GroupBy(chunked-array) (#9522)

* GroupBy(chunked-array)

Closes #757
Closes #2852

* Optimizations

* Optimize multi-index construction

* Add tests

* Add whats-new

* Raise errors

* Add docstring

* preserve attrs

* Add test for #757

* Typing fixes

* Handle multiple groupers

* Backcompat

* better backcompat

* fix

* Handle edge case

* comment

* type: ignore

doc/user-guide/groupby.rst

@@ -294,6 +294,15 @@ is identical to
     ds.resample(time=TimeResampler("ME"))
 
 
+The :py:class:`groupers.UniqueGrouper` accepts an optional ``labels`` kwarg that is not present
+in :py:meth:`DataArray.groupby` or :py:meth:`Dataset.groupby`.
+Specifying ``labels`` is required when grouping by a lazy array type (e.g. dask or cubed).
+The ``labels`` are used to construct the output coordinate (say for a reduction), and aggregations
+will only be run over the specified labels.
+You may use ``labels`` to also specify the ordering of groups to be used during iteration.
+The order will be preserved in the output.
+
+
 .. _groupby.multiple:
 
 Grouping by multiple variables

doc/whats-new.rst

@@ -23,14 +23,21 @@ New Features
 ~~~~~~~~~~~~
 - Added :py:meth:`DataTree.persist` method (:issue:`9675`, :pull:`9682`).
   By `Sam Levang <https://github.com/slevang>`_.
+- Support lazy grouping by dask arrays, and allow specifying ordered groups with ``UniqueGrouper(labels=["a", "b", "c"])``
+  (:issue:`2852`, :issue:`757`).
+  By `Deepak Cherian <https://github.com/dcherian>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
 
 Deprecations
 ~~~~~~~~~~~~
-
+- Grouping by a chunked array (e.g. dask or cubed) currently eagerly loads that variable in to
+  memory. This behaviour is deprecated. If eager loading was intended, please load such arrays
+  manually using ``.load()`` or ``.compute()``. Else pass ``eagerly_compute_group=False``, and
+  provide expected group labels using the ``labels`` kwarg to a grouper object such as
+  :py:class:`grouper.UniqueGrouper` or :py:class:`grouper.BinGrouper`.
 
 Bug fixes
 ~~~~~~~~~
@@ -94,14 +101,6 @@ New Features
   (:issue:`9427`, :pull: `9428`).
   By `Alfonso Ladino <https://github.com/aladinor>`_.
 
-Breaking changes
-~~~~~~~~~~~~~~~~
-
-
-Deprecations
-~~~~~~~~~~~~
-
-
 Bug fixes
 ~~~~~~~~~
 

xarray/core/common.py

@@ -1094,7 +1094,7 @@ def _resample(
                 f"Received {type(freq)} instead."
             )
 
-        rgrouper = ResolvedGrouper(grouper, group, self)
+        rgrouper = ResolvedGrouper(grouper, group, self, eagerly_compute_group=False)
 
         return resample_cls(
             self,

xarray/core/dataarray.py

@@ -6748,6 +6748,7 @@ def groupby(
         *,
         squeeze: Literal[False] = False,
         restore_coord_dims: bool = False,
+        eagerly_compute_group: bool = True,
         **groupers: Grouper,
     ) -> DataArrayGroupBy:
         """Returns a DataArrayGroupBy object for performing grouped operations.
@@ -6763,6 +6764,11 @@ def groupby(
         restore_coord_dims : bool, default: False
             If True, also restore the dimension order of multi-dimensional
             coordinates.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
         **groupers : Mapping of str to Grouper or Resampler
             Mapping of variable name to group by to :py:class:`Grouper` or :py:class:`Resampler` object.
             One of ``group`` or ``groupers`` must be provided.
@@ -6877,7 +6883,9 @@ def groupby(
         )
 
         _validate_groupby_squeeze(squeeze)
-        rgroupers = _parse_group_and_groupers(self, group, groupers)
+        rgroupers = _parse_group_and_groupers(
+            self, group, groupers, eagerly_compute_group=eagerly_compute_group
+        )
         return DataArrayGroupBy(self, rgroupers, restore_coord_dims=restore_coord_dims)
 
     @_deprecate_positional_args("v2024.07.0")
@@ -6892,6 +6900,7 @@ def groupby_bins(
         squeeze: Literal[False] = False,
         restore_coord_dims: bool = False,
         duplicates: Literal["raise", "drop"] = "raise",
+        eagerly_compute_group: bool = True,
     ) -> DataArrayGroupBy:
         """Returns a DataArrayGroupBy object for performing grouped operations.
 
@@ -6928,6 +6937,11 @@ def groupby_bins(
             coordinates.
         duplicates : {"raise", "drop"}, default: "raise"
             If bin edges are not unique, raise ValueError or drop non-uniques.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
 
         Returns
         -------
@@ -6965,7 +6979,9 @@ def groupby_bins(
             precision=precision,
             include_lowest=include_lowest,
         )
-        rgrouper = ResolvedGrouper(grouper, group, self)
+        rgrouper = ResolvedGrouper(
+            grouper, group, self, eagerly_compute_group=eagerly_compute_group
+        )
 
         return DataArrayGroupBy(
             self,

xarray/core/dataset.py

@@ -10379,6 +10379,7 @@ def groupby(
         *,
         squeeze: Literal[False] = False,
         restore_coord_dims: bool = False,
+        eagerly_compute_group: bool = True,
         **groupers: Grouper,
     ) -> DatasetGroupBy:
         """Returns a DatasetGroupBy object for performing grouped operations.
@@ -10394,6 +10395,11 @@ def groupby(
         restore_coord_dims : bool, default: False
             If True, also restore the dimension order of multi-dimensional
             coordinates.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
         **groupers : Mapping of str to Grouper or Resampler
             Mapping of variable name to group by to :py:class:`Grouper` or :py:class:`Resampler` object.
             One of ``group`` or ``groupers`` must be provided.
@@ -10476,7 +10482,9 @@ def groupby(
         )
 
         _validate_groupby_squeeze(squeeze)
-        rgroupers = _parse_group_and_groupers(self, group, groupers)
+        rgroupers = _parse_group_and_groupers(
+            self, group, groupers, eagerly_compute_group=eagerly_compute_group
+        )
 
         return DatasetGroupBy(self, rgroupers, restore_coord_dims=restore_coord_dims)
 
@@ -10492,6 +10500,7 @@ def groupby_bins(
         squeeze: Literal[False] = False,
         restore_coord_dims: bool = False,
         duplicates: Literal["raise", "drop"] = "raise",
+        eagerly_compute_group: bool = True,
     ) -> DatasetGroupBy:
         """Returns a DatasetGroupBy object for performing grouped operations.
 
@@ -10528,6 +10537,11 @@ def groupby_bins(
             coordinates.
         duplicates : {"raise", "drop"}, default: "raise"
             If bin edges are not unique, raise ValueError or drop non-uniques.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
 
         Returns
         -------
@@ -10565,7 +10579,9 @@ def groupby_bins(
             precision=precision,
             include_lowest=include_lowest,
         )
-        rgrouper = ResolvedGrouper(grouper, group, self)
+        rgrouper = ResolvedGrouper(
+            grouper, group, self, eagerly_compute_group=eagerly_compute_group
+        )
 
         return DatasetGroupBy(
             self,