Support node_labels in write_nexus via TRANSLATE

python/CHANGELOG.rst

@@ -11,7 +11,8 @@ In development
 
 - Add ``TreeSequence.ld_matrix`` stats method and documentation, for computing
   two-locus statistics in site and branch mode.
-  (:user:`lkirk`, :user:`apragsdale`, :pr:`3416`) 
+  (:user:`lkirk`, :user:`apragsdale`, :pr:`3416`)
+- Add `node_labels` parameter to `write_nexus`. (:user:`kaathewisegit`, :pr:`3442`)
 
 --------------------
 [1.0.2] - 2026-03-06

python/tests/test_phylo_formats.py

@@ -26,6 +26,7 @@
 
 import functools
 import io
+import random
 import textwrap
 
 import dendropy
@@ -334,6 +335,97 @@ def test_nexus_no_trees_or_alignments(self):
         )
 
 
+class TestNexusNodeLabels:
+    @tests.cached_example
+    def balanced_tree(self):
+        #   4
+        # ┏━┻┓
+        # ┃  3
+        # ┃ ┏┻┓
+        # 0 1 2
+        return tskit.Tree.generate_balanced(3)
+
+    def test_as_nexus_labels_basic(self):
+        ts = self.balanced_tree().tree_sequence
+        labels = {0: "human", 1: "chimp", 2: "bonobo"}
+        expected = textwrap.dedent(
+            """\
+            #NEXUS
+            BEGIN TAXA;
+              DIMENSIONS NTAX=3;
+              TAXLABELS human chimp bonobo;
+            END;
+            BEGIN TREES;
+              TRANSLATE n0 human, n1 chimp, n2 bonobo;
+              TREE t0^1 = [&R] (n0:2,(n1:1,n2:1):1);
+            END;
+            """
+        )
+        assert expected == ts.as_nexus(include_alignments=False, node_labels=labels)
+
+    def test_as_nexus_labels_partial(self):
+        ts = self.balanced_tree().tree_sequence
+        labels = {0: "human", 2: "bonobo"}
+        expected = textwrap.dedent(
+            """\
+            #NEXUS
+            BEGIN TAXA;
+              DIMENSIONS NTAX=3;
+              TAXLABELS human n1 bonobo;
+            END;
+            BEGIN TREES;
+              TRANSLATE n0 human, n2 bonobo;
+              TREE t0^1 = [&R] (n0:2,(n1:1,n2:1):1);
+            END;
+            """
+        )
+        assert expected == ts.as_nexus(include_alignments=False, node_labels=labels)
+
+    def test_as_nexus_labels_none(self):
+        ts = self.balanced_tree().tree_sequence
+        expected = textwrap.dedent(
+            """\
+            #NEXUS
+            BEGIN TAXA;
+              DIMENSIONS NTAX=3;
+              TAXLABELS n0 n1 n2;
+            END;
+            BEGIN TREES;
+              TREE t0^1 = [&R] (n0:2,(n1:1,n2:1):1);
+            END;
+            """
+        )
+        assert expected == ts.as_nexus(include_alignments=False, node_labels=None)
+
+    @pytest.mark.parametrize("ts", get_example_tree_sequences())
+    def test_parseable(self, ts):
+        def all_samples_are_leaves(ts):
+            internal_nodes = np.unique(ts.edges_parent)
+            is_internal_sample = np.isin(ts.samples(), internal_nodes)
+            return not np.any(is_internal_sample)
+
+        if not all_samples_are_leaves(ts):
+            # TRANSLATE doesn't support translating internal nodes
+            return
+
+        for tree in ts.trees():
+            if not tree.has_single_root:
+                return
+
+        labels = {}
+        samples = ts.samples()
+        k = random.randint(1, len(samples))
+        for node in random.sample(list(samples), k):
+            labels[node] = f"new_node_which_was_{node}"
+
+        nexus = ts.as_nexus(include_alignments=False, node_labels=labels)
+        ds = dendropy.DataSet.get(data=nexus, schema="nexus")
+        tree = ds.tree_lists[0][0]
+        dendropy_labels = [node.taxon.label for node in tree.nodes() if node.taxon]
+        for label in labels.values():
+            assert label.replace("_", " ") in dendropy_labels
+
+
 class TestNewickCodePaths:
     """
     Test that the different code paths we use under the hood lead to

python/tskit/text_formats.py

@@ -120,6 +120,7 @@ def write_nexus(
     include_alignments,
     reference_sequence,
     missing_data_character,
+    node_labels,
     isolated_as_missing=None,
 ):
     # See TreeSequence.write_nexus for documentation on parameters.
@@ -134,7 +135,13 @@ def write_nexus(
     print("#NEXUS", file=out)
     print("BEGIN TAXA;", file=out)
     print("", f"DIMENSIONS NTAX={ts.num_samples};", sep=indent, file=out)
-    taxlabels = " ".join(f"n{u}" for u in ts.samples())
+
+    if node_labels is not None:
+        taxlabels = " ".join(
+            node_labels[u] if u in node_labels else f"n{u}" for u in ts.samples()
+        )
+    else:
+        taxlabels = " ".join(f"n{u}" for u in ts.samples())
     print("", f"TAXLABELS {taxlabels};", sep=indent, file=out)
     print("END;", file=out)
 
@@ -166,6 +173,11 @@ def write_nexus(
     include_trees = True if include_trees is None else include_trees
     if include_trees:
         print("BEGIN TREES;", file=out)
+
+        if node_labels is not None:
+            translations = ", ".join(f"n{u} {name}" for u, name in node_labels.items())
+            print(f"  TRANSLATE {translations};", file=out)
+
         for tree in ts.trees():
             start_interval = "{0:.{1}f}".format(tree.interval.left, pos_precision)
             end_interval = "{0:.{1}f}".format(tree.interval.right, pos_precision)

python/tskit/trees.py

@@ -6797,6 +6797,7 @@ def write_nexus(
         reference_sequence=None,
         missing_data_character=None,
         isolated_as_missing=None,
+        node_labels=None,
     ):
         """
         Returns a `nexus encoding <https://en.wikipedia.org/wiki/Nexus_file>`_
@@ -6896,6 +6897,10 @@ def write_nexus(
         :param str missing_data_character: As for the :meth:`.alignments` method,
             but defaults to "?".
         :param bool isolated_as_missing: As for the :meth:`.alignments` method.
+        :param node_labels: A map of type `{node_id: name}`.  Samples present
+            in the map will have the given name instead of `n{node_id}`.  Note
+            that the names must not have whitespace (spaces should be replaced
+            by underscores) or puncuation in them.
         :return: A nexus representation of this :class:`TreeSequence`
         :rtype: str
         """
@@ -6908,6 +6913,7 @@ def write_nexus(
             reference_sequence=reference_sequence,
             missing_data_character=missing_data_character,
             isolated_as_missing=isolated_as_missing,
+            node_labels=node_labels,
         )
 
     def as_nexus(self, **kwargs):