Remove the references to _file_obj outside low level code paths, ch…

…ange to `_close` (#4809)

* Move from _file_obj object to _close function

* Remove all references to _close outside of low level

* Fix type hints

* Cleanup code style

* Fix non-trivial type hint problem

* Revert adding the `close` argument and add a set_close instead

* Remove helper class for an easier helper function + code style

* Add set_close docstring

* Code style

* Revert changes in _replace to keep cose as an exception

See: https://github.com/pydata/xarray/pull/4809/files#r557628298

* One more bit to revert

* One more bit to revert

* Add What's New entry

* Use set_close setter

* Apply suggestions from code review

Co-authored-by: Stephan Hoyer <shoyer@google.com>

* Rename user-visible argument

* Sync wording in docstrings.

Co-authored-by: Stephan Hoyer <shoyer@google.com>

doc/whats-new.rst

@@ -108,6 +108,8 @@ Internal Changes
   By `Maximilian Roos <https://github.com/max-sixty>`_.
 - Speed up attribute style access (e.g. ``ds.somevar`` instead of ``ds["somevar"]``) and tab completion
   in ipython (:issue:`4741`, :pull:`4742`). By `Richard Kleijn <https://github.com/rhkleijn>`_.
+- Added the ``set_close`` method to ``Dataset`` and ``DataArray`` for beckends to specify how to voluntary release
+  all resources. (:pull:`#4809`), By `Alessandro Amici <https://github.com/alexamici>`_.
 
 .. _whats-new.0.16.2:
 

xarray/backends/api.py

@@ -522,7 +522,7 @@ def maybe_decode_store(store, chunks):
 
         else:
             ds2 = ds
-        ds2._file_obj = ds._file_obj
+        ds2.set_close(ds._close)
         return ds2
 
     filename_or_obj = _normalize_path(filename_or_obj)
@@ -701,7 +701,7 @@ def open_dataarray(
     else:
         (data_array,) = dataset.data_vars.values()
 
-    data_array._file_obj = dataset._file_obj
+    data_array.set_close(dataset._close)
 
     # Reset names if they were changed during saving
     # to ensure that we can 'roundtrip' perfectly
@@ -715,17 +715,6 @@ def open_dataarray(
     return data_array
 
 
-class _MultiFileCloser:
-    __slots__ = ("file_objs",)
-
-    def __init__(self, file_objs):
-        self.file_objs = file_objs
-
-    def close(self):
-        for f in self.file_objs:
-            f.close()
-
-
 def open_mfdataset(
     paths,
     chunks=None,
@@ -918,14 +907,14 @@ def open_mfdataset(
         getattr_ = getattr
 
     datasets = [open_(p, **open_kwargs) for p in paths]
-    file_objs = [getattr_(ds, "_file_obj") for ds in datasets]
+    closers = [getattr_(ds, "_close") for ds in datasets]
     if preprocess is not None:
         datasets = [preprocess(ds) for ds in datasets]
 
     if parallel:
         # calling compute here will return the datasets/file_objs lists,
         # the underlying datasets will still be stored as dask arrays
-        datasets, file_objs = dask.compute(datasets, file_objs)
+        datasets, closers = dask.compute(datasets, closers)
 
     # Combine all datasets, closing them in case of a ValueError
     try:
@@ -963,7 +952,11 @@ def open_mfdataset(
             ds.close()
         raise
 
-    combined._file_obj = _MultiFileCloser(file_objs)
+    def multi_file_closer():
+        for closer in closers:
+            closer()
+
+    combined.set_close(multi_file_closer)
 
     # read global attributes from the attrs_file or from the first dataset
     if attrs_file is not None:

xarray/backends/apiv2.py

@@ -90,7 +90,7 @@ def _dataset_from_backend_dataset(
             **extra_tokens,
         )
 
-    ds._file_obj = backend_ds._file_obj
+    ds.set_close(backend_ds._close)
 
     # Ensure source filename always stored in dataset object (GH issue #2550)
     if "source" not in ds.encoding:

xarray/backends/rasterio_.py

@@ -361,6 +361,6 @@ def open_rasterio(filename, parse_coordinates=None, chunks=None, cache=None, loc
         result = result.chunk(chunks, name_prefix=name_prefix, token=token)
 
     # Make the file closeable
-    result._file_obj = manager
+    result.set_close(manager.close)
 
     return result

xarray/backends/store.py

@@ -19,7 +19,6 @@ def open_backend_dataset_store(
     decode_timedelta=None,
 ):
     vars, attrs = store.load()
-    file_obj = store
     encoding = store.get_encoding()
 
     vars, attrs, coord_names = conventions.decode_cf_variables(
@@ -36,7 +35,7 @@ def open_backend_dataset_store(
 
     ds = Dataset(vars, attrs=attrs)
     ds = ds.set_coords(coord_names.intersection(vars))
-    ds._file_obj = file_obj
+    ds.set_close(store.close)
     ds.encoding = encoding
 
     return ds

xarray/conventions.py

@@ -576,12 +576,12 @@ def decode_cf(
         vars = obj._variables
         attrs = obj.attrs
         extra_coords = set(obj.coords)
-        file_obj = obj._file_obj
+        close = obj._close
         encoding = obj.encoding
     elif isinstance(obj, AbstractDataStore):
         vars, attrs = obj.load()
         extra_coords = set()
-        file_obj = obj
+        close = obj.close
         encoding = obj.get_encoding()
     else:
         raise TypeError("can only decode Dataset or DataStore objects")
@@ -599,7 +599,7 @@ def decode_cf(
     )
     ds = Dataset(vars, attrs=attrs)
     ds = ds.set_coords(coord_names.union(extra_coords).intersection(vars))
-    ds._file_obj = file_obj
+    ds.set_close(close)
     ds.encoding = encoding
 
     return ds

xarray/core/common.py

@@ -11,6 +11,7 @@
     Iterator,
     List,
     Mapping,
+    Optional,
     Tuple,
     TypeVar,
     Union,
@@ -330,7 +331,9 @@ def get_squeeze_dims(
 class DataWithCoords(SupportsArithmetic, AttrAccessMixin):
     """Shared base class for Dataset and DataArray."""
 
-    __slots__ = ()
+    _close: Optional[Callable[[], None]]
+
+    __slots__ = ("_close",)
 
     _rolling_exp_cls = RollingExp
 
@@ -1263,11 +1266,27 @@ def where(self, cond, other=dtypes.NA, drop: bool = False):
 
         return ops.where_method(self, cond, other)
 
+    def set_close(self, close: Optional[Callable[[], None]]) -> None:
+        """Register the function that releases any resources linked to this object.
+
+        This method controls how xarray cleans up resources associated
+        with this object when the ``.close()`` method is called. It is mostly
+        intended for backend developers and it is rarely needed by regular
+        end-users.
+
+        Parameters
+        ----------
+        close : callable
+            The function that when called like ``close()`` releases
+            any resources linked to this object.
+        """
+        self._close = close
+
     def close(self: Any) -> None:
-        """Close any files linked to this object"""
-        if self._file_obj is not None:
-            self._file_obj.close()
-        self._file_obj = None
+        """Release any resources linked to this object."""
+        if self._close is not None:
+            self._close()
+        self._close = None
 
     def isnull(self, keep_attrs: bool = None):
         """Test each value in the array for whether it is a missing value.

xarray/core/dataarray.py

@@ -344,14 +344,15 @@ class DataArray(AbstractArray, DataWithCoords):
 
     _cache: Dict[str, Any]
     _coords: Dict[Any, Variable]
+    _close: Optional[Callable[[], None]]
     _indexes: Optional[Dict[Hashable, pd.Index]]
     _name: Optional[Hashable]
     _variable: Variable
 
     __slots__ = (
         "_cache",
         "_coords",
-        "_file_obj",
+        "_close",
         "_indexes",
         "_name",
         "_variable",
@@ -421,7 +422,7 @@ def __init__(
         # public interface.
         self._indexes = indexes
 
-        self._file_obj = None
+        self._close = None
 
     def _replace(
         self,

xarray/core/dataset.py

@@ -636,6 +636,7 @@ class Dataset(Mapping, ImplementsDatasetReduce, DataWithCoords):
     _coord_names: Set[Hashable]
     _dims: Dict[Hashable, int]
     _encoding: Optional[Dict[Hashable, Any]]
+    _close: Optional[Callable[[], None]]
     _indexes: Optional[Dict[Hashable, pd.Index]]
     _variables: Dict[Hashable, Variable]
 
@@ -645,7 +646,7 @@ class Dataset(Mapping, ImplementsDatasetReduce, DataWithCoords):
         "_coord_names",
         "_dims",
         "_encoding",
-        "_file_obj",
+        "_close",
         "_indexes",
         "_variables",
         "__weakref__",
@@ -687,7 +688,7 @@ def __init__(
         )
 
         self._attrs = dict(attrs) if attrs is not None else None
-        self._file_obj = None
+        self._close = None
         self._encoding = None
         self._variables = variables
         self._coord_names = coord_names
@@ -703,7 +704,7 @@ def load_store(cls, store, decoder=None) -> "Dataset":
         if decoder:
             variables, attributes = decoder(variables, attributes)
         obj = cls(variables, attrs=attributes)
-        obj._file_obj = store
+        obj.set_close(store.close)
         return obj
 
     @property
@@ -876,7 +877,7 @@ def __dask_postcompute__(self):
             self._attrs,
             self._indexes,
             self._encoding,
-            self._file_obj,
+            self._close,
         )
         return self._dask_postcompute, args
 
@@ -896,7 +897,7 @@ def __dask_postpersist__(self):
             self._attrs,
             self._indexes,
             self._encoding,
-            self._file_obj,
+            self._close,
         )
         return self._dask_postpersist, args
 
@@ -1007,7 +1008,7 @@ def _construct_direct(
         attrs=None,
         indexes=None,
         encoding=None,
-        file_obj=None,
+        close=None,
     ):
         """Shortcut around __init__ for internal use when we want to skip
         costly validation
@@ -1020,7 +1021,7 @@ def _construct_direct(
         obj._dims = dims
         obj._indexes = indexes
         obj._attrs = attrs
-        obj._file_obj = file_obj
+        obj._close = close
         obj._encoding = encoding
         return obj
 
@@ -2122,7 +2123,7 @@ def isel(
             attrs=self._attrs,
             indexes=indexes,
             encoding=self._encoding,
-            file_obj=self._file_obj,
+            close=self._close,
         )
 
     def _isel_fancy(