From 4848345a61e0a61c70e5c96a6f9071639a1866cd Mon Sep 17 00:00:00 2001
From: Aaron Ragsdale <aaron.ragsdale@mail.mcgill.ca>
Date: Thu, 21 Jan 2021 15:53:24 -0600
Subject: [PATCH] update to follow spec

---
 demes/demes.py                |  66 ++++++------
 examples/browning_america.yml |   4 +
 examples/gutenkunst_ooa.yml   |   3 +
 examples/jacobs_papuans.yml   |   4 +
 examples/offshoots.yml        |   3 +
 tests/test_demes.py           | 187 ++++++++++++++++++----------------
 tests/test_load_dump.py       |  56 ++++++----
 tests/test_moments.py         |  88 ++++++++++++----
 8 files changed, 255 insertions(+), 156 deletions(-)

diff --git a/demes/demes.py b/demes/demes.py
index 18c8bfac..ffd572a5 100644
--- a/demes/demes.py
+++ b/demes/demes.py
@@ -1366,60 +1366,68 @@ def deme(
             This must be provided.
         :param final_size: The final population size of the deme. If ``None``,
             the deme has a constant ``initial_size`` population size.
-        :param float selfing_rate: The default selfing rate for this deme.
-            May be ``None``.
-        :param float cloning_rate: The default cloning rate for this deme.
-            May be ``None``.
         :param epochs: Epochs that define population sizes, selfing rates, and
             cloning rates, for the deme over various time periods.
             If not specified, a single epoch will be created for the deme that
             spans from ``start_time`` to ``end_time``, using the ``initial_size``,
             ``final_size``, ``selfing_rate`` and ``cloning_rate`` provided.
+        :param defaults: Default attributes for epochs, including cloning_rate
+            and selfing_rate.
         :return: Newly created deme.
         :rtype: :class:`.Deme`
         """
+        # some basic deme property checks
         if id in self:
             raise ValueError(f"deme {id} already exists in this graph")
+        if epochs is None:
+            raise ValueError(f"deme {id} must have at least one specified epoch")
         if initial_size is None and epochs is not None:
             initial_size = epochs[0].initial_size
         if initial_size is None:
-            raise ValueError(f"must set initial_size for {id}")
+            raise ValueError(f"must set initial_size for deme {id}")
         if ancestors is None:
             ancestors = []
         if not isinstance(ancestors, list):
-            raise TypeError("ancestors must be a list of deme IDs")
+            raise TypeError(f"ancestors must be a list of deme IDs for deme {id}")
         for ancestor in ancestors:
             if ancestor not in self:
-                raise ValueError(f"ancestor deme {ancestor} not in graph")
+                raise ValueError(f"ancestor deme {ancestor} not in graph for deme {id}")
         if proportions is None:
             if len(ancestors) == 1:
                 proportions = [1.0]
             else:
                 proportions = []
-        if epochs is None:
-            raise ValueError("Demes must have at least one specified epoch")
 
-        # set the start time to first epoch's start time, to inf or to
-        # the ancestor's end time, if not given
+        # set the start time to first epoch's start time.
+        # if first epoch does not have a start time, set to inf or to
+        # the ancestor's end time
         if start_time is None:
-            if epochs is not None and epochs[0].start_time is not None:
+            if epochs[0].start_time is not None:
                 start_time = epochs[0].start_time
             elif len(ancestors) > 0:
                 if len(ancestors) > 1:
                     raise ValueError(
-                        "with multiple ancestors, start_time must be specified"
+                        f"with multiple ancestors, start_time must be specified for deme {id}"
                     )
                 start_time = self[ancestors[0]].end_time
             else:
                 start_time = float("inf")
-        # set the end time to the last epoch's end time or to zero if not given
+        # the first epoch's start time is set to deme's start time
+        if epochs[0].start_time is None:
+            epochs[0].start_time = start_time
+        elif epochs[0].start_time != start_time:
+            raise ValueError(
+                f"deme and first epoch start times do not align for deme {id}"
+            )
+
+        # set the end time to the last epoch's end time
         if end_time is None:
-            if epochs[-1].end_time is not None:
-                end_time = epochs[-1].end_time
-            else:
-                end_time = 0
+            end_time = epochs[-1].end_time
         if epochs[-1].end_time is not None and epochs[-1].end_time != end_time:
-            raise ValueError("deme and final epoch end times do not align")
+            raise ValueError(
+                f"deme and final epoch end times do not align for deme {id}"
+            )
+
         # check start time is valid wrt ancestor time intervals
         for ancestor in ancestors:
             anc = self[ancestor]
@@ -1429,6 +1437,7 @@ def deme(
                     f"of existence for ancestor {ancestor} "
                     f"({anc.start_time}, {anc.end_time})"
                 )
+
         # set default cloning and selfing rates
         if "selfing_rate" in defaults:
             selfing_rate = defaults["selfing_rate"]
@@ -1443,24 +1452,17 @@ def deme(
         else:
             cloning_rate = None
 
-        # the last epoch's end time is set to deme's end time
-        if epochs[-1].end_time is None:
-            epochs[-1].end_time = end_time
-        # the first epoch's start time is set to deme's start time
-        if epochs[0].start_time is None:
-            epochs[0].start_time = start_time
-        elif epochs[0].start_time != start_time:
-            raise ValueError("deme and first epoch start times do not align")
-
         # fill in all attributes of epochs
         for i in range(len(epochs)):
-            # set the start and end times based on surrounding demes
+            # set the start time based on the last epoch's end time
             if epochs[i].start_time is None:
                 epochs[i].start_time = epochs[i - 1].end_time
             if epochs[i].end_time is None:
-                if epochs[i + 1].start_time is None:
-                    raise ValueError("ambiguity about epochs' start/end times")
-                epochs[i].end_time = epochs[i + 1].start_time
+                raise ValueError("all epochs must specify the end time")
+            if i > 0 and epochs[i].start_time != epochs[i - 1].end_time:
+                raise ValueError(
+                    "epoch start and end times do not align for deme {id}, epochs {i - 1} and {i}"
+                )
             # for each subsequent epoch, fill in start size, final size,
             # and size function as necessary based on last epoch
             if epochs[i].initial_size is None:
diff --git a/examples/browning_america.yml b/examples/browning_america.yml
index 0c5d06b2..8912d448 100644
--- a/examples/browning_america.yml
+++ b/examples/browning_america.yml
@@ -26,18 +26,21 @@ demes:
     ancestors: [AMH]
     epochs:
     - initial_size: 14474
+      end_time: 0
   - id: EUR
     description: European population
     ancestors: [OOA]
     epochs:
     - initial_size: 1000
       final_size: 34039
+      end_time: 0
   - id: EAS
     description: East Asian population
     ancestors: [OOA]
     epochs:
     - initial_size: 510
       final_size: 45852
+      end_time: 0
   - id: ADMIX
     description: Admixed America
     ancestors: [AFR, EUR, EAS]
@@ -46,6 +49,7 @@ demes:
     - start_time: 12
       initial_size: 30000
       final_size: 54664
+      end_time: 0
 migrations:
   symmetric:
   - demes: [AFR, OOA]
diff --git a/examples/gutenkunst_ooa.yml b/examples/gutenkunst_ooa.yml
index 553d103e..dd4eecae 100644
--- a/examples/gutenkunst_ooa.yml
+++ b/examples/gutenkunst_ooa.yml
@@ -31,18 +31,21 @@ demes:
     ancestors: [AMH]
     epochs:
     - initial_size: 12300
+      end_time: 0
   - id: CEU
     description: Utah Residents (CEPH) with Northern and Western European Ancestry
     ancestors: [OOA]
     epochs:
     - initial_size: 1000
       final_size: 29725
+      end_time: 0
   - id: CHB
     description: Han Chinese in Beijing, China
     ancestors: [OOA]
     epochs:
     - initial_size: 510
       final_size: 54090
+      end_time: 0
 migrations:
   symmetric:
   - demes: [YRI, OOA]
diff --git a/examples/jacobs_papuans.yml b/examples/jacobs_papuans.yml
index 3c1a3560..4341f673 100644
--- a/examples/jacobs_papuans.yml
+++ b/examples/jacobs_papuans.yml
@@ -43,16 +43,19 @@ demes:
     epochs:
     - start_time: 12500.0
       initial_size: 13249.0
+      end_time: 0
   - id: Den1
     ancestors: [DenA]
     epochs:
     - start_time: 9750.0
       initial_size: 13249.0
+      end_time: 0
   - id: Nea1
     ancestors: [NeaA]
     epochs:
     - start_time: 3375.0
       initial_size: 13249.0
+      end_time: 0
   - id: Ghost
     ancestors: [YRI]
     start_time: 2218.0
@@ -84,6 +87,7 @@ demes:
     epochs:
     - start_time: 1293.0
       initial_size: 6962.0
+      end_time: 0
 migrations:
   symmetric:
   - demes: [YRI, Ghost]
diff --git a/examples/offshoots.yml b/examples/offshoots.yml
index 85654b16..d34768b2 100644
--- a/examples/offshoots.yml
+++ b/examples/offshoots.yml
@@ -7,18 +7,21 @@ demes:
     description: Main population
     epochs:
     - initial_size: 1000
+      end_time: 0
   - id: offshoot1
     description: More recent offshoot population
     ancestors: [ancestral]
     epochs:
     - start_time: 500
       initial_size: 100
+      end_time: 0
   - id: offshoot2
     description: More ancient offshoot population
     ancestors: [ancestral]
     epochs:
     - start_time: 1000
       initial_size: 200
+      end_time: 0
 migrations:
   asymmetric:
   - source: ancestral
diff --git a/tests/test_demes.py b/tests/test_demes.py
index 1b84e271..acd20aac 100644
--- a/tests/test_demes.py
+++ b/tests/test_demes.py
@@ -1318,7 +1318,7 @@ def test_in_generations(self):
 
     def test_bad_migration_time(self):
         dg = demes.Graph(description="test bad migration", time_units="generations")
-        dg.deme("deme1", epochs=[Epoch(initial_size=1000)])
+        dg.deme("deme1", epochs=[Epoch(initial_size=1000, end_time=0)])
         dg.deme("deme2", epochs=[Epoch(end_time=100, initial_size=1000)])
         with self.assertRaises(ValueError):
             dg.migration(
@@ -1327,8 +1327,8 @@ def test_bad_migration_time(self):
 
     def test_overlapping_migrations(self):
         dg = demes.Graph(description="test", time_units="generations")
-        dg.deme("A", epochs=[Epoch(initial_size=1)])
-        dg.deme("B", epochs=[Epoch(initial_size=1)])
+        dg.deme("A", epochs=[Epoch(initial_size=1, end_time=0)])
+        dg.deme("B", epochs=[Epoch(initial_size=1, end_time=0)])
         dg.migration(source="A", dest="B", rate=0.01)
         with self.assertRaises(ValueError):
             dg.migration(source="A", dest="B", start_time=10, rate=0.02)
@@ -1346,27 +1346,34 @@ def test_overlapping_migrations(self):
 
     def test_bad_pulse_time(self):
         dg = demes.Graph(description="test bad pulse time", time_units="generations")
-        dg.deme("deme1", epochs=[Epoch(initial_size=1000)])
+        dg.deme("deme1", epochs=[Epoch(initial_size=1000, end_time=0)])
         dg.deme("deme2", epochs=[Epoch(end_time=100, initial_size=1000)])
         with self.assertRaises(ValueError):
             dg.pulse(source="deme1", dest="deme2", proportion=0.1, time=10)
 
     def test_bad_deme(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("b", epochs=[Epoch(initial_size=1)])
+        dg.deme("b", epochs=[Epoch(initial_size=1, end_time=0)])
         with self.assertRaises(ValueError):
-            # no initial_size
+            # no epochs given
             dg.deme("a")
+        with self.assertRaises(ValueError):
+            # no initial_size
+            dg.deme("a", epochs=[Epoch(end_time=1)])
         with self.assertRaises(TypeError):
             # ancestors must be a list
-            dg.deme("a", ancestors="b", epochs=[Epoch(initial_size=1, start_time=10)])
+            dg.deme(
+                "a",
+                ancestors="b",
+                epochs=[Epoch(initial_size=1, start_time=10, end_time=0)],
+            )
         with self.assertRaises(ValueError):
             # ancestor c doesn't exist
             dg.deme(
                 "a",
                 ancestors=["b", "c"],
                 proportions=[0.5, 0.5],
-                epochs=[Epoch(initial_size=1)],
+                epochs=[Epoch(initial_size=1, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # end_time and final epoch end_time are different
@@ -1396,9 +1403,9 @@ def test_bad_deme(self):
 
     def test_duplicate_deme(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", epochs=[Epoch(initial_size=1)])
+        dg.deme("a", epochs=[Epoch(initial_size=1, end_time=0)])
         with self.assertRaises(ValueError):
-            dg.deme("a", epochs=[Epoch(initial_size=1)])
+            dg.deme("a", epochs=[Epoch(initial_size=1, end_time=0)])
 
     def test_ancestor_not_in_graph(self):
         dg = demes.Graph(description="a", time_units="generations")
@@ -1419,16 +1426,20 @@ def test_duplicate_ancestors(self):
     def test_bad_start_time_wrt_ancestors(self):
         dg = demes.Graph(description="a", time_units="generations")
         dg.deme("a", epochs=[Epoch(initial_size=100, start_time=100, end_time=50)])
-        dg.deme("b", epochs=[Epoch(initial_size=100)])
+        dg.deme("b", epochs=[Epoch(initial_size=100, end_time=0)])
         with self.assertRaises(ValueError):
             # start_time too old
             dg.deme(
-                "c", ancestors=["a"], epochs=[Epoch(initial_size=100, start_time=200)]
+                "c",
+                ancestors=["a"],
+                epochs=[Epoch(initial_size=100, start_time=200, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # start_time too young
             dg.deme(
-                "c", ancestors=["a"], epochs=[Epoch(initial_size=100, start_time=20)]
+                "c",
+                ancestors=["a"],
+                epochs=[Epoch(initial_size=100, start_time=20, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # start_time too old
@@ -1436,7 +1447,7 @@ def test_bad_start_time_wrt_ancestors(self):
                 "c",
                 ancestors=["a", "b"],
                 proportions=[0.5, 0.5],
-                epochs=[Epoch(start_time=200, initial_size=100)],
+                epochs=[Epoch(start_time=200, initial_size=100, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # start_time too young
@@ -1444,7 +1455,7 @@ def test_bad_start_time_wrt_ancestors(self):
                 "c",
                 ancestors=["a", "b"],
                 proportions=[0.5, 0.5],
-                epochs=[Epoch(initial_size=100, start_time=20)],
+                epochs=[Epoch(initial_size=100, start_time=20, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # start_time not provided
@@ -1452,7 +1463,7 @@ def test_bad_start_time_wrt_ancestors(self):
                 "c",
                 ancestors=["a", "b"],
                 proportions=[0.5, 0.5],
-                epochs=[Epoch(initial_size=100)],
+                epochs=[Epoch(initial_size=100, end_time=0)],
             )
 
     def test_proportions_default(self):
@@ -1464,7 +1475,7 @@ def test_proportions_default(self):
             dg.deme(
                 "c",
                 ancestors=["a", "b"],
-                epochs=[Epoch(initial_size=100, start_time=100)],
+                epochs=[Epoch(initial_size=100, start_time=100, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # proportions wrong length
@@ -1472,7 +1483,7 @@ def test_proportions_default(self):
                 "c",
                 ancestors=["a", "b"],
                 proportions=[1],
-                epochs=[Epoch(initial_size=100, start_time=100)],
+                epochs=[Epoch(initial_size=100, start_time=100, end_time=0)],
             )
         with self.assertRaises(ValueError):
             # proportions wrong length
@@ -1480,9 +1491,9 @@ def test_proportions_default(self):
                 "c",
                 ancestors=["a", "b"],
                 proportions=[1 / 3, 1 / 3, 1 / 3],
-                epochs=[Epoch(initial_size=100, start_time=100)],
+                epochs=[Epoch(initial_size=100, start_time=100, end_time=0)],
             )
-        dg.deme("c", ancestors=["b"], epochs=[Epoch(initial_size=100)])
+        dg.deme("c", ancestors=["b"], epochs=[Epoch(initial_size=100, end_time=0)])
         self.assertEqual(len(dg["c"].proportions), 1)
         self.assertEqual(dg["c"].proportions[0], 1.0)
 
@@ -1503,7 +1514,7 @@ def test_bad_migration(self):
             dg.symmetric_migration(demes=["a"], rate=0.1)
         with self.assertRaises(ValueError):
             dg.migration(source="a", dest="b", rate=0.1)
-        dg.deme("a", epochs=[Epoch(initial_size=100)])
+        dg.deme("a", epochs=[Epoch(initial_size=100, end_time=0)])
         with self.assertRaises(ValueError):
             dg.migration(source="a", dest="b", rate=0.1)
         with self.assertRaises(ValueError):
@@ -1511,7 +1522,7 @@ def test_bad_migration(self):
 
     def test_bad_pulse(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", epochs=[Epoch(initial_size=100)])
+        dg.deme("a", epochs=[Epoch(initial_size=100, end_time=0)])
         with self.assertRaises(ValueError):
             dg.pulse(source="a", dest="b", proportion=0.1, time=10)
         with self.assertRaises(ValueError):
@@ -1520,7 +1531,7 @@ def test_bad_pulse(self):
     def test_pulse_same_time(self):
         g1 = Graph(description="test", time_units="generations")
         for j in range(4):
-            g1.deme(f"d{j}", epochs=[Epoch(initial_size=1000)])
+            g1.deme(f"d{j}", epochs=[Epoch(initial_size=1000, end_time=0)])
 
         T = 100  # time of pulses
 
@@ -1576,7 +1587,7 @@ def test_isclose(self):
             time_units="generations",
         )
         g2 = copy.deepcopy(g1)
-        g1.deme("d1", epochs=[Epoch(initial_size=1000)])
+        g1.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
         self.assertTrue(g1.isclose(g1))
         self.assertTrue(g1.isclose(demes.loads(demes.dumps(g1))))
 
@@ -1585,7 +1596,7 @@ def test_isclose(self):
             description="some other description",
             time_units="generations",
         )
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
         self.assertTrue(g1.isclose(g3))
 
         # Don't care about doi for equality.
@@ -1594,16 +1605,16 @@ def test_isclose(self):
             time_units="generations",
             doi=["https://example.com/foo.bar"],
         )
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
         self.assertTrue(g1.isclose(g3))
 
         # The order in which demes are added shouldn't matter.
         g3 = copy.deepcopy(g2)
         g4 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d2", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d1", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
         self.assertTrue(g3.isclose(g4))
 
         # The order in which migrations are added shouldn't matter.
@@ -1626,78 +1637,80 @@ def test_isclose(self):
 
         self.assertFalse(g1 == g2)
         g3 = copy.deepcopy(g2)
-        g3.deme("dX", epochs=[Epoch(initial_size=1000)])
+        g3.deme("dX", epochs=[Epoch(initial_size=1000, end_time=0)])
         self.assertFalse(g1.isclose(g3))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1001)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1001, end_time=0)])
         self.assertFalse(g1.isclose(g3))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         self.assertFalse(g1.isclose(g3))
 
         g3 = copy.deepcopy(g1)
         g4 = copy.deepcopy(g1)
-        g3.deme("d2", epochs=[Epoch(initial_size=1000, start_time=50)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, start_time=50, end_time=0)])
         g4.deme(
-            "d2", ancestors=["d1"], epochs=[Epoch(initial_size=1000, start_time=50)]
+            "d2",
+            ancestors=["d1"],
+            epochs=[Epoch(initial_size=1000, start_time=50, end_time=0)],
         )
         self.assertFalse(g3.isclose(g4))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4 = copy.deepcopy(g2)
-        g4.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g4.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4.migration(source="d2", dest="d1", rate=1e-5)
         self.assertFalse(g3.isclose(g4))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g3.migration(source="d1", dest="d2", rate=1e-5)
         g4 = copy.deepcopy(g2)
-        g4.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g4.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4.migration(source="d2", dest="d1", rate=1e-5)
         self.assertFalse(g3.isclose(g4))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g3.migration(source="d2", dest="d1", rate=1e-5)
         g4 = copy.deepcopy(g2)
-        g4.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g4.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4.symmetric_migration(demes=["d2", "d1"], rate=1e-5)
         self.assertFalse(g3.isclose(g4))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4 = copy.deepcopy(g2)
-        g4.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g4.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4.pulse(source="d1", dest="d2", proportion=0.01, time=100)
         self.assertFalse(g3.isclose(g4))
 
         g3 = copy.deepcopy(g2)
-        g3.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g3.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g3.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g3.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g3.pulse(source="d2", dest="d1", proportion=0.01, time=100)
         g4 = copy.deepcopy(g2)
-        g4.deme("d1", epochs=[Epoch(initial_size=1000)])
-        g4.deme("d2", epochs=[Epoch(initial_size=1000)])
+        g4.deme("d1", epochs=[Epoch(initial_size=1000, end_time=0)])
+        g4.deme("d2", epochs=[Epoch(initial_size=1000, end_time=0)])
         g4.pulse(source="d1", dest="d2", proportion=0.01, time=100)
         self.assertFalse(g3.isclose(g4))
 
     def test_validate(self):
         g1 = demes.Graph(description="test", time_units="generations")
         g1.deme("a", epochs=[Epoch(initial_size=1, end_time=100)])
-        g1.deme("b", epochs=[Epoch(initial_size=1, start_time=50)])
+        g1.deme("b", epochs=[Epoch(initial_size=1, start_time=50, end_time=0)])
         g1.validate()
 
         #
@@ -1734,9 +1747,9 @@ def test_validate(self):
 
     def test_newly_created_objects_return(self):
         g = demes.Graph(description="test", time_units="generations")
-        d1 = g.deme("a", epochs=[Epoch(initial_size=1)])
+        d1 = g.deme("a", epochs=[Epoch(initial_size=1, end_time=0)])
         self.assertIsInstance(d1, Deme)
-        d2 = g.deme("b", epochs=[Epoch(initial_size=1, start_time=50)])
+        d2 = g.deme("b", epochs=[Epoch(initial_size=1, start_time=50, end_time=0)])
         self.assertIsInstance(d2, Deme)
 
         mig = g.migration(source="a", dest="b", rate=1e-4, end_time=25)
@@ -1753,9 +1766,20 @@ def test_deme_end_time(self):
         g = demes.Graph(description="test", time_units="generations")
         g.deme(
             "a",
-            end_time=10,
-            epochs=[Epoch(end_time=100, initial_size=10), Epoch(initial_size=20)],
+            epochs=[
+                Epoch(end_time=100, initial_size=10),
+                Epoch(initial_size=20, end_time=0),
+            ],
         )
+        with self.assertRaises(ValueError):
+            g.deme(
+                "a",
+                end_time=10,
+                epochs=[
+                    Epoch(end_time=100, initial_size=10),
+                    Epoch(initial_size=20, end_time=0),
+                ],
+            )
         with self.assertRaises(ValueError):
             g.deme("b", epochs=[Epoch(initial_size=100, start_time=100, end_time=100)])
         with self.assertRaises(ValueError):
@@ -1772,37 +1796,28 @@ def test_ambiguous_epoch_times(self):
                 ],
             )
 
-    def test_epoch_end_time_from_start_time(self):
-        g = demes.Graph(description="test", time_units="generations")
-        g.deme(
-            "a",
-            end_time=10,
-            epochs=[Epoch(initial_size=10), Epoch(start_time=10, initial_size=20)],
-        )
-        self.assertTrue(g["a"].epochs[0].end_time == 10)
-
 
 class TestGraphToDict(unittest.TestCase):
     def test_finite_start_time(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", epochs=[Epoch(initial_size=100, start_time=100)])
+        dg.deme("a", epochs=[Epoch(initial_size=100, start_time=100, end_time=0)])
         d = dg.asdict()
         self.assertTrue(d["demes"][0]["epochs"][0]["start_time"] == dg["a"].start_time)
 
     def test_deme_selfing_rate(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", epochs=[Epoch(initial_size=100, selfing_rate=0.1)])
+        dg.deme("a", epochs=[Epoch(initial_size=100, selfing_rate=0.1, end_time=0)])
         d = dg.asdict()
         self.assertTrue(d["demes"][0]["epochs"][0]["selfing_rate"] == 0.1)
 
     def test_deme_cloning_rate(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", epochs=[Epoch(initial_size=100, cloning_rate=0.1)])
+        dg.deme("a", epochs=[Epoch(initial_size=100, cloning_rate=0.1, end_time=0)])
         d = dg.asdict()
         self.assertTrue(d["demes"][0]["epochs"][0]["cloning_rate"] == 0.1)
         d = dg.asdict_simplified()
         self.assertTrue(d["demes"][0]["epochs"][0]["cloning_rate"] == 0.1)
-        dg.deme("b", epochs=[Epoch(initial_size=200)])
+        dg.deme("b", epochs=[Epoch(initial_size=200, end_time=0)])
         d = dg.asdict_simplified()
         self.assertTrue("cloning_rate" not in d["demes"][1])
         dg.deme(
@@ -1881,14 +1896,16 @@ def test_fill_epoch_cloning_rates(self):
 
     def test_fill_description(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", description="described", epochs=[Epoch(initial_size=100)])
+        dg.deme(
+            "a", description="described", epochs=[Epoch(initial_size=100, end_time=0)]
+        )
         d = dg.asdict()
         self.assertTrue(d["demes"][0]["description"] == dg["a"].description)
 
     def test_fill_migration_bounds(self):
         dg = demes.Graph(description="a", time_units="generations")
-        dg.deme("a", epochs=[Epoch(initial_size=100)])
-        dg.deme("b", epochs=[Epoch(initial_size=100)])
+        dg.deme("a", epochs=[Epoch(initial_size=100, end_time=0)])
+        dg.deme("b", epochs=[Epoch(initial_size=100, end_time=0)])
         dg.migration(source="a", dest="b", rate=0.01, start_time=20, end_time=10)
         d = dg.asdict()
         self.assertTrue(d["migrations"]["asymmetric"][0]["start_time"] == 20)
@@ -1912,15 +1929,15 @@ def test_bad_custom_attributes(self):
 
     def test_multiple_symmetric_migrations(self):
         g = demes.Graph(description="descr", time_units="generations")
-        g.deme("a", epochs=[Epoch(initial_size=100)])
-        g.deme("b", epochs=[Epoch(initial_size=200)])
-        g.deme("c", epochs=[Epoch(initial_size=300)])
+        g.deme("a", epochs=[Epoch(initial_size=100, end_time=0)])
+        g.deme("b", epochs=[Epoch(initial_size=200, end_time=0)])
+        g.deme("c", epochs=[Epoch(initial_size=300, end_time=0)])
         g.symmetric_migration(demes=["a", "b", "c"], rate=0.01)
         d = g.asdict_simplified()
         self.assertTrue(len(d["migrations"]["symmetric"]) == 1)
         self.assertTrue("asymmetric" not in d["migrations"])
 
-        g.deme("d", epochs=[Epoch(initial_size=400)])
+        g.deme("d", epochs=[Epoch(initial_size=400, end_time=0)])
         g.symmetric_migration(demes=["a", "d"], rate=0.01)
         d = g.asdict_simplified()
         self.assertTrue(len(d["migrations"]["symmetric"]) == 2)
@@ -1931,10 +1948,10 @@ def test_multiple_symmetric_migrations(self):
 
     def test_mix_sym_asym_migrations(self):
         g = demes.Graph(description="a", time_units="generations")
-        g.deme("a", epochs=[Epoch(initial_size=1, start_time=100)])
-        g.deme("b", epochs=[Epoch(initial_size=1)])
-        g.deme("c", epochs=[Epoch(initial_size=1)])
-        g.deme("d", epochs=[Epoch(initial_size=1)])
+        g.deme("a", epochs=[Epoch(initial_size=1, start_time=100, end_time=0)])
+        g.deme("b", epochs=[Epoch(initial_size=1, end_time=0)])
+        g.deme("c", epochs=[Epoch(initial_size=1, end_time=0)])
+        g.deme("d", epochs=[Epoch(initial_size=1, end_time=0)])
         g.symmetric_migration(demes=["a", "b"], rate=0.01)
         g.symmetric_migration(demes=["b", "c"], rate=0.01)
         g.symmetric_migration(demes=["a", "c", "d"], rate=0.01)
diff --git a/tests/test_load_dump.py b/tests/test_load_dump.py
index d3ca967b..e6805e2a 100644
--- a/tests/test_load_dump.py
+++ b/tests/test_load_dump.py
@@ -53,10 +53,16 @@ def jacobs_papuans():
         epochs=[Epoch(end_time=9750, initial_size=N_DeniAnc)],
     )
     # Altai Denisovan (sampling lineage)
-    g.deme("DenAltai", ancestors=["Den2"], epochs=[Epoch(initial_size=5083)])
+    g.deme(
+        "DenAltai", ancestors=["Den2"], epochs=[Epoch(initial_size=5083, end_time=0)]
+    )
     # Introgressing Denisovan lineages 1 and 2
-    g.deme("DenI1", ancestors=["Den2"], epochs=[Epoch(initial_size=N_archaic)])
-    g.deme("DenI2", ancestors=["Den1"], epochs=[Epoch(initial_size=N_archaic)])
+    g.deme(
+        "DenI1", ancestors=["Den2"], epochs=[Epoch(initial_size=N_archaic, end_time=0)]
+    )
+    g.deme(
+        "DenI2", ancestors=["Den1"], epochs=[Epoch(initial_size=N_archaic, end_time=0)]
+    )
 
     g.deme(
         "Nea",
@@ -64,7 +70,7 @@ def jacobs_papuans():
         epochs=[Epoch(end_time=3375, initial_size=N_archaic)],
     )
     # Altai Neanderthal (sampling lineage)
-    g.deme("NeaAltai", ancestors=["Nea"], epochs=[Epoch(initial_size=826)])
+    g.deme("NeaAltai", ancestors=["Nea"], epochs=[Epoch(initial_size=826, end_time=0)])
     # Introgressing Neanderthal lineage
     g.deme(
         "NeaI", ancestors=["Nea"], epochs=[Epoch(end_time=883, initial_size=N_archaic)]
@@ -75,7 +81,7 @@ def jacobs_papuans():
         ancestors=["ancestral_hominin"],
         epochs=[Epoch(end_time=2218, initial_size=41563)],
     )
-    g.deme("Africa", ancestors=["AMH"], epochs=[Epoch(initial_size=48433)])
+    g.deme("Africa", ancestors=["AMH"], epochs=[Epoch(initial_size=48433, end_time=0)])
     g.deme(
         "Ghost1",
         ancestors=["AMH"],
@@ -91,7 +97,9 @@ def jacobs_papuans():
         ancestors=["Ghost1"],
         epochs=[Epoch(end_time=1758, initial_size=N_ghost)],
     )
-    g.deme("Ghost3", ancestors=["Ghost2"], epochs=[Epoch(initial_size=N_ghost)])
+    g.deme(
+        "Ghost3", ancestors=["Ghost2"], epochs=[Epoch(initial_size=N_ghost, end_time=0)]
+    )
     g.deme(
         "Papua",
         ancestors=["Ghost1"],
@@ -110,8 +118,14 @@ def jacobs_papuans():
             demes.Epoch(end_time=1293, initial_size=12971),
         ],
     )
-    g.deme("WestEurasia", ancestors=["Eurasia"], epochs=[Epoch(initial_size=6962)])
-    g.deme("EastAsia", ancestors=["Eurasia"], epochs=[Epoch(initial_size=9025)])
+    g.deme(
+        "WestEurasia",
+        ancestors=["Eurasia"],
+        epochs=[Epoch(initial_size=6962, end_time=0)],
+    )
+    g.deme(
+        "EastAsia", ancestors=["Eurasia"], epochs=[Epoch(initial_size=9025, end_time=0)]
+    )
 
     g.symmetric_migration(
         demes=["Africa", "Ghost3"], rate=1.79e-4, start_time=T_Eu_bottleneck
@@ -174,7 +188,7 @@ def test_bad_format_param(self):
 
     def test_bad_filename_param(self):
         g = demes.Graph(description="test", time_units="generations")
-        g.deme("A", epochs=[Epoch(initial_size=1000)])
+        g.deme("A", epochs=[Epoch(initial_size=1000, end_time=0)])
 
         class F:
             pass
@@ -201,7 +215,7 @@ def check_dumps_simple(self, *, format, simplified):
             generation_time=42,
         )
         for id, N in zip("ABCD", [100, 200, 300, 400]):
-            g.deme(id, epochs=[Epoch(initial_size=N)])
+            g.deme(id, epochs=[Epoch(initial_size=N, end_time=0)])
         string = demes.dumps(g, format=format, simplified=simplified)
         assert "description" in string
         assert g.description in string
@@ -230,7 +244,7 @@ def check_dumps_simple(self, *, format, simplified):
             assert "cloning_rate" not in string
 
         g1 = copy.deepcopy(g)
-        g1.deme("E", epochs=[Epoch(initial_size=100, selfing_rate=0.1)])
+        g1.deme("E", epochs=[Epoch(initial_size=100, selfing_rate=0.1, end_time=0)])
         string = demes.dumps(g1, format=format, simplified=simplified)
         assert "selfing_rate" in string
         assert "0.1" in string
@@ -238,7 +252,7 @@ def check_dumps_simple(self, *, format, simplified):
             assert "cloning_rate" not in string
 
         g1 = copy.deepcopy(g)
-        g1.deme("E", epochs=[Epoch(initial_size=100, cloning_rate=0.1)])
+        g1.deme("E", epochs=[Epoch(initial_size=100, cloning_rate=0.1, end_time=0)])
         string = demes.dumps(g1, format=format, simplified=simplified)
         if simplified:
             assert "selfing_rate" not in string
@@ -313,7 +327,8 @@ def test_loads_json_simple(self):
                         "id": "A",
                         "epochs": [
                         {
-                            "initial_size": 100
+                            "initial_size": 100,
+                            "end_time": 0
                         }
                         ]
                     },
@@ -321,7 +336,8 @@ def test_loads_json_simple(self):
                         "id": "B",
                         "epochs": [
                         {
-                            "initial_size": 100
+                            "initial_size": 100,
+                            "end_time": 0
                         }
                         ]
                     },
@@ -332,7 +348,8 @@ def test_loads_json_simple(self):
                         "epochs": [
                         {
                             "start_time": 500,
-                            "initial_size": 100
+                            "initial_size": 100,
+                            "end_time": 0
                         }
                         ]
                     }
@@ -359,13 +376,16 @@ def test_loads_yaml_simple(self):
                 -   id: A
                     epochs:
                         -   initial_size: 100
+                            end_time: 0
                 -   id: B
                     epochs:
                         -   initial_size: 100
+                            end_time: 0
                 -   id: C
                     epochs:
                         -   initial_size: 100
                             start_time: 500
+                            end_time: 0
                     ancestors: [A, B]
                     proportions: [0.1, 0.9]
             """
@@ -401,7 +421,7 @@ def check_dump_and_load_simple(self, *, format, simplified):
             generation_time=42,
         )
         for id, N in zip("ABCD", [100, 200, 300, 400]):
-            g1.deme(id, epochs=[Epoch(initial_size=N)])
+            g1.deme(id, epochs=[Epoch(initial_size=N, end_time=0)])
         with tempfile.TemporaryDirectory() as tmpdir:
             tmpfile = pathlib.Path(tmpdir) / "temp.txt"
             demes.dump(g1, tmpfile, format=format, simplified=simplified)
@@ -547,12 +567,12 @@ def test_float_subclass(self):
         )
         self.check_dump_load_roundtrip(g)
 
-        g.deme("B", epochs=[Epoch(initial_size=N[0])])
+        g.deme("B", epochs=[Epoch(initial_size=N[0], end_time=0)])
         g.deme(
             "C",
             ancestors=["A", "B"],
             proportions=[fractions.Fraction(1, 3), fractions.Fraction(2, 3)],
-            epochs=[Epoch(initial_size=N[0], start_time=T[1])],
+            epochs=[Epoch(initial_size=N[0], start_time=T[1], end_time=0)],
         )
         self.check_dump_load_roundtrip(g)
 
diff --git a/tests/test_moments.py b/tests/test_moments.py
index c99fec88..66a50dce 100644
--- a/tests/test_moments.py
+++ b/tests/test_moments.py
@@ -95,9 +95,15 @@ def test_num_lineages(self):
         # simple admix model
         g = demes.Graph(description="test", time_units="generations")
         g.deme(id="anc", epochs=[Epoch(initial_size=100, end_time=100)])
-        g.deme(id="pop1", epochs=[Epoch(initial_size=100)], ancestors=["anc"])
-        g.deme(id="pop2", epochs=[Epoch(initial_size=100)], ancestors=["anc"])
-        g.deme(id="pop3", epochs=[Epoch(initial_size=100)], ancestors=["anc"])
+        g.deme(
+            id="pop1", epochs=[Epoch(initial_size=100, end_time=0)], ancestors=["anc"]
+        )
+        g.deme(
+            id="pop2", epochs=[Epoch(initial_size=100, end_time=0)], ancestors=["anc"]
+        )
+        g.deme(
+            id="pop3", epochs=[Epoch(initial_size=100, end_time=0)], ancestors=["anc"]
+        )
         g.deme(
             id="pop",
             initial_size=100,
@@ -121,7 +127,7 @@ def test_num_lineages(self):
     # test basic results against moments implementation
     def test_one_pop(self):
         g = demes.Graph(description="test", time_units="generations")
-        g.deme(id="Pop", epochs=[Epoch(initial_size=1000)])
+        g.deme(id="Pop", epochs=[Epoch(initial_size=1000, end_time=0)])
         fs = moments_.SFS(g, ["Pop"], [20])
         fs_m = moments.Demographics1D.snm([20])
         self.assertTrue(np.allclose(fs.data, fs_m.data))
@@ -143,14 +149,22 @@ def test_more_than_5_demes(self):
         g = demes.Graph(description="test", time_units="generations")
         g.deme(id="anc", epochs=[Epoch(initial_size=1000, end_time=1000)])
         for i in range(6):
-            g.deme(id=f"pop{i}", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
+            g.deme(
+                id=f"pop{i}",
+                epochs=[Epoch(initial_size=1000, end_time=0)],
+                ancestors=["anc"],
+            )
         with self.assertRaises(ValueError):
             moments_.SFS(g, ["pop{i}" for i in range(6)], [10 for i in range(6)])
 
         g = demes.Graph(description="test", time_units="generations")
         g.deme(id="anc", epochs=[Epoch(initial_size=1000, end_time=1000)])
         for i in range(3):
-            g.deme(id=f"pop{i}", ancestors=["anc"], epochs=[Epoch(initial_size=1000)])
+            g.deme(
+                id=f"pop{i}",
+                ancestors=["anc"],
+                epochs=[Epoch(initial_size=1000, end_time=0)],
+            )
         with self.assertRaises(ValueError):
             moments_.SFS(
                 g,
@@ -161,7 +175,7 @@ def test_more_than_5_demes(self):
 
     def test_one_pop_ancient_samples(self):
         g = demes.Graph(description="test", time_units="generations")
-        g.deme(id="Pop", epochs=[Epoch(initial_size=1000)])
+        g.deme(id="Pop", epochs=[Epoch(initial_size=1000, end_time=0)])
         fs = moments_.SFS(g, ["Pop", "Pop"], [20, 4], sample_times=[0, 100])
         fs_m = moments.Demographics1D.snm([24])
         fs_m = moments.Manips.split_1D_to_2D(fs_m, 20, 4)
@@ -196,7 +210,7 @@ def test_simple_merge(self):
             id="Pop",
             ancestors=["Source1", "Source2"],
             proportions=[0.8, 0.2],
-            epochs=[Epoch(initial_size=4000, start_time=10)],
+            epochs=[Epoch(initial_size=4000, start_time=10, end_time=0)],
         )
         fs = moments_.SFS(g, ["Pop"], [20])
 
@@ -210,13 +224,21 @@ def test_simple_merge(self):
     def test_simple_admixture(self):
         g = demes.Graph(description="test", time_units="generations")
         g.deme(id="Anc", epochs=[Epoch(initial_size=1000, end_time=100)])
-        g.deme(id="Source1", epochs=[Epoch(initial_size=2000)], ancestors=["Anc"])
-        g.deme(id="Source2", epochs=[Epoch(initial_size=3000)], ancestors=["Anc"])
+        g.deme(
+            id="Source1",
+            epochs=[Epoch(initial_size=2000, end_time=0)],
+            ancestors=["Anc"],
+        )
+        g.deme(
+            id="Source2",
+            epochs=[Epoch(initial_size=3000, end_time=0)],
+            ancestors=["Anc"],
+        )
         g.deme(
             id="Pop",
             ancestors=["Source1", "Source2"],
             proportions=[0.8, 0.2],
-            epochs=[Epoch(initial_size=4000, start_time=10)],
+            epochs=[Epoch(initial_size=4000, start_time=10, end_time=0)],
         )
         fs = moments_.SFS(g, ["Source1", "Source2", "Pop"], [10, 10, 10])
 
@@ -272,8 +294,14 @@ def nu_func(t):
     def test_simple_pulse_model(self):
         g = demes.Graph(description="test", time_units="generations")
         g.deme(id="anc", epochs=[Epoch(initial_size=1000, end_time=100)])
-        g.deme(id="source", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
-        g.deme(id="dest", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
+        g.deme(
+            id="source",
+            epochs=[Epoch(initial_size=1000, end_time=0)],
+            ancestors=["anc"],
+        )
+        g.deme(
+            id="dest", epochs=[Epoch(initial_size=1000, end_time=0)], ancestors=["anc"]
+        )
         g.pulse(source="source", dest="dest", time=10, proportion=0.1)
         fs = moments_.SFS(g, ["source", "dest"], [20, 20])
 
@@ -287,9 +315,15 @@ def test_simple_pulse_model(self):
     def test_n_way_split(self):
         g = demes.Graph(description="three-way", time_units="generations")
         g.deme(id="anc", epochs=[Epoch(initial_size=1000, end_time=10)])
-        g.deme(id="deme1", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
-        g.deme(id="deme2", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
-        g.deme(id="deme3", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
+        g.deme(
+            id="deme1", epochs=[Epoch(initial_size=1000, end_time=0)], ancestors=["anc"]
+        )
+        g.deme(
+            id="deme2", epochs=[Epoch(initial_size=1000, end_time=0)], ancestors=["anc"]
+        )
+        g.deme(
+            id="deme3", epochs=[Epoch(initial_size=1000, end_time=0)], ancestors=["anc"]
+        )
         ns = [10, 15, 20]
         fs = moments_.SFS(g, ["deme1", "deme2", "deme3"], ns)
         self.assertTrue(np.all([fs.sample_sizes[i] == ns[i] for i in range(len(ns))]))
@@ -330,7 +364,7 @@ def test_n_way_admixture(self):
             id="merged",
             ancestors=["source1", "source2", "source3"],
             proportions=[0.5, 0.2, 0.3],
-            epochs=[Epoch(initial_size=1000, start_time=10)],
+            epochs=[Epoch(initial_size=1000, start_time=10, end_time=0)],
         )
         ns = [10]
         fs = moments_.SFS(g, ["merged"], ns)
@@ -347,14 +381,26 @@ def test_n_way_admixture(self):
 
         g = demes.Graph(description="three-way admix", time_units="generations")
         g.deme(id="anc", epochs=[Epoch(initial_size=1000, end_time=100)])
-        g.deme(id="source1", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
-        g.deme(id="source2", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
-        g.deme(id="source3", epochs=[Epoch(initial_size=1000)], ancestors=["anc"])
+        g.deme(
+            id="source1",
+            epochs=[Epoch(initial_size=1000, end_time=0)],
+            ancestors=["anc"],
+        )
+        g.deme(
+            id="source2",
+            epochs=[Epoch(initial_size=1000, end_time=0)],
+            ancestors=["anc"],
+        )
+        g.deme(
+            id="source3",
+            epochs=[Epoch(initial_size=1000, end_time=0)],
+            ancestors=["anc"],
+        )
         g.deme(
             id="admixed",
             ancestors=["source1", "source2", "source3"],
             proportions=[0.5, 0.2, 0.3],
-            epochs=[Epoch(initial_size=1000, start_time=10)],
+            epochs=[Epoch(initial_size=1000, start_time=10, end_time=0)],
         )
         ns = [10]
         fs = moments_.SFS(g, ["admixed"], ns)