Switch back default netCDF engine

doc/whats-new.rst

@@ -17,6 +17,14 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+- Xarray's default engine for reading/writing netCDF files has been reverted to
+  prefer netCDF4, which was the default before v2025.09.1. This change
+  had larger implications for the ecosystem than we anticipated. We are still
+  considering changing the default in the future, but will be a bit more careful
+  about the implications. See :issue:`10657` and linked issues for discussion.
+  The behavior can still be customized, e.g., with
+  ``xr.set_options(netcdf_engine_order=['h5netcdf', 'netcdf4', 'scipy'])``.
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 
 Deprecations
 ~~~~~~~~~~~~

xarray/core/options.py

@@ -59,7 +59,7 @@ class T_Options(TypedDict):
         enable_cftimeindex: bool
         file_cache_maxsize: int
         keep_attrs: Literal["default"] | bool
-        netcdf_engine_order: Sequence[Literal["h5netcdf", "scipy", "netcdf4"]]
+        netcdf_engine_order: Sequence[Literal["netcdf4", "h5netcdf", "scipy"]]
         warn_for_unclosed_files: bool
         use_bottleneck: bool
         use_flox: bool