issue 1340: changed mrca signature to take more than 2 args.

python/CHANGELOG.rst

@@ -12,6 +12,8 @@
 - The JSON metadata codec now interprets the empty string as an empty object. This means
   that applying a schema to an existing table will no longer necessitate modifying the
   existing rows. (:user:`benjeffery`, :issue:`2064`, :pr:`2104`)
+- ``tree.mrca`` now takes 2 or more arguments.
+  (:user:`savitakartik`, :issue:`1340`, :pr:`2121`)
 
 
 ----------------------

python/tests/test_highlevel.py

@@ -851,6 +851,40 @@ def test_minlex_postorder_multiple_roots(self):
         assert tree_orders == expected_result
 
 
+class TestMRCA:
+    t = tskit.Tree.generate_balanced(3)
+    #  4
+    # ┏━┻┓
+    # ┃  3
+    # ┃ ┏┻┓
+    # 0 1 2
+
+    def test_two_or_more_args(self):
+        assert self.t.mrca(2, 1) == 3
+        assert self.t.mrca(0, 1, 2) == 4
+
+    def test_less_than_two_args(self):
+        with pytest.raises(ValueError):
+            self.t.mrca(1)
+
+    def test_no_args(self):
+        with pytest.raises(ValueError):
+            self.t.mrca()
+
+    def test_same_args(self):
+        assert self.t.mrca(0, 0, 0, 0) == 0
+
+    def test_different_tree_levels(self):
+        assert self.t.mrca(0, 3) == 4
+
+    def test_out_of_bounds_args(self):
+        with pytest.raises(ValueError):
+            self.t.mrca(0, 6)
+
+    def test_virtual_root_arg(self):
+        assert self.t.mrca(0, 5) == 5
+
+
 class TestMRCACalculator:
     """
     Class to test the Schieber-Vishkin algorithm.

python/tskit/trees.py

@@ -1017,16 +1017,17 @@ def get_mrca(self, u, v):
         # Deprecated alias for mrca
         return self.mrca(u, v)
 
-    def mrca(self, u, v):
+    def mrca(self, *args):
         """
         Returns the most recent common ancestor of the specified nodes.
 
-        :param int u: The first node.
-        :param int v: The second node.
-        :return: The most recent common ancestor of u and v.
+        :param int `*args`: input node IDs, must be at least 2.
+        :return: The most recent common ancestor of input nodes.
         :rtype: int
         """
-        return self._ll_tree.get_mrca(u, v)
+        if len(args) < 2:
+            raise ValueError("Must supply at least two arguments")
+        return functools.reduce(self._ll_tree.get_mrca, args)
 
     def get_tmrca(self, u, v):
         # Deprecated alias for tmrca