replace asserts with ValueErrors, add tests for empty ensemble or mis…

…sing nworkers

libensemble/ensemble.py

@@ -303,13 +303,13 @@ def __init__(
                 self._libE_specs = LibeSpecs(**self._libE_specs)
             self._known_comms = self._libE_specs.comms
 
-        assert (
-            self._known_comms is not None
-        ), "comms must be specified, either by setting parse_args=True or in libE_specs.comms"
+        if self._known_comms is None:
+            raise ValueError("comms must be specified, either by setting parse_args=True or in libE_specs.comms")
 
         if self._known_comms == "local":
             self.is_manager = True
-            assert self.nworkers, "nworkers must be specified if comms is 'local'"
+            if not self.nworkers:
+                raise ValueError("nworkers must be specified if comms is 'local'")
 
     def _parse_args(self) -> (int, bool, LibeSpecs):
         self.nworkers, self.is_manager, libE_specs_parsed, self.extra_args = parse_args_f()

libensemble/tests/unit_tests/test_ensemble.py

@@ -212,6 +212,30 @@ def test_ensemble_prevent_comms_overwrite():
     warnings.resetwarnings()
 
 
+def test_empty_ensemble():
+    """Test that an empty ensemble can't be created, plus that nworkers must be specified"""
+    from libensemble.ensemble import Ensemble
+    from libensemble.specs import LibeSpecs
+
+    flag = 1
+    try:
+        Ensemble()
+    except ValueError:
+        flag = 0
+
+    assert not flag, "Empty ensemble, without comms, should not be created"
+
+    flag = 1
+    try:
+        Ensemble(
+            libE_specs=LibeSpecs(comms="local"),
+        )
+    except ValueError:
+        flag = 0
+
+    assert not flag, "'local' ensemble without nworkers should not be created"
+
+
 if __name__ == "__main__":
     test_ensemble_init()
     test_ensemble_parse_args_false()
@@ -221,3 +245,4 @@ def test_ensemble_prevent_comms_overwrite():
     test_flakey_workflow()
     test_ensemble_specs_update_libE_specs()
     test_ensemble_prevent_comms_overwrite()
+    test_empty_ensemble()