fix h5netcdf backend for format=None, use same rule as netcdf4 backend

doc/whats-new.rst

@@ -16,6 +16,8 @@ New Features
 - :py:func:`merge` and :py:func:`concat` now support :py:class:`DataTree`
   objects (:issue:`9790`, :issue:`9778`).
   By `Stephan Hoyer <https://github.com/shoyer>`_.
+- 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 with version 1.7.0. (:issue:`10676`, :pull:`10686`).
+  By `David Huard <https://github.com/huard>`_.
 
 Breaking Changes
 ~~~~~~~~~~~~~~~~
@@ -27,6 +29,8 @@ Deprecations
 
 Bug Fixes
 ~~~~~~~~~
+- Fix h5netcdf backend for format=None, use same rule as netcdf4 backend (:pull:`10859`).
+  By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_
 
 
 Documentation
@@ -285,8 +289,6 @@ 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 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>`_.
 

xarray/backends/h5netcdf_.py

@@ -204,7 +204,10 @@ def open(
                     f"{magic_number!r} is not the signature of a valid netCDF4 file"
                 )
 
-        if format not in [None, "NETCDF4", "NETCDF4_CLASSIC"]:
+        if format is None:
+            format = "NETCDF4"
+
+        if format not in ["NETCDF4", "NETCDF4_CLASSIC"]:
             raise ValueError(f"invalid format for h5netcdf backend: {format}")
 
         kwargs = {