is_hollow method (#272)

* tests

* implementation

* API docs

* narrative docs

* whatsnew

datatree/datatree.py

@@ -505,6 +505,11 @@ def is_empty(self) -> bool:
         """False if node contains any data or attrs. Does not look at children."""
         return not (self.has_data or self.has_attrs)
 
+    @property
+    def is_hollow(self) -> bool:
+        """True if only leaf nodes contain data."""
+        return not any(node.has_data for node in self.subtree if not node.is_leaf)
+
     @property
     def variables(self) -> Mapping[Hashable, Variable]:
         """Low level interface to node contents as dict of Variable objects.

datatree/tests/test_datatree.py

@@ -136,6 +136,16 @@ def test_has_data(self):
         john = DataTree(name="john", data=None)
         assert not john.has_data
 
+    def test_is_hollow(self):
+        john = DataTree(data=xr.Dataset({"a": 0}))
+        assert john.is_hollow
+
+        eve = DataTree(children={"john": john})
+        assert eve.is_hollow
+
+        eve.ds = xr.Dataset({"a": 1})
+        assert not eve.is_hollow
+
 
 class TestVariablesChildrenNameCollisions:
     def test_parent_already_has_variable_with_childs_name(self):

docs/source/api.rst

@@ -64,6 +64,7 @@ This interface echoes that of ``xarray.Dataset``.
    DataTree.has_data
    DataTree.has_attrs
    DataTree.is_empty
+   DataTree.is_hollow
 
 ..
 

docs/source/hierarchical-data.rst

@@ -369,6 +369,25 @@ You can see this tree is similar to the ``dt`` object above, except that it is m
 
 (If you want to keep the name of the root node, you will need to add the ``name`` kwarg to :py:class:`from_dict`, i.e. ``DataTree.from_dict(non_empty_nodes, name=dt.root.name)``.)
 
+.. _Tree Contents:
+
+Tree Contents
+-------------
+
+Hollow Trees
+~~~~~~~~~~~~
+
+A concept that can sometimes be useful is that of a "Hollow Tree", which means a tree with data stored only at the leaf nodes.
+This is useful because certain useful tree manipulation operations only make sense for hollow trees.
+
+You can check if a tree is a hollow tree by using the :py:meth:`~DataTree.is_hollow` property.
+We can see that the Simpson's family is not hollow because the data variable ``"age"`` is present at some nodes which
+have children (i.e. Abe and Homer).
+
+.. ipython:: python
+
+    simpsons.is_hollow
+
 .. _manipulating trees:
 
 Manipulating Trees

docs/source/whats-new.rst

@@ -25,6 +25,8 @@ New Features
 
 - New :py:meth:`DataTree.match` method for glob-like pattern matching of node paths. (:pull:`267`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
+- New :py:meth:`DataTree.is_hollow` property for checking if data is only contained at the leaf nodes. (:pull:`272`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 - Indicate which node caused the problem if error encountered while applying user function using :py:func:`map_over_subtree`
   (:issue:`190`, :pull:`264`). Only works when using python 3.11 or later.
   By `Tom Nicholas <https://github.com/TomNicholas>`_.