Improve handling when no MPI found.

libensemble/executors/mpi_executor.py

@@ -47,29 +47,32 @@
         information using the ``custom_info`` argument. This takes
         a dictionary of values.
 
-        The allowable fields are::
+        The allowable fields are:
 
-            'mpi_runner' [string]:
-                Select runner: 'mpich', 'openmpi', 'aprun', 'srun', 'jsrun', 'custom'
-                All except 'custom' relate to runner classes in libEnsemble.
+        .. parsed-literal::
+
+            **'mpi_runner'** [string]:
+                Select runner: `'mpich'`, `'openmpi'`, `'aprun'`, `'srun'`, `'jsrun'`, `'custom'`
+                All except `'custom'` relate to runner classes in libEnsemble.
                 Custom allows user to define their own run-lines but without parsing
                 arguments or making use of auto-resources.
-            'runner_name' [string]:
-                Runner name: Replaces run command if present. All runners have a default
-                except for 'custom'.
-            'subgroup_launch' [bool]:
+            **'runner_name'** [string]:
+                The literal string that appears at the front of the run command.
+                This is typically 'mpirun', 'srun', etc., and can be a full path.
+                Defaults exist for all runners except 'custom'.
+            **'subgroup_launch'** [bool]:
                 Whether MPI runs should be initiated in a new process group. This needs
                 to be correct for kills to work correctly. Use the standalone test at
-                libensemble/tests/standalone_tests/kill_test to determine correct value
+                `libensemble/tests/standalone_tests/kill_test` to determine correct value
                 for a system.
 
-        For example::
+    For example::
 
-            customizer = {'mpi_runner': 'mpich',
-                          'runner_name': 'wrapper -x mpich'}
+        customizer = {'mpi_runner': 'mpich',
+                      'runner_name': 'wrapper -x mpich'}
 
-            from libensemble.executors.mpi_executor import MPIExecutor
-            exctr = MPIExecutor(custom_info=customizer)
+        from libensemble.executors.mpi_executor import MPIExecutor
+        exctr = MPIExecutor(custom_info=customizer)
 
 
     """
@@ -336,6 +339,9 @@
         else:
             mpi_runner_obj = self.mpi_runner_obj or self._create_mpi_runner_from_attr()
 
+        if env_script is None and mpi_runner_obj is None:
+            raise ExecutorException("No valid MPI runner was found")
+
         mpi_specs = mpi_runner_obj.get_mpi_specs(
             task,
             num_procs,

libensemble/executors/mpi_runner.py

@@ -21,11 +21,13 @@ def get_runner(mpi_runner_type, runner_name=None, platform_info=None):
             "msmpi": MSMPI_MPIRunner,
             "custom": MPIRunner,
         }
-        mpi_runner = mpi_runners[mpi_runner_type]
-        if runner_name is not None:
-            runner = mpi_runner(run_command=runner_name, platform_info=platform_info)
-        else:
-            runner = mpi_runner(platform_info=platform_info)
+        runner = None
+        if mpi_runner_type is not None:
+            mpi_runner = mpi_runners[mpi_runner_type]
+            if runner_name is not None:
+                runner = mpi_runner(run_command=runner_name, platform_info=platform_info)
+            else:
+                runner = mpi_runner(platform_info=platform_info)
         return runner
 
     def __init__(self, run_command="mpiexec", platform_info=None):