(self, tmp_path) -> None:
+        import netCDF4
+
+        data = create_test_data()
+
+        # Create CDL representation using netCDF4
+        fn = tmp_path / "tmp.nc"
+        data.to_netcdf(fn, engine="netcdf4", format=self.file_format)
+        with netCDF4.Dataset(fn) as ds:
+            expected = ds.tocdl()
+
+        # Write using h5netcdf
+        data.to_netcdf(fn, engine=self.engine, format=self.file_format)
+
+        # Read using netCDF4
+        with netCDF4.Dataset(fn) as ds:
+            actual = ds.tocdl()
 
-# TODO: add cross-engine tests for NETCDF4_CLASSIC
+        assert expected == actual
 
 
 @requires_scipy_or_netCDF4

From b142d38c7ac8a59f96c64dea21c92a174fa689fb Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Wed, 3 Sep 2025 01:02:39 -0400
Subject: [PATCH 07/16] Added global attribute to test data. Apply CLASSIC
 conversion to variable attributes as well.

---
 xarray/backends/h5netcdf_.py | 9 +++++++--
 xarray/tests/__init__.py     | 1 +
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 8490d105fcc..166f007698b 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -318,11 +318,16 @@ def set_dimension(self, name, length, is_unlimited=False):
         else:
             self.ds.dimensions[name] = length
 
-    def set_attribute(self, key, value):
+    def convert_string(self, value):
+        """If format is NETCDF4_CLASSIC, convert strings to char arrays."""
         if self.format != "NETCDF4":
             value = encode_nc3_attr_value(value)
             if isinstance(value, bytes):
                 value = np.bytes_(value)
+        return value
+
+    def set_attribute(self, key, value):
+        value = self.convert_string(value)
         self.ds.attrs[key] = value
 
     def encode_variable(self, variable, name=None):
@@ -402,7 +407,7 @@ def prepare_variable(
             nc4_var = self.ds[name]
 
         for k, v in attrs.items():
-            nc4_var.attrs[k] = v
+            nc4_var.attrs[k] = self.convert_string(v)
 
         target = H5NetCDFArrayWrapper(name, self)
 
diff --git a/xarray/tests/__init__.py b/xarray/tests/__init__.py
index 3b4e49c64d8..67e24122852 100644
--- a/xarray/tests/__init__.py
+++ b/xarray/tests/__init__.py
@@ -397,6 +397,7 @@ def create_test_data(
     else:
         numbers_values = rs.integers(0, 3, _dims["dim3"], dtype="int64")
     obj.coords["numbers"] = ("dim3", numbers_values)
+    obj.attrs = {"foo": "bar"}
     obj.encoding = {"foo": "bar"}
     assert_writeable(obj)
     return obj

From 909a96c3c676268b20006a69d4be16089d830b5e Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Wed, 3 Sep 2025 13:35:12 -0400
Subject: [PATCH 08/16] Use h5dump to compare file content instead of CDL. Add
 _nc3_strict attribute.

---
 doc/whats-new.rst             |  2 +-
 xarray/backends/h5netcdf_.py  | 20 ++++++++++++++++++++
 xarray/tests/test_backends.py | 21 ++++++++-------------
 3 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 47dc27d9a51..ceeffb38102 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -24,7 +24,7 @@ New Features
   :py:meth:`DataTree.to_zarr`.
   By `Stephan Hoyer <https://github.com/shoyer>`_.
 - The ``h5netcdf`` engine can now write ``NETCDF4_CLASSIC`` files
-  (:issue:`10676`, :pull:``).
+  (:issue:`10676`, :pull:`10686`).
   By `David Huard <https://github.com/huard>`_.
 
 Breaking changes
diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 166f007698b..17d7986784b 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -3,6 +3,7 @@
 import functools
 import io
 import os
+import subprocess
 from collections.abc import Iterable
 from typing import TYPE_CHECKING, Any, Self
 
@@ -101,6 +102,19 @@ def _read_attributes(h5netcdf_var):
 )
 
 
+def _h5dump(fn: str):
+    """Call h5dump on an h5netcdf file."""
+    out = subprocess.run(
+        ["h5dump", "-A", "-B", fn], check=False, capture_output=True
+    ).stdout.decode()
+
+    # Strip first and last line
+    # HDF5 "file" {
+    # ...
+    # }
+    return "\n".join(out.splitlines()[1:-1])
+
+
 def _h5netcdf_create_group(dataset, name):
     return dataset.create_group(name)
 
@@ -231,6 +245,12 @@ def _acquire(self, needs_lock=True):
             )
         return ds
 
+    def encode(self, variables, attributes):
+        """Overload encode to set _nc3_strict flag."""
+        if self.format != "NETCDF4":
+            self.ds._h5file.attrs["_nc3_strict"] = np.int32(1)
+        return super().encode(variables, attributes)
+
     @property
     def ds(self):
         return self._acquire()
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index b6c1120c375..b0244c4a312 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -45,7 +45,7 @@
     save_mfdataset,
 )
 from xarray.backends.common import robust_getitem
-from xarray.backends.h5netcdf_ import H5netcdfBackendEntrypoint
+from xarray.backends.h5netcdf_ import H5netcdfBackendEntrypoint, _h5dump
 from xarray.backends.netcdf3 import _nc3_dtype_coercions
 from xarray.backends.netCDF4_ import (
     NetCDF4BackendEntrypoint,
@@ -4601,23 +4601,18 @@ def create_store(self):
                 yield store
 
     @requires_netCDF4
-    def test_cdl_representation(self, tmp_path) -> None:
-        import netCDF4
-
+    def test_h5dump(self, tmp_path) -> None:
         data = create_test_data()
 
-        # Create CDL representation using netCDF4
-        fn = tmp_path / "tmp.nc"
+        # Dump the representation of the netCDF4-generated file
+        fn = tmp_path / "netcdf4.nc"
         data.to_netcdf(fn, engine="netcdf4", format=self.file_format)
-        with netCDF4.Dataset(fn) as ds:
-            expected = ds.tocdl()
+        expected = _h5dump(fn)
 
-        # Write using h5netcdf
+        # Dump the representation of the h5netcdf-generated file
+        fn = tmp_path / "h5netcdf.nc"
         data.to_netcdf(fn, engine=self.engine, format=self.file_format)
-
-        # Read using netCDF4
-        with netCDF4.Dataset(fn) as ds:
-            actual = ds.tocdl()
+        actual = _h5dump(fn)
 
         assert expected == actual
 

From 35b50cea7fa3a2297ab5511e6ead1a7c5d921cb9 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Wed, 3 Sep 2025 16:54:16 -0400
Subject: [PATCH 09/16] raise error if writing groups to CLASSIC file.

---
 xarray/backends/h5netcdf_.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 17d7986784b..1538b051ac3 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -151,6 +151,10 @@ def __init__(
                     raise ValueError(
                         "must supply a h5netcdf.File if the group argument is provided"
                     )
+                if format == "NETCDF4_CLASSIC":
+                    raise ValueError(
+                        "Cannot create sub-groups in `NETCDF4_CLASSIC` format."
+                    )
                 root = manager
             manager = DummyFileManager(root)
 
@@ -215,6 +219,9 @@ def open(
         if format not in [None, "NETCDF4", "NETCDF4_CLASSIC"]:
             raise ValueError("invalid format for h5netcdf backend")
 
+        if format == "NETCDF4_CLASSIC" and group is not None:
+            raise ValueError("Cannot create sub-groups in `NETCDF4_CLASSIC` format.")
+
         kwargs = {
             "invalid_netcdf": invalid_netcdf,
             "decode_vlen_strings": decode_vlen_strings,

From a187c8d9ba2959a33d1006c105a18f3dcfa4594b Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Fri, 5 Sep 2025 13:14:28 -0400
Subject: [PATCH 10/16] remove h5dump test. Remove _nc3_strict attribute
 (should go into h5netcdf).

---
 xarray/backends/h5netcdf_.py  | 23 +++--------------------
 xarray/tests/test_backends.py | 30 +++++++++++++++---------------
 2 files changed, 18 insertions(+), 35 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 1538b051ac3..a43015766bc 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -3,11 +3,11 @@
 import functools
 import io
 import os
-import subprocess
 from collections.abc import Iterable
 from typing import TYPE_CHECKING, Any, Self
 
 import numpy as np
+from packaging.version import Version
 
 from xarray.backends.common import (
     BACKEND_ENTRYPOINTS,
@@ -102,19 +102,6 @@ def _read_attributes(h5netcdf_var):
 )
 
 
-def _h5dump(fn: str):
-    """Call h5dump on an h5netcdf file."""
-    out = subprocess.run(
-        ["h5dump", "-A", "-B", fn], check=False, capture_output=True
-    ).stdout.decode()
-
-    # Strip first and last line
-    # HDF5 "file" {
-    # ...
-    # }
-    return "\n".join(out.splitlines()[1:-1])
-
-
 def _h5netcdf_create_group(dataset, name):
     return dataset.create_group(name)
 
@@ -231,6 +218,8 @@ def open(
             kwargs.update(driver_kwds)
         if phony_dims is not None:
             kwargs["phony_dims"] = phony_dims
+        if format is not None and Version(h5netcdf.__version__) > Version("1.6"):
+            kwargs["format"] = format
 
         if lock is None:
             if mode == "r":
@@ -252,12 +241,6 @@ def _acquire(self, needs_lock=True):
             )
         return ds
 
-    def encode(self, variables, attributes):
-        """Overload encode to set _nc3_strict flag."""
-        if self.format != "NETCDF4":
-            self.ds._h5file.attrs["_nc3_strict"] = np.int32(1)
-        return super().encode(variables, attributes)
-
     @property
     def ds(self):
         return self._acquire()
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 96d82bca413..82916d5662a 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -45,7 +45,7 @@
     save_mfdataset,
 )
 from xarray.backends.common import robust_getitem
-from xarray.backends.h5netcdf_ import H5netcdfBackendEntrypoint, _h5dump
+from xarray.backends.h5netcdf_ import H5netcdfBackendEntrypoint
 from xarray.backends.netcdf3 import _nc3_dtype_coercions
 from xarray.backends.netCDF4_ import (
     NetCDF4BackendEntrypoint,
@@ -4581,6 +4581,7 @@ def test_string_attributes_stored_as_char(self, tmp_path):
         with h5netcdf.File(store_path, "r") as ds:
             # Check that the attribute is stored as a char array
             assert ds._h5file.attrs["foo"].dtype == np.dtype("S3")
+            # assert ds._h5file.attrs["_nc3_strict"] == 1
 
 
 @requires_h5netcdf
@@ -4597,20 +4598,19 @@ def create_store(self):
                 yield store
 
     @requires_netCDF4
-    def test_h5dump(self, tmp_path) -> None:
-        data = create_test_data()
-
-        # Dump the representation of the netCDF4-generated file
-        fn = tmp_path / "netcdf4.nc"
-        data.to_netcdf(fn, engine="netcdf4", format=self.file_format)
-        expected = _h5dump(fn)
-
-        # Dump the representation of the h5netcdf-generated file
-        fn = tmp_path / "h5netcdf.nc"
-        data.to_netcdf(fn, engine=self.engine, format=self.file_format)
-        actual = _h5dump(fn)
-
-        assert expected == actual
+    def test_cross_engine_read_write_netcdf4(self) -> None:
+        # Drop dim3, because its labels include strings. These appear to be
+        # not properly read with python-netCDF4, which converts them into
+        # unicode instead of leaving them as bytes.
+        data = create_test_data().drop_vars("dim3")
+        data.attrs["foo"] = "bar"
+        valid_engines: list[T_NetcdfEngine] = ["netcdf4", "h5netcdf"]
+        for write_engine in valid_engines:
+            with create_tmp_file() as tmp_file:
+                data.to_netcdf(tmp_file, engine=write_engine, format=self.file_format)
+                for read_engine in valid_engines:
+                    with open_dataset(tmp_file, engine=read_engine) as actual:
+                        assert_identical(data, actual)
 
 
 @requires_scipy_or_netCDF4

From 0c1bc086beea37b379b544e0814ae1d438b1568c Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Mon, 8 Sep 2025 11:38:16 -0400
Subject: [PATCH 11/16] fix h5netcdf version check.

---
 xarray/backends/h5netcdf_.py  | 19 +++++++++----------
 xarray/tests/test_backends.py |  1 -
 2 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index a43015766bc..ff0c5c0d17d 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -125,6 +125,7 @@ def __init__(
         manager: FileManager | h5netcdf.File | h5netcdf.Group,
         group=None,
         mode=None,
+        format=None,
         lock=HDF5_LOCK,
         autoclose=False,
     ):
@@ -138,17 +139,13 @@ def __init__(
                     raise ValueError(
                         "must supply a h5netcdf.File if the group argument is provided"
                     )
-                if format == "NETCDF4_CLASSIC":
-                    raise ValueError(
-                        "Cannot create sub-groups in `NETCDF4_CLASSIC` format."
-                    )
                 root = manager
             manager = DummyFileManager(root)
 
         self._manager = manager
         self._group = group
         self._mode = mode
-        self.format = None
+        self.format = format or "NETCDF4"
         # todo: utilizing find_root_and_group seems a bit clunky
         #  making filename available on h5netcdf.Group seems better
         self._filename = find_root_and_group(self.ds)[0].filename
@@ -218,7 +215,7 @@ def open(
             kwargs.update(driver_kwds)
         if phony_dims is not None:
             kwargs["phony_dims"] = phony_dims
-        if format is not None and Version(h5netcdf.__version__) > Version("1.6"):
+        if format is not None and Version(h5netcdf.__version__) > Version("1.6.4"):
             kwargs["format"] = format
 
         if lock is None:
@@ -330,7 +327,7 @@ def set_dimension(self, name, length, is_unlimited=False):
 
     def convert_string(self, value):
         """If format is NETCDF4_CLASSIC, convert strings to char arrays."""
-        if self.format != "NETCDF4":
+        if self.format == "NETCDF4_CLASSIC":
             value = encode_nc3_attr_value(value)
             if isinstance(value, bytes):
                 value = np.bytes_(value)
@@ -341,10 +338,12 @@ def set_attribute(self, key, value):
         self.ds.attrs[key] = value
 
     def encode_variable(self, variable, name=None):
-        if self.format == "NETCDF4":
+        if self.format == "NETCDF4_CLASSIC":
+            return encode_nc3_variable(variable, name=name)
+        elif self.format == "NETCDF4":
             return _encode_nc4_variable(variable, name=name)
-
-        return encode_nc3_variable(variable, name=name)
+        else:
+            raise ValueError(f"unexpected format: {self.format}")
 
     def prepare_variable(
         self, name, variable, check_encoding=False, unlimited_dims=None
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 82916d5662a..aa8a1f2f15c 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4581,7 +4581,6 @@ def test_string_attributes_stored_as_char(self, tmp_path):
         with h5netcdf.File(store_path, "r") as ds:
             # Check that the attribute is stored as a char array
             assert ds._h5file.attrs["foo"].dtype == np.dtype("S3")
-            # assert ds._h5file.attrs["_nc3_strict"] == 1
 
 
 @requires_h5netcdf

From 03ea2de56738577b9e1314d870b5ea47cfb1717a Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Mon, 8 Sep 2025 16:05:42 -0400
Subject: [PATCH 12/16] try to fix tests

---
 xarray/backends/h5netcdf_.py  | 19 ++++++++++++-------
 xarray/tests/test_backends.py |  3 ++-
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index ff0c5c0d17d..1cfa4dfc635 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -125,7 +125,7 @@ def __init__(
         manager: FileManager | h5netcdf.File | h5netcdf.Group,
         group=None,
         mode=None,
-        format=None,
+        format="NETCDF4",
         lock=HDF5_LOCK,
         autoclose=False,
     ):
@@ -145,7 +145,7 @@ def __init__(
         self._manager = manager
         self._group = group
         self._mode = mode
-        self.format = format or "NETCDF4"
+        self.format = format
         # todo: utilizing find_root_and_group seems a bit clunky
         #  making filename available on h5netcdf.Group seems better
         self._filename = find_root_and_group(self.ds)[0].filename
@@ -229,7 +229,14 @@ def open(
             if isinstance(filename, str)
             else h5netcdf.File(filename, mode=mode, **kwargs)
         )
-        return cls(manager, group=group, mode=mode, lock=lock, autoclose=autoclose)
+        return cls(
+            manager,
+            group=group,
+            format=format,
+            mode=mode,
+            lock=lock,
+            autoclose=autoclose,
+        )
 
     def _acquire(self, needs_lock=True):
         with self._manager.acquire_context(needs_lock) as root:
@@ -340,10 +347,8 @@ def set_attribute(self, key, value):
     def encode_variable(self, variable, name=None):
         if self.format == "NETCDF4_CLASSIC":
             return encode_nc3_variable(variable, name=name)
-        elif self.format == "NETCDF4":
-            return _encode_nc4_variable(variable, name=name)
-        else:
-            raise ValueError(f"unexpected format: {self.format}")
+
+        return _encode_nc4_variable(variable, name=name)
 
     def prepare_variable(
         self, name, variable, check_encoding=False, unlimited_dims=None
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 8ee804ada59..1722ce3b387 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -460,6 +460,7 @@ def roundtrip(
             save_kwargs = {}
         if open_kwargs is None:
             open_kwargs = {}
+
         with create_tmp_file(allow_cleanup_failure=allow_cleanup_failure) as path:
             self.save(data, path, **save_kwargs)
             with self.open(path, **open_kwargs) as ds:
@@ -4799,7 +4800,7 @@ def test_encoding_unlimited_dims(self) -> None:
     @requires_scipy
     def test_roundtrip_via_bytes(self) -> None:
         original = create_test_data()
-        netcdf_bytes = original.to_netcdf()
+        netcdf_bytes = original.to_netcdf(engine="scipy")
         roundtrip = load_dataset(netcdf_bytes)
         assert_identical(roundtrip, original)
 

From 592b98b027f38205f230b151aa80e560d9b2aab3 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Mon, 8 Sep 2025 16:50:49 -0400
Subject: [PATCH 13/16] Set default format to NETCDF4 instead of None, because
 passing None to `netCDF4_.get_datatype` skips required conversions. Remove
 global attribute from create_test_data because it impacts other tests in
 other files.

---
 doc/whats-new.rst             |  3 +--
 xarray/backends/h5netcdf_.py  | 18 +++++++++---------
 xarray/tests/__init__.py      |  1 -
 xarray/tests/test_backends.py |  6 ++++++
 4 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index a841467d4b7..b54115b92e4 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -62,8 +62,7 @@ New Features
 - ``compute=False`` is now supported by :py:meth:`DataTree.to_netcdf` and
   :py:meth:`DataTree.to_zarr`.
   By `Stephan Hoyer <https://github.com/shoyer>`_.
-- The ``h5netcdf`` engine can now write ``NETCDF4_CLASSIC`` files
-  (:issue:`10676`, :pull:`10686`).
+- The ``h5netcdf`` engine has support for pseudo ``NETCDF4_CLASSIC`` files, meaning variables and attributes are cast to supported types. Note that the saved files won't be recognized as genuine ``NETCDF4_CLASSIC`` files until ``h5netcdf`` adds support.   (:issue:`10676`, :pull:`10686`).
   By `David Huard <https://github.com/huard>`_.
 - ``open_dataset`` will now correctly infer a path ending in ``.zarr/`` as zarr
   By `Ian Hunt-Isaak <https://github.com/ianhi>`_.
diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 1cfa4dfc635..1b8b40963db 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -142,10 +142,13 @@ def __init__(
                 root = manager
             manager = DummyFileManager(root)
 
+        if format == "NETCDF4_CLASSIC" and group is not None:
+            raise ValueError("Cannot create sub-groups in `NETCDF4_CLASSIC` format.")
+
         self._manager = manager
         self._group = group
         self._mode = mode
-        self.format = format
+        self.format = format or "NETCDF4"
         # todo: utilizing find_root_and_group seems a bit clunky
         #  making filename available on h5netcdf.Group seems better
         self._filename = find_root_and_group(self.ds)[0].filename
@@ -169,7 +172,7 @@ def open(
         cls,
         filename,
         mode="r",
-        format=None,
+        format="NETCDF4",
         group=None,
         lock=None,
         autoclose=False,
@@ -203,9 +206,6 @@ def open(
         if format not in [None, "NETCDF4", "NETCDF4_CLASSIC"]:
             raise ValueError("invalid format for h5netcdf backend")
 
-        if format == "NETCDF4_CLASSIC" and group is not None:
-            raise ValueError("Cannot create sub-groups in `NETCDF4_CLASSIC` format.")
-
         kwargs = {
             "invalid_netcdf": invalid_netcdf,
             "decode_vlen_strings": decode_vlen_strings,
@@ -358,7 +358,7 @@ def prepare_variable(
         _ensure_no_forward_slash_in_name(name)
         attrs = variable.attrs.copy()
         dtype = _get_datatype(
-            variable, self.format, raise_on_invalid_encoding=check_encoding
+            variable, nc_format=self.format, raise_on_invalid_encoding=check_encoding
         )
 
         fillvalue = attrs.pop("_FillValue", None)
@@ -510,7 +510,7 @@ def open_dataset(
         drop_variables: str | Iterable[str] | None = None,
         use_cftime=None,
         decode_timedelta=None,
-        format=None,
+        format="NETCDF4",
         group=None,
         lock=None,
         invalid_netcdf=None,
@@ -570,7 +570,7 @@ def open_datatree(
         drop_variables: str | Iterable[str] | None = None,
         use_cftime=None,
         decode_timedelta=None,
-        format=None,
+        format="NETCDF4",
         group: str | None = None,
         lock=None,
         invalid_netcdf=None,
@@ -613,7 +613,7 @@ def open_groups_as_dict(
         drop_variables: str | Iterable[str] | None = None,
         use_cftime=None,
         decode_timedelta=None,
-        format=None,
+        format="NETCDF4",
         group: str | None = None,
         lock=None,
         invalid_netcdf=None,
diff --git a/xarray/tests/__init__.py b/xarray/tests/__init__.py
index 67e24122852..3b4e49c64d8 100644
--- a/xarray/tests/__init__.py
+++ b/xarray/tests/__init__.py
@@ -397,7 +397,6 @@ def create_test_data(
     else:
         numbers_values = rs.integers(0, 3, _dims["dim3"], dtype="int64")
     obj.coords["numbers"] = ("dim3", numbers_values)
-    obj.attrs = {"foo": "bar"}
     obj.encoding = {"foo": "bar"}
     assert_writeable(obj)
     return obj
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 1722ce3b387..094d0c74988 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4715,6 +4715,12 @@ def test_cross_engine_read_write_netcdf4(self) -> None:
                     with open_dataset(tmp_file, engine=read_engine) as actual:
                         assert_identical(data, actual)
 
+    def test_group_fails(self):
+        # Check writing group data fails with CLASSIC format
+        original = create_test_data()
+        with pytest.raises(ValueError):
+            original.to_netcdf(group="sub", format=self.file_format, engine=self.engine)
+
 
 @requires_scipy_or_netCDF4
 class TestGenericNetCDFData(NetCDF3Only, CFEncodedBase):

From 13b60d005613d7e71b6648ed07d56a81e9e7b5f3 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 9 Sep 2025 11:21:30 -0400
Subject: [PATCH 14/16] Apply suggestions from code review

Co-authored-by: Stephan Hoyer <shoyer@google.com>
---
 xarray/backends/h5netcdf_.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 1b8b40963db..af527dc7b3b 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -204,7 +204,7 @@ def open(
                 )
 
         if format not in [None, "NETCDF4", "NETCDF4_CLASSIC"]:
-            raise ValueError("invalid format for h5netcdf backend")
+            raise ValueError(f"invalid format for h5netcdf backend: {format}")
 
         kwargs = {
             "invalid_netcdf": invalid_netcdf,

From cf8b4be409d8a7a99b54abbed0e66203e94af527 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Tue, 9 Sep 2025 11:28:17 -0400
Subject: [PATCH 15/16] Suggestions from review.

---
 xarray/backends/h5netcdf_.py  | 10 +++++++---
 xarray/tests/test_backends.py |  6 ++++--
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 1b8b40963db..9b3115b2c8c 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -333,7 +333,11 @@ def set_dimension(self, name, length, is_unlimited=False):
             self.ds.dimensions[name] = length
 
     def convert_string(self, value):
-        """If format is NETCDF4_CLASSIC, convert strings to char arrays."""
+        """If format is NETCDF4_CLASSIC, convert strings to fixed width char
+        arrays to ensure they can be read by legacy software.
+
+        CLASSIC attributes are read by third party software as fixed width char arrays
+        """
         if self.format == "NETCDF4_CLASSIC":
             value = encode_nc3_attr_value(value)
             if isinstance(value, bytes):
@@ -347,8 +351,8 @@ def set_attribute(self, key, value):
     def encode_variable(self, variable, name=None):
         if self.format == "NETCDF4_CLASSIC":
             return encode_nc3_variable(variable, name=name)
-
-        return _encode_nc4_variable(variable, name=name)
+        else:
+            return _encode_nc4_variable(variable, name=name)
 
     def prepare_variable(
         self, name, variable, check_encoding=False, unlimited_dims=None
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 094d0c74988..d9a8b85aaac 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -4718,7 +4718,9 @@ def test_cross_engine_read_write_netcdf4(self) -> None:
     def test_group_fails(self):
         # Check writing group data fails with CLASSIC format
         original = create_test_data()
-        with pytest.raises(ValueError):
+        with pytest.raises(
+            ValueError, match=r"Cannot create sub-groups in `NETCDF4_CLASSIC` format."
+        ):
             original.to_netcdf(group="sub", format=self.file_format, engine=self.engine)
 
 
@@ -4806,7 +4808,7 @@ def test_encoding_unlimited_dims(self) -> None:
     @requires_scipy
     def test_roundtrip_via_bytes(self) -> None:
         original = create_test_data()
-        netcdf_bytes = original.to_netcdf(engine="scipy")
+        netcdf_bytes = original.to_netcdf()
         roundtrip = load_dataset(netcdf_bytes)
         assert_identical(roundtrip, original)
 

From b2a21d22a39829879b98006bf64e38101eba02a0 Mon Sep 17 00:00:00 2001
From: David Huard <huard.david@ouranos.ca>
Date: Mon, 22 Sep 2025 11:11:43 -0400
Subject: [PATCH 16/16] Raise an error within `get_child_store` rather that
 `__init__` with format=NETCDF4_CLASSIC group not None.

---
 xarray/backends/h5netcdf_.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xarray/backends/h5netcdf_.py b/xarray/backends/h5netcdf_.py
index 9c1bb16cba6..d3993590e56 100644
--- a/xarray/backends/h5netcdf_.py
+++ b/xarray/backends/h5netcdf_.py
@@ -143,9 +143,6 @@ def __init__(
                 root = manager
             manager = DummyFileManager(root)
 
-        if format == "NETCDF4_CLASSIC" and group is not None:
-            raise ValueError("Cannot create sub-groups in `NETCDF4_CLASSIC` format.")
-
         self._manager = manager
         self._group = group
         self._mode = mode
@@ -158,6 +155,9 @@ def __init__(
         self.autoclose = autoclose
 
     def get_child_store(self, group: str) -> Self:
+        if self.format == "NETCDF4_CLASSIC":
+            raise ValueError("Cannot create sub-groups in `NETCDF4_CLASSIC` format.")
+
         if self._group is not None:
             group = os.path.join(self._group, group)
         return type(self)(
@@ -231,7 +231,7 @@ def open(
             else PickleableFileManager
         )
         manager = manager_cls(h5netcdf.File, filename, mode=mode, kwargs=kwargs)
-        
+
         return cls(
             manager,
             group=group,