More correct deprecation warning for lock argument

doc/whats-new.rst

@@ -131,6 +131,11 @@ Deprecations
   :py:func:`xarray.open_mfdataset` when `combine='by_coords'` is specified.
   Fixes (:issue:`5230`), via (:pull:`5231`, :pull:`5255`).
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
+- The `lock` keyword argument to :py:func:`open_dataset` and :py:func:`open_dataarray` is now
+  a backend specific option. It will give a warning if passed to a backend that doesn't support it
+  instead of being silently ignored. From the next version it will raise an error.
+  This is part of the refactor to support external backends (:issue:`5073`).
+  By `Tom Nicholas <https://github.com/TomNicholas>`_ and `Alessandro Amici <https://github.com/alexamici>`_.
 
 
 Bug fixes

xarray/backends/api.py

@@ -449,7 +449,7 @@ def open_dataset(
           relevant when using dask or another form of parallelism. By default,
           appropriate locks are chosen to safely read and write files with the
           currently active dask scheduler. Supported by "netcdf4", "h5netcdf",
-          "pynio", "pseudonetcdf", "cfgrib".
+          "scipy", "pynio", "pseudonetcdf", "cfgrib".
 
         See engine open function for kwargs accepted by each specific engine.
 
@@ -633,7 +633,7 @@ def open_dataarray(
           relevant when using dask or another form of parallelism. By default,
           appropriate locks are chosen to safely read and write files with the
           currently active dask scheduler. Supported by "netcdf4", "h5netcdf",
-          "pynio", "pseudonetcdf", "cfgrib".
+          "scipy", "pynio", "pseudonetcdf", "cfgrib".
 
         See engine open function for kwargs accepted by each specific engine.
 

xarray/backends/pydap_.py

@@ -1,3 +1,5 @@
+import warnings
+
 import numpy as np
 
 from ..core import indexing
@@ -122,7 +124,16 @@ def open_dataset(
         use_cftime=None,
         decode_timedelta=None,
         session=None,
+        lock=None,
     ):
+        # TODO remove after v0.19
+        if lock is not None:
+            warnings.warn(
+                "The kwarg 'lock' has been deprecated for this backend, and is now "
+                "ignored. In the future passing lock will raise an error.",
+                DeprecationWarning,
+            )
+
         store = PydapDataStore.open(
             filename_or_obj,
             session=session,

xarray/backends/zarr.py

@@ -1,5 +1,6 @@
 import os
 import pathlib
+import warnings
 from distutils.version import LooseVersion
 
 import numpy as np
@@ -721,7 +722,15 @@ def open_dataset(
         consolidate_on_close=False,
         chunk_store=None,
         storage_options=None,
+        lock=None,
     ):
+        # TODO remove after v0.19
+        if lock is not None:
+            warnings.warn(
+                "The kwarg 'lock' has been deprecated for this backend, and is now "
+                "ignored. In the future passing lock will raise an error.",
+                DeprecationWarning,
+            )
 
         filename_or_obj = _normalize_path(filename_or_obj)
         store = ZarrStore.open_group(