Apply suggestions from code review

Co-authored-by: Ronnie Dutta <61982285+MetRonnie@users.noreply.github.com>

CHANGES.md

@@ -36,7 +36,7 @@ Fourth Release Candidate for Cylc 8 suitable for acceptance testing.
 ### Enhancements
 
 [#4896](https://github.com/cylc/cylc-flow/pull/4896) - Allow the setting of
-default job runner directives for plaforms.
+default job runner directives for platforms.
 
 [#4887](https://github.com/cylc/cylc-flow/pull/4887) - Disallow relative paths
 in `global.cylc[install]source dirs`.

cylc/flow/cfgspec/globalcfg.py

@@ -1314,10 +1314,10 @@
                       ``hosts = main, backup, failsafe``.
                 ''')
             with Conf('directives', desc='''
-                Job runner (batch scheduler) directives.
-
-                Default directives for :cylc:conf:
+                :Defaults for: :cylc:conf:
                 `flow.cylc[runtime][<namespace>][directives]`
+
+                Job runner (batch scheduler) directives.
             '''):
                 Conf('<directive>', VDR.V_STRING, desc='''
                     Example directives for the built-in job runner handlers

cylc/flow/task_job_mgr.py

@@ -1278,27 +1278,6 @@ def _prep_submit_task_job_impl(self, workflow, itask, rtconfig):
             job_d=job_d
         )
 
-    @staticmethod
-    def default_dict_merger(defaults, tdef):
-        """Use directive from platform if not set in rtconfig.
-
-        For item in defaults add item to output if not in tdef.
-
-        Examples:
-        >>> defaults = {'use': 1, 'override': 1}
-        >>> tdef = {'override': 2, 'nodefault': 1}
-        >>> result = TaskJobManager.default_dict_merger(defaults, tdef)
-        >>> {i: result[i] for i in sorted(result)}
-        {'nodefault': 1, 'override': 2, 'use': 1}
-        """
-        output = {}
-        for key in set(list(defaults.keys()) + list(tdef.keys())):
-            if key in defaults and key not in tdef:
-                output[key] = defaults[key]
-            else:
-                output[key] = tdef[key]
-        return output
-
     def get_job_conf(
         self,
         workflow,
@@ -1320,9 +1299,9 @@ def get_job_conf(
                 itask.platform['job runner command template']
             ),
             'dependencies': itask.state.get_resolved_dependencies(),
-            'directives': self.default_dict_merger(
-                itask.platform['directives'], rtconfig['directives']
-            ),
+            'directives': {
+                **itask.platform['directives'], **rtconfig['directives']
+            },
             'environment': rtconfig['environment'],
             'execution_time_limit': itask.summary[self.KEY_EXECUTE_TIME_LIMIT],
             'env-script': rtconfig['env-script'],