Add integration test.

cylc/flow/scripts/dump.py

@@ -313,7 +313,7 @@ async def dump(workflow_id, options, write=print):
                         if attrs:
                             result += " (" + ",".join(attrs) + ")"
                         if options.show_flows:
-                            result += f" flows={item['flowNums']}"
+                            result += f" flows={item['flowNums'].replace(' ','')}"
                         write(result)
 
     except Exception as exc:

tests/integration/scripts/test_dump.py

@@ -24,7 +24,6 @@
     get_option_parser,
 )
 
-
 DumpOptions = Options(get_option_parser())
 
 
@@ -54,3 +53,46 @@ async def test_dump_tasks(flow, scheduler, start):
             write=ret.append
         )
         assert ret == ['a, 1, waiting, not-held, queued, not-runahead']
+
+
+async def test_dump_format(flow, scheduler, start):
+    """Check the new "cylc dump" output format, i.e. task IDs.
+
+    See: https://github.com/cylc/cylc-flow/pull/6440
+    """
+    id_ = flow({
+        'scheduler': {
+            'allow implicit tasks': 'true',
+        },
+        'scheduling': {
+            'graph': {
+                'R1': 'a',
+            },
+        },
+    })
+    schd = scheduler(id_)
+    async with start(schd):
+        [itask] = schd.pool.get_tasks()
+
+        itask.state_reset(is_held=True)
+        schd.pool.data_store_mgr.delta_task_held(itask)
+
+        itask.state_reset(is_runahead=True)
+        schd.pool.data_store_mgr.delta_task_runahead(itask)
+
+        itask.state_reset(is_queued=True)
+        schd.pool.data_store_mgr.delta_task_queued(itask)
+
+        itask.flow_nums = set([1,2])
+        schd.pool.data_store_mgr.delta_task_flow_nums(itask)
+
+        await schd.update_data_structure()
+        ret = []
+        await dump(
+            id_,
+            DumpOptions(disp_form='tasks', show_flows=True),
+            write=ret.append
+        )
+        assert ret == [
+            '1/a:waiting (held,queued,runahead) flows=[1,2]',
+        ]