From d5348e2ac6c250b1f713961d0dc5bffb80c1fae9 Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Wed, 25 Oct 2023 13:47:17 -0400
Subject: [PATCH 1/5] tests

---
 datatree/tests/test_datatree.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/datatree/tests/test_datatree.py b/datatree/tests/test_datatree.py
index 9dd601c8..e9f373d7 100644
--- a/datatree/tests/test_datatree.py
+++ b/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):

From 2ab6cc8f1e48f83710c32403c12e54202464362e Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Wed, 25 Oct 2023 13:47:28 -0400
Subject: [PATCH 2/5] implementation

---
 datatree/datatree.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/datatree/datatree.py b/datatree/datatree.py
index f2618d2a..52049f47 100644
--- a/datatree/datatree.py
+++ b/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.

From 05aa145decda00e68025d05b80e82cc9297344a2 Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Wed, 25 Oct 2023 13:47:40 -0400
Subject: [PATCH 3/5] API docs

---
 docs/source/api.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/source/api.rst b/docs/source/api.rst
index 54a98639..215105ef 100644
--- a/docs/source/api.rst
+++ b/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
 
 ..
 

From c48325399585cb0e6660a3c94da4278d3110c515 Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Wed, 25 Oct 2023 13:47:54 -0400
Subject: [PATCH 4/5] narrative docs

---
 docs/source/hierarchical-data.rst | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/docs/source/hierarchical-data.rst b/docs/source/hierarchical-data.rst
index f74a635d..51bcea56 100644
--- a/docs/source/hierarchical-data.rst
+++ b/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

From cdeadd957458df60a77975e1c173abb775a9e107 Mon Sep 17 00:00:00 2001
From: Thomas Nicholas <thomas.nicholas@columbia.edu>
Date: Wed, 25 Oct 2023 13:53:10 -0400
Subject: [PATCH 5/5] whatsnew

---
 docs/source/whats-new.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/docs/source/whats-new.rst b/docs/source/whats-new.rst
index eb2c0340..e62c4379 100644
--- a/docs/source/whats-new.rst
+++ b/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>`_.