Implement non-breaking solution and update tests accordingly

fenicsprecice/fenicsprecice.py

@@ -459,26 +459,17 @@ def retrieve_checkpoint(self):
         -------
         u : FEniCS Function
             Current state of the physical variable of interest for this participant.
-        t : double (optional)
-            Current simulation time.
-        n : int (optional)
-            Current time window (iteration) number.
+        t : double 
+            Current simulation time or None if not specified in store_checkpoint
+        n : int 
+            Current time window (iteration) number or None if not specified in store_checkpoint
         """
         assert (not self.is_time_window_complete())
         logger.debug("Restore solver state")
 
         # since t and n are optional, they should not be returned, if not specified
-        payload, t, n = self._checkpoint.get_state()
-        match (t, n):
-            case (None, None):
-                return payload
-            case (_, None):
-                return payload, t
-            case (None, _):
-                return payload, n
-            case _:
-                return payload, t, n
-
+        return self._checkpoint.get_state()
+
     def advance(self, dt):
         """
         Advances coupling in preCICE.

tests/integration/test_write_read.py

@@ -221,15 +221,17 @@ def test_opt_parameters(adapter):
             adapter.store_checkpoint(u, t) # without n
             res = adapter.retrieve_checkpoint()
             self.assertEqual(len(res), 2) # correct number of return values
-            res_u, res_t = res
+            res_u, res_t, res_n = res
             self.assertEqual(res_t, t)
+            self.assertEqual(res_n, None)
             np.testing.assert_array_equal(res_u.vector(), u.vector())
 
             adapter.store_checkpoint(u, n) # without t
             res = adapter.retrieve_checkpoint()
             self.assertEqual(len(res), 2) # correct number of return values
-            res_u, res_n = res
+            res_u, res_t, res_n = res
             self.assertEqual(res_n, n)
+            self.assertEqual(res_t, None)
             np.testing.assert_array_equal(res_u.vector(), u.vector())
 
         def test_payload_only(adapter):
@@ -241,8 +243,10 @@ def test_payload_only(adapter):
             u = interpolate(E, V)
             # test adapter
             adapter.store_checkpoint(u) # no optional parameters
-            res = adapter.retrieve_checkpoint()
-            np.testing.assert_array_equal(res.vector(), u.vector())
+            res_u, res_t, res_n = adapter.retrieve_checkpoint()
+            self.assertEqual(res_t, None)
+            self.assertEqual(res_n, None)
+            np.testing.assert_array_equal(res_u.vector(), u.vector())