Make get_launcher return None when no launcher

My initial though was that get_launcher should raise an error when no
launcher is found on the system since there cannot be any job launcher.
I realized that this would break the --doNotLaunch option that users may
want to use on system with no launcher, just to create the files.

Author: bouthilx

smartdispatch/tests/test_utils.py

@@ -198,24 +198,16 @@ def test_get_launcher(self):
                 lambda command: launcher if launcher == command else None)
 
             for cluster_name in self.CLUSTER_NAMES:
-                if cluster_name in utils.MSUB_CLUSTERS:
-                    continue
-                with self.assertRaises(RuntimeError):
-                    utils.get_launcher(cluster_name)
+                self._assert_launcher(None, cluster_name)
 
             for idx in range(self.N_RANDOM):
-                with self.assertRaises(RuntimeError):
-                    utils.get_launcher(self._get_random_string())
+                self._assert_launcher(None, self._get_random_string())
 
             # Test if command_is_available only returns None
             mock_distutils.spawn.find_executable.return_value = None
 
             for cluster_name in self.CLUSTER_NAMES:
-                if cluster_name in utils.MSUB_CLUSTERS:
-                    continue
-                with self.assertRaises(RuntimeError):
-                    utils.get_launcher(cluster_name)
+                self._assert_launcher(None, cluster_name)
 
             for idx in range(self.N_RANDOM):
-                with self.assertRaises(RuntimeError):
-                    utils.get_launcher(self._get_random_string())
+                self._assert_launcher(None, self._get_random_string())

smartdispatch/utils.py

@@ -193,4 +193,4 @@ def get_launcher(cluster_name):
         if command_is_available(launcher):
             return launcher
 
-    raise RuntimeError("No compatible launcher found on the system")
+    return None