Convert to xarray

Changelog.rst

@@ -3,13 +3,21 @@ Version NEXTVERSION
 
 **2026-06-??**
 
+* New methods to convert to `xarray`: `cf.Field.to_xarray`,
+  `cf.FieldList.to_xarray`, `cf.Domain.to_xarray`, and
+  `cf.DomainList.to_xarray`
+  (https://github.com/NCAS-CMS/cf-python/issues/933)
+* New output format for `cf.write` that creates an `xarray` dataset in
+  memory: ``'XARRAY'``
+  (https://github.com/NCAS-CMS/cf-python/issues/933)
 * New keyword parameter to `cf.Field.regrids` and `cf.Field.regridc`:
   ``mtol`` (https://github.com/NCAS-CMS/cf-python/issues/949)
 * Fix bug in `cf.read` that prevented some OPeNDAP URLS being read
   (https://github.com/NCAS-CMS/cf-python/issues/948)
 * Fix bug that caused weighted `cf.collapse` and `cf.weights` to fail
   when a cell meausures has a size 1 axis
   (https://github.com/NCAS-CMS/cf-python/issues/952)
+* New optional dependency: ``xarray>=2026.2.0``
 
 ----
 

cf/mixin/fielddomain.py

@@ -3884,6 +3884,43 @@ def set_coordinate_reference(
 
         return self.set_construct(ref, key=key, copy=False)
 
+    def to_xarray(self, group=True):
+        """Convert the {{class}} to an `xarray` dataset.
+
+        {{cf_xarray description}}
+
+        Note that ``ds = f.to_xarray()`` is identical to ``ds =
+        cf.write(f, fmt='XARRAY')``; and multiple {{class_lower}}s may
+        be written to the same `xarray` dataset with
+        `cf.{{class}}List.to_xarray`, or with `cf.write` (e.g. ``ds =
+        cf.write([f, g], fmt='XARRAY')``). Also, `cf.write` allows a
+        mixture a mixture of fields and domains to be written to the
+        same `xarray` dataset.
+
+        .. versionadded:: NEXTVERSION
+
+        .. seealso:: `cf.{{class}}List.to_xarray`, `cf.write`
+
+        :Parameter:
+
+            group: `bool`, optional
+
+                If False then create a "flat" dataset, i.e. one with
+                only the root group, regardless of any group structure
+                specified by the netCDF interfaces of the
+                {{class_lower}} and its components. If True (the
+                default) then any sub-groups will be created and
+                populated.
+
+        :Returns:
+
+            {{Returns xarray}}
+
+        """
+        from cf.read_write import write
+
+        return write(self, fmt="XARRAY", group=group)
+
     # ----------------------------------------------------------------
     # Aliases
     # ----------------------------------------------------------------

cf/mixin/fielddomainlist.py

@@ -331,3 +331,35 @@ def select_by_rank(self, *ranks):
         """
 
         return type(self)(f for f in self if f.match_by_rank(*ranks))
+
+    def to_xarray(self, group=True):
+        """Convert the list elements to an `xarray` Dataset.
+
+        {{cf_xarray description}}
+
+        Note that ``ds = fl.to_xarray()`` is identical to ``ds =
+        cf.write(fl, fmt='XARRAY')``. Also, `cfdm.write` allows a
+        mixture a mixture of fields and domains to be written to the
+        same `xarray` dataset.
+
+        .. versionadded:: NEXTVERSION
+
+        .. seealso:: `cf.write`
+
+        :Parameter:
+
+            group: `bool`, optional
+                If False then create a "flat" dataset, i.e. one with
+                only the root group, regardless of any group structure
+                specified by the netCDF interfaces of the list
+                elements and their components. If True (the default)
+                then any sub-groups will be created and populated.
+
+        :Returns:
+
+            {{Returns xarray}}
+
+        """
+        from cf.read_write import write
+
+        return write(self, fmt="XARRAY", group=group)

cf/test/test_xarray.py

@@ -0,0 +1,178 @@
+import datetime
+import faulthandler
+import unittest
+
+faulthandler.enable()  # to debug seg faults and timeouts
+
+import xarray as xr
+
+import cf
+
+
+class xarrayTest(unittest.TestCase):
+    """Unit test for converting to xarray."""
+
+    def setUp(self):
+        """Preparations called immediately before each test method."""
+        # Disable log messages to silence expected warnings
+        cf.log_level("DISABLE")
+        # Note: to enable all messages for given methods, lines or
+        # calls (those without a 'verbose' option to do the same)
+        # e.g. to debug them, wrap them (for methods, start-to-end
+        # internally) as follows:
+        #
+        # cf.LOG_LEVEL('DEBUG')
+        # < ... test code ... >
+        # cf.log_level('DISABLE')
+
+    def test_Field_to_xarray(self):
+        """Test Field.to_xarray."""
+        fields = cf.example_fields()
+
+        # Write each field to a different xarray dataset
+        for f in fields:
+            ds = f.to_xarray()
+            self.assertIsInstance(ds, xr.Dataset)
+            str(ds)
+            self.assertIn("Conventions", ds.attrs)
+
+        # Write all fields to one xarray dataset
+        ds = cf.write(fields, fmt="XARRAY")
+        self.assertIsInstance(ds, xr.Dataset)
+        str(ds)
+
+    def test_Domain_to_xarray(self):
+        """Test Domain.to_xarray."""
+        domains = [f.domain for f in cf.example_fields()]
+
+        # Write each domain to a different xarray dataset
+        for d in domains:
+            ds = d.to_xarray()
+            self.assertIsInstance(ds, xr.Dataset)
+            str(ds)
+
+        # Write all domains to one xarray dataset
+        ds = cf.write(domains, fmt="XARRAY")
+        self.assertIsInstance(ds, xr.Dataset)
+        str(ds)
+
+    def test_FieldList_to_xarray(self):
+        """Test Field.to_xarray."""
+        fields = cf.example_fields()
+        ds = fields.to_xarray()
+        self.assertIsInstance(ds, xr.Dataset)
+        str(ds)
+
+    def test_DomainList_to_xarray(self):
+        """Test DomainList.to_xarray."""
+        domains = cf.DomainList([f.domain for f in cf.example_fields()])
+        ds = domains.to_xarray()
+        self.assertIsInstance(ds, xr.Dataset)
+        str(ds)
+
+    def test_FieldList_to_xarray_from_disk(self):
+        """Test FieldList.to_xarray from datasets read from disk."""
+        for dataset in (
+            "example_field_0.nc",
+            "example_field_0.zarr2",
+            "example_field_0.zarr3",
+            "gathered.nc",
+            "DSG_timeSeries_contiguous.nc",
+            "DSG_timeSeries_indexed.nc",
+            "DSG_timeSeriesProfile_indexed_contiguous.nc",
+            "parent.nc",
+            "external.nc",
+            "external_missing.nc",
+            "combined.nc",
+            "geometry_1.nc",
+            "geometry_2.nc",
+            "geometry_3.nc",
+            "geometry_4.nc",
+            "geometry_interior_ring.nc",
+            "geometry_interior_ring_2.nc",
+            "string_char.nc",
+            "subsampled_2.nc",
+            "ugrid_1.nc",
+            "ugrid_2.nc",
+            "ugrid_3.nc",
+            "test_file.nc",
+            "extra_data.pp",
+            "file1.pp",
+            "umfile.pp",
+            "wgdos_packed.pp",
+        ):
+            f = cf.read(dataset)
+            ds = f.to_xarray()
+            self.assertIsInstance(ds, xr.Dataset)
+            str(ds)
+
+    def test_Field_to_xarray_groups(self):
+        """Test Field.to_xarray with groups."""
+        f = cf.example_field(0)
+        g = f.copy()
+
+        ds = f.to_xarray()
+        self.assertIsInstance(ds, xr.Dataset)
+
+        f.nc_set_variable("/forecast/model/q2")
+        ds = f.to_xarray()
+        self.assertIsInstance(ds, xr.DataTree)
+        self.assertIn("q2", ds["/forecast/model"])
+        str(ds)
+
+        # group=True
+        ds = cf.write([f, g], fmt="XARRAY")
+        self.assertIsInstance(ds, xr.DataTree)
+        str(ds)
+
+        self.assertIn("q", ds)
+        self.assertIn("q2", ds["/forecast/model"])
+
+        # group=False
+        ds = f.to_xarray(group=False)
+        self.assertIsInstance(ds, xr.Dataset)
+        self.assertIn("q2", ds)
+        str(ds)
+
+        ds = cf.write([f, g], fmt="XARRAY", group=False)
+        self.assertIsInstance(ds, xr.Dataset)
+        str(ds)
+
+        self.assertIn("q", ds)
+        self.assertIn("q2", ds)
+
+    def test_FieldList_to_xarray_groups(self):
+        """Test Field.to_xarray with groups."""
+        f = cf.example_fields(0)
+
+        ds = f.to_xarray()
+        self.assertIsInstance(ds, xr.Dataset)
+
+        f[0].nc_set_variable("/forecast/model/q2")
+        ds = f.to_xarray()
+        self.assertIsInstance(ds, xr.DataTree)
+        self.assertIn("q2", ds["/forecast/model"])
+        str(ds)
+
+        ds = f.to_xarray(group=False)
+        self.assertIsInstance(ds, xr.Dataset)
+        self.assertIn("q2", ds)
+        str(ds)
+
+    def test_Field_to_xarray_aggregation(self):
+        """Test Field.to_xarray with aggregated data."""
+        f = cf.read("example_field_0.nc")[0]
+        dsf = f.to_xarray()
+
+        self.assertEqual(f.shape[0], 5)
+        g = cf.aggregate([f[:3], f[3:]])[0]
+        dsg = g.to_xarray()
+
+        self.assertTrue(dsg.equals(dsf))
+
+
+if __name__ == "__main__":
+    print("Run date:", datetime.datetime.now())
+    cf.environment()
+    print("")
+    unittest.main(verbosity=2)

docs/source/class/cf.Domain.rst

@@ -262,6 +262,20 @@ NetCDF
    ~cf.Domain.nc_set_global_attribute
    ~cf.Domain.nc_set_global_attributes
 
+.. _Domain-xarray:
+
+xarray
+------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Domain.to_xarray
+
 Groups
 ^^^^^^
 

docs/source/class/cf.Field.rst

@@ -440,6 +440,20 @@ NetCDF
    ~cf.Field.nc_dataset_chunksizes
    ~cf.Field.nc_set_dataset_chunksizes
 
+.. _Field-xarray:
+
+xarray
+------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Field.to_xarray
+
 Groups
 ^^^^^^
 

docs/source/class/cf.FieldList.rst

@@ -41,6 +41,20 @@ Comparison
 
    ~cf.FieldList.equals
 
+.. _FieldList-xarray:
+
+xarray
+------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.FieldList.to_xarray
+
 Miscellaneous
 -------------
 

docs/source/installation.rst

@@ -273,6 +273,12 @@ environments for which these features are not required.
 
   For reading and writing Zarr datasets.
 
+.. rubric:: xarray
+
+* `xarray <https://pypi.org/project/xarray>`_, version 2026.2.0 or newer.
+
+  For converting fields and domains to `xarray` datasets in memory.
+
 .. rubric:: Regridding
 
 * `esmpy <https://earthsystemmodeling.org/esmpy/>`_, version 8.7.0 or