Move io tests, remove undefined reference to documentation.

Also starts fixing simple mypy errors

xarray/backends/common.py

@@ -463,7 +463,7 @@ class BackendEntrypoint:
       decoding and it returns an instance of :py:class:`~datatree.DataTree`.
       It shall take in input at least ``filename_or_obj`` argument. The
       implementation of this method is not mandatory.  For more details see
-      :ref:`RST open_datatree`.
+      <reference to open_datatree documentation>.
 
     Attributes
     ----------

xarray/datatree_/datatree/io.py

@@ -3,14 +3,14 @@
 
 def _get_nc_dataset_class(engine):
     if engine == "netcdf4":
-        from netCDF4 import Dataset  # type: ignore
+        from netCDF4 import Dataset
     elif engine == "h5netcdf":
-        from h5netcdf.legacyapi import Dataset  # type: ignore
+        from h5netcdf.legacyapi import Dataset
     elif engine is None:
         try:
             from netCDF4 import Dataset
         except ImportError:
-            from h5netcdf.legacyapi import Dataset  # type: ignore
+            from h5netcdf.legacyapi import Dataset
     else:
         raise ValueError(f"unsupported engine: {engine}")
     return Dataset
@@ -78,7 +78,7 @@ def _datatree_to_netcdf(
 
 
 def _create_empty_zarr_group(store, group, mode):
-    import zarr  # type: ignore
+    import zarr
 
     root = zarr.open_group(store, mode=mode)
     root.create_group(group, overwrite=True)
@@ -92,7 +92,7 @@ def _datatree_to_zarr(
     consolidated: bool = True,
     **kwargs,
 ):
-    from zarr.convenience import consolidate_metadata  # type: ignore
+    from zarr.convenience import consolidate_metadata
 
     if kwargs.get("group", None) is not None:
         raise NotImplementedError(

xarray/tests/datatree/conftest.py

@@ -0,0 +1,65 @@
+import pytest
+
+import xarray as xr
+from xarray.datatree_.datatree import DataTree
+
+
+@pytest.fixture(scope="module")
+def create_test_datatree():
+    """
+    Create a test datatree with this structure:
+
+    <datatree.DataTree>
+    |-- set1
+    |   |-- <xarray.Dataset>
+    |   |   Dimensions:  ()
+    |   |   Data variables:
+    |   |       a        int64 0
+    |   |       b        int64 1
+    |   |-- set1
+    |   |-- set2
+    |-- set2
+    |   |-- <xarray.Dataset>
+    |   |   Dimensions:  (x: 2)
+    |   |   Data variables:
+    |   |       a        (x) int64 2, 3
+    |   |       b        (x) int64 0.1, 0.2
+    |   |-- set1
+    |-- set3
+    |-- <xarray.Dataset>
+    |   Dimensions:  (x: 2, y: 3)
+    |   Data variables:
+    |       a        (y) int64 6, 7, 8
+    |       set0     (x) int64 9, 10
+
+    The structure has deliberately repeated names of tags, variables, and
+    dimensions in order to better check for bugs caused by name conflicts.
+    """
+
+    def _create_test_datatree(modify=lambda ds: ds):
+        set1_data = modify(xr.Dataset({"a": 0, "b": 1}))
+        set2_data = modify(xr.Dataset({"a": ("x", [2, 3]), "b": ("x", [0.1, 0.2])}))
+        root_data = modify(xr.Dataset({"a": ("y", [6, 7, 8]), "set0": ("x", [9, 10])}))
+
+        # Avoid using __init__ so we can independently test it
+        root = DataTree(data=root_data)
+        set1 = DataTree(name="set1", parent=root, data=set1_data)
+        DataTree(name="set1", parent=set1)
+        DataTree(name="set2", parent=set1)
+        set2 = DataTree(name="set2", parent=root, data=set2_data)
+        DataTree(name="set1", parent=set2)
+        DataTree(name="set3", parent=root)
+
+        return root
+
+    return _create_test_datatree
+
+
+@pytest.fixture(scope="module")
+def simple_datatree(create_test_datatree):
+    """
+    Invoke create_test_datatree fixture (callback).
+
+    Returns a DataTree.
+    """
+    return create_test_datatree()

xarray/datatree_/datatree/tests/test_io.py → xarray/tests/datatree/test_io.py

@@ -3,7 +3,11 @@
 
 from xarray.backends.api import open_datatree
 from xarray.datatree_.datatree.testing import assert_equal
-from xarray.datatree_.datatree.tests import requires_h5netcdf, requires_netCDF4, requires_zarr
+from xarray.tests import (
+    requires_h5netcdf,
+    requires_netCDF4,
+    requires_zarr,
+)
 
 
 class TestIO: