Add order argument to ts.nodes()

Fixes tskit-dev#2370

python/CHANGELOG.rst

@@ -2,6 +2,11 @@
 [0.5.3] - 2022-XX-XX
 --------------------
 
+**Features**
+
+ - The ``ts.nodes`` method now takes an ``order`` parameter so that nodes
+   can be visited in time order (:user:`hyanwong`, :pr:`2471`, :issue:`2370`)
+
 **Changes**
 
  - Single statistics computed with ``TreeSequence.general_stat`` are now

python/tests/test_highlevel.py

@@ -1441,6 +1441,33 @@ def verify_pairwise_diversity(self, ts):
     def test_pairwise_diversity(self, ts):
         self.verify_pairwise_diversity(ts)
 
+    def test_bad_node_order(self):
+        ts = tskit.TableCollection(1).tree_sequence()
+        for order in ["abc", 0, 1, False]:
+            with pytest.raises(ValueError, match="order"):
+                ts.nodes(order=order)
+
+    def test_node_order(self):
+        ts = tskit.Tree.generate_balanced(8).tree_sequence
+        tables = ts.dump_tables()
+        tables.subset(np.arange(ts.num_nodes - 1, -1, -1))  # reverse the node order
+        tables.sort()
+        ts = tables.tree_sequence()
+        order = [n.id for n in ts.nodes()]
+        assert order == list(range(ts.num_nodes))
+        order = [n.id for n in ts.nodes(order="id")]
+        assert order == list(range(ts.num_nodes))
+        order = [n.id for n in ts.nodes(order="timeasc")]
+        assert order != list(range(ts.num_nodes))
+        order = np.array(order)
+        assert np.all(ts.nodes_time[order] == np.sort(ts.nodes_time))
+        # Check it conforms to the order of parents in the edge table
+        parent_only_order = order[np.isin(order, ts.edges_parent)]
+        edge_parents = np.concatenate(
+            (ts.edges_parent[:-1][np.diff(ts.edges_parent) != 0], ts.edges_parent[-1:])
+        )
+        assert np.all(parent_only_order == edge_parents)
+
     def verify_edgesets(self, ts):
         """
         Verifies that the edgesets we return are equivalent to the original edges.

python/tskit/trees.py

@@ -3800,10 +3800,16 @@ class SimpleContainerSequence:
     Simple wrapper to allow arrays of SimpleContainers (e.g. edges, nodes) that have a
     function allowing access by index (e.g. ts.edge(i), ts.node(i)) to be treated as a
     python sequence, allowing forward and reverse iteration.
+
+    To generate a sequence of items in a different order, the ``order`` parameter allows
+    an array of indexes to be passed in, such as returned from np.argsort or np.lexsort.
     """
 
-    def __init__(self, getter, length):
-        self.getter = getter
+    def __init__(self, getter, length, order=None):
+        if order is None:
+            self.getter = getter
+        else:
+            self.getter = lambda index: getter(order[index])
         self.length = length
 
     def __len__(self):
@@ -4463,15 +4469,31 @@ def individuals(self):
         """
         return SimpleContainerSequence(self.individual, self.num_individuals)
 
-    def nodes(self):
+    def nodes(self, *, order=None):
         """
         Returns an iterable sequence of all the :ref:`nodes <sec_node_table_definition>`
         in this tree sequence.
 
+        .. note:: Although node ids are commonly ordered by node time, this is not a
+            formal tree sequence requirement. If you wish to iterate over nodes in
+            time order, you should therefore use ``order="timeasc"`` (and wrap the
+            resulting sequence in the standard Python :func:`python:reversed` function
+            if you wish to iterate over older nodes before younger ones)
+
+        :param str order: The order in which the nodes should be returned: must be
+            one of "id" (default) or "timeasc" (ascending order of time, then by
+            ascending node id, matching the first two ordering requirements of
+            parent nodes in a :meth:`sorted <TableCollection.sort>` edge table).
         :return: An iterable sequence of all nodes.
         :rtype: Sequence(:class:`.Node`)
         """
-        return SimpleContainerSequence(self.node, self.num_nodes)
+        order = "id" if order is None else order
+        if order not in ["id", "timeasc"]:
+            raise ValueError('order must be "id" or "timeasc"')
+        odr = None
+        if order == "timeasc":
+            odr = np.lexsort((np.arange(self.num_nodes), self.nodes_time))
+        return SimpleContainerSequence(self.node, self.num_nodes, order=odr)
 
     def edges(self):
         """