Refactor backends to separate writers from readers (#10771)

* move datatree_io to backends directory

* split backends/api.py

* lint imports

* fix bad import mypy noticed

* make type available at runtime

* whatsnew

* lint

Author: TomNicholas

doc/whats-new.rst

@@ -77,6 +77,8 @@ Documentation
 Internal Changes
 ~~~~~~~~~~~~~~~~
 
+- Refactor structure of ``backends`` module to separate code for reading data from code for writing data (:pull:`10771`).
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 - All test files now have full mypy type checking enabled (``check_untyped_defs = true``),
   improving type safety and making the test suite a better reference for type annotations.
   (:pull:`10768`)

xarray/__init__.py

@@ -10,8 +10,8 @@
     open_datatree,
     open_groups,
     open_mfdataset,
-    save_mfdataset,
 )
+from xarray.backends.writers import save_mfdataset
 from xarray.backends.zarr import open_zarr
 from xarray.coding.cftime_offsets import cftime_range, date_range, date_range_like
 from xarray.coding.cftimeindex import CFTimeIndex