diff --git a/cylc/flow/broadcast_report.py b/cylc/flow/broadcast_report.py
index 72fedb4cbef..cb44f2212f3 100644
--- a/cylc/flow/broadcast_report.py
+++ b/cylc/flow/broadcast_report.py
@@ -72,7 +72,7 @@ def get_broadcast_change_iter(modified_settings, is_cancel=False):
         value = setting
         keys_str = ""
         while isinstance(value, dict):
-            key, value = list(value.items())[0]
+            key, value = next(iter(value.items()))
             if isinstance(value, dict):
                 keys_str += "[" + key + "]"
             else:
diff --git a/cylc/flow/clean.py b/cylc/flow/clean.py
index b38f01b12fc..b4597780fd3 100644
--- a/cylc/flow/clean.py
+++ b/cylc/flow/clean.py
@@ -129,7 +129,7 @@ def _clean_check(opts: 'Values', id_: str, run_dir: Path) -> None:
     except ContactFileExists as exc:
         raise ServiceFileError(
             f"Cannot clean running workflow {id_}.\n\n{exc}"
-        )
+        ) from None
 
 
 def init_clean(id_: str, opts: 'Values') -> None:
@@ -173,7 +173,7 @@ def init_clean(id_: str, opts: 'Values') -> None:
             try:
                 platform_names = get_platforms_from_db(local_run_dir)
             except ServiceFileError as exc:
-                raise ServiceFileError(f"Cannot clean {id_} - {exc}")
+                raise ServiceFileError(f"Cannot clean {id_} - {exc}") from None
             except sqlite3.OperationalError as exc:
                 # something went wrong with the query
                 # e.g. the table/field we need isn't there
@@ -186,7 +186,7 @@ def init_clean(id_: str, opts: 'Values') -> None:
                     ' local files (you may need to remove files on other'
                     ' platforms manually).'
                 )
-                raise ServiceFileError(f"Cannot clean {id_} - {exc}")
+                raise ServiceFileError(f"Cannot clean {id_} - {exc}") from exc
 
         if platform_names and platform_names != {'localhost'}:
             remote_clean(
@@ -361,7 +361,8 @@ def remote_clean(
     except PlatformLookupError as exc:
         raise PlatformLookupError(
             f"Cannot clean {id_} on remote platforms as the workflow database "
-            f"is out of date/inconsistent with the global config - {exc}")
+            f"is out of date/inconsistent with the global config - {exc}"
+        ) from None
 
     queue: Deque[RemoteCleanQueueTuple] = deque()
     remote_clean_cmd = partial(
diff --git a/cylc/flow/command_validation.py b/cylc/flow/command_validation.py
index 1c57d452c43..ea67f00206e 100644
--- a/cylc/flow/command_validation.py
+++ b/cylc/flow/command_validation.py
@@ -70,7 +70,7 @@ def flow_opts(flows: List[str], flow_wait: bool) -> None:
             try:
                 int(val)
             except ValueError:
-                raise InputError(ERR_OPT_FLOW_VAL.format(val))
+                raise InputError(ERR_OPT_FLOW_VAL.format(val)) from None
 
     if flow_wait and flows[0] in [FLOW_NEW, FLOW_NONE]:
         raise InputError(ERR_OPT_FLOW_WAIT)
diff --git a/cylc/flow/commands.py b/cylc/flow/commands.py
index 134681bdfd5..173984f17e0 100644
--- a/cylc/flow/commands.py
+++ b/cylc/flow/commands.py
@@ -211,7 +211,7 @@ async def stop(
         try:
             mode = StopMode(mode)
         except ValueError:
-            raise CommandFailedError(f"Invalid stop mode: '{mode}'")
+            raise CommandFailedError(f"Invalid stop mode: '{mode}'") from None
         schd._set_stop(mode)
         if mode is StopMode.REQUEST_KILL:
             schd.time_next_kill = time()
@@ -309,7 +309,7 @@ async def set_verbosity(schd: 'Scheduler', level: Union[int, str]):
         lvl = int(level)
         LOG.setLevel(lvl)
     except (TypeError, ValueError) as exc:
-        raise CommandFailedError(exc)
+        raise CommandFailedError(exc) from None
     cylc.flow.flags.verbosity = log_level_to_verbosity(lvl)
 
 
diff --git a/cylc/flow/config.py b/cylc/flow/config.py
index 5b7738f3e6c..fdcc10adb05 100644
--- a/cylc/flow/config.py
+++ b/cylc/flow/config.py
@@ -199,11 +199,11 @@ def interpolate_template(tmpl, params_dict):
     try:
         return tmpl % params_dict
     except KeyError:
-        raise ParamExpandError('bad parameter')
+        raise ParamExpandError('bad parameter') from None
     except TypeError:
-        raise ParamExpandError('wrong data type for parameter')
+        raise ParamExpandError('wrong data type for parameter') from None
     except ValueError:
-        raise ParamExpandError('bad template syntax')
+        raise ParamExpandError('bad template syntax') from None
 
 
 class WorkflowConfig:
@@ -480,8 +480,8 @@ def __init__(
                             get_interval(offset_string).standardise())
                     except IntervalParsingError:
                         raise WorkflowConfigError(
-                            "Illegal %s spec: %s" % (
-                                s_type, offset_string))
+                            "Illegal %s spec: %s" % (s_type, offset_string)
+                        ) from None
                     extn = "(" + offset_string + ")"
 
                 # Replace family names with members.
@@ -709,7 +709,7 @@ def process_initial_cycle_point(self) -> None:
                 try:
                     icp = ingest_time(orig_icp, get_current_time_string())
                 except IsodatetimeError as exc:
-                    raise WorkflowConfigError(str(exc))
+                    raise WorkflowConfigError(str(exc)) from None
         if orig_icp != icp:
             # now/next()/previous() was used, need to store
             # evaluated point in DB
@@ -761,7 +761,7 @@ def process_start_cycle_point(self) -> None:
                     for taskid in self.options.starttask
                 ]
             except ValueError as exc:
-                raise InputError(str(exc))
+                raise InputError(str(exc)) from None
             self.start_point = min(
                 get_point(cycle).standardise()
                 for cycle in cycle_points if cycle
@@ -1114,7 +1114,7 @@ def _check_completion_expression(self, task_name: str, expr: str) -> None:
                     f'\n  {expr}'
                     '\nThe "finished" output cannot be used in completion'
                     ' expressions, use "succeeded or failed".'
-                )
+                ) from None
 
             for alt_qualifier, qualifier in ALT_QUALIFIERS.items():
                 _alt_compvar = trigger_to_completion_variable(alt_qualifier)
@@ -1125,21 +1125,21 @@ def _check_completion_expression(self, task_name: str, expr: str) -> None:
                         f'\n  {expr}'
                         f'\nUse "{_compvar}" not "{_alt_compvar}" '
                         'in completion expressions.'
-                    )
+                    ) from None
 
             raise WorkflowConfigError(
                 # NOTE: str(exc) == "name 'x' is not defined" tested in
                 # tests/integration/test_optional_outputs.py
                 f'Error in [runtime][{task_name}]completion:'
                 f'\n{error}'
-            )
+            ) from None
         except Exception as exc:  # includes InvalidCompletionExpression
             # expression contains non-whitelisted syntax or any other error in
             # the expression e.g. SyntaxError
             raise WorkflowConfigError(
                 f'Error in [runtime][{task_name}]completion:'
                 f'\n{str(exc)}'
-            )
+            ) from None
 
         # ensure consistency between the graph and the completion expression
         for compvar in (
@@ -1415,11 +1415,12 @@ def compute_family_tree(self):
                     c3_single.mro(name))
             except RecursionError:
                 raise WorkflowConfigError(
-                    "circular [runtime] inheritance?")
+                    "circular [runtime] inheritance?"
+                ) from None
             except Exception as exc:
                 # catch inheritance errors
                 # TODO - specialise MRO exceptions
-                raise WorkflowConfigError(str(exc))
+                raise WorkflowConfigError(str(exc)) from None
 
         for name in self.cfg['runtime']:
             ancestors = self.runtime['linearized ancestors'][name]
@@ -1758,7 +1759,7 @@ def _check_task_event_handlers(self):
                                     f' {taskdef.name}:'
                                     f' {handler_template}:'
                                     f' {repr(exc)}'
-                                )
+                                ) from None
 
     def _check_special_tasks(self):
         """Check declared special tasks are valid, and detect special
@@ -1865,7 +1866,9 @@ def generate_triggers(self, lexpression, left_nodes, right, seq,
         try:
             expr_list = listify(lexpression)
         except SyntaxError:
-            raise WorkflowConfigError('Error in expression "%s"' % lexpression)
+            raise WorkflowConfigError(
+                'Error in expression "%s"' % lexpression
+            ) from None
 
         triggers = {}
         xtrig_labels = set()
@@ -1942,7 +1945,9 @@ def generate_triggers(self, lexpression, left_nodes, right, seq,
                 xtrig = xtrigs[label]
             except KeyError:
                 if label != 'wall_clock':
-                    raise WorkflowConfigError(f"xtrigger not defined: {label}")
+                    raise WorkflowConfigError(
+                        f"xtrigger not defined: {label}"
+                    ) from None
                 else:
                     # Allow "@wall_clock" in graph as implicit zero-offset.
                     xtrig = SubFuncContext('wall_clock', 'wall_clock', [], {})
@@ -2276,7 +2281,7 @@ def load_graph(self):
                 msg += ' (final cycle point=%s)' % fcp
                 if isinstance(exc, CylcError):
                     msg += ' %s' % exc.args[0]
-                raise WorkflowConfigError(msg)
+                raise WorkflowConfigError(msg) from None
             self.sequences.append(seq)
             parser = GraphParser(
                 family_map,
@@ -2431,7 +2436,7 @@ def get_taskdef(
             except TaskDefError as exc:
                 if orig_expr:
                     LOG.error(orig_expr)
-                raise WorkflowConfigError(str(exc))
+                raise WorkflowConfigError(str(exc)) from None
             else:
                 # Record custom message outputs from [runtime].
                 for output, message in (
@@ -2443,14 +2448,14 @@ def get_taskdef(
                             f'Invalid task output "'
                             f'[runtime][{name}][outputs]'
                             f'{output} = {message}" - {msg}'
-                        )
+                        ) from None
                     valid, msg = TaskMessageValidator.validate(message)
                     if not valid:
                         raise WorkflowConfigError(
                             f'Invalid task message "'
                             f'[runtime][{name}][outputs]'
                             f'{output} = {message}" - {msg}'
-                        )
+                        ) from None
                     self.taskdefs[name].add_output(output, message)
 
         return self.taskdefs[name]
@@ -2462,7 +2467,7 @@ def _get_taskdef(self, name: str) -> TaskDef:
         try:
             rtcfg = self.cfg['runtime'][name]
         except KeyError:
-            raise WorkflowConfigError("Task not defined: %s" % name)
+            raise WorkflowConfigError("Task not defined: %s" % name) from None
         # We may want to put in some handling for cases of changing the
         # initial cycle via restart (accidentally or otherwise).
 
@@ -2554,7 +2559,9 @@ def process_metadata_urls(self):
                     'workflow': self.workflow,
                 }
             except (KeyError, ValueError):
-                raise InputError(f'Invalid template [meta]URL: {url}')
+                raise InputError(
+                    f'Invalid template [meta]URL: {url}'
+                ) from None
             else:
                 LOG.warning(
                     'Detected deprecated template variables in [meta]URL.'
@@ -2590,7 +2597,9 @@ def process_metadata_urls(self):
                         'task': name,
                     }
                 except (KeyError, ValueError):
-                    raise InputError(f'Invalid template [meta]URL: {url}')
+                    raise InputError(
+                        f'Invalid template [meta]URL: {url}'
+                    ) from None
                 else:
                     LOG.warning(
                         'Detected deprecated template variables in'
diff --git a/cylc/flow/cycling/integer.py b/cylc/flow/cycling/integer.py
index 749c651fc08..20cc3765670 100644
--- a/cylc/flow/cycling/integer.py
+++ b/cylc/flow/cycling/integer.py
@@ -150,7 +150,7 @@ def standardise(self):
         try:
             self.value = str(int(self))
         except (TypeError, ValueError) as exc:
-            raise PointParsingError(type(self), self.value, exc)
+            raise PointParsingError(type(self), self.value, exc) from None
         return self
 
     def __int__(self):
diff --git a/cylc/flow/cycling/iso8601.py b/cylc/flow/cycling/iso8601.py
index a66ce3f5ba0..2ab311df425 100644
--- a/cylc/flow/cycling/iso8601.py
+++ b/cylc/flow/cycling/iso8601.py
@@ -102,7 +102,7 @@ def standardise(self):
                     WorkflowSpecifics.NUM_EXPANDED_YEAR_DIGITS)
             else:
                 message = str(exc)
-            raise PointParsingError(type(self), self.value, message)
+            raise PointParsingError(type(self), self.value, message) from None
         return self
 
     def sub(self, other):
@@ -176,7 +176,7 @@ def standardise(self):
         try:
             self.value = str(interval_parse(self.value))
         except IsodatetimeError:
-            raise IntervalParsingError(type(self), self.value)
+            raise IntervalParsingError(type(self), self.value) from None
         return self
 
     def add(self, other):
@@ -782,7 +782,7 @@ def prev_next(
             raise WorkflowConfigError(
                 f'Invalid offset: {my_time}:'
                 f' Offset lists are semicolon separated, try {suggest}'
-            )
+            ) from None
 
         timepoints.append(parsed_point + now)
 
diff --git a/cylc/flow/dbstatecheck.py b/cylc/flow/dbstatecheck.py
index b38c394ccdb..1890c697c71 100644
--- a/cylc/flow/dbstatecheck.py
+++ b/cylc/flow/dbstatecheck.py
@@ -78,7 +78,7 @@ def __init__(self, rund, workflow, db_path=None):
         try:
             self.db_point_fmt = self._get_db_point_format()
             self.c7_back_compat_mode = False
-        except sqlite3.OperationalError as exc:
+        except sqlite3.OperationalError:
             # BACK COMPAT: Cylc 7 DB (see method below).
             try:
                 self.db_point_fmt = self._get_db_point_format_compat()
@@ -86,7 +86,7 @@ def __init__(self, rund, workflow, db_path=None):
             except sqlite3.OperationalError:
                 with suppress(Exception):
                     self.conn.close()
-                raise exc  # original error
+                raise
 
     def __enter__(self):
         return self
@@ -137,7 +137,7 @@ def adjust_point_to_db(self, cycle, offset):
             raise InputError(
                 f'Cycle point "{cycle}" is not compatible'
                 f' with DB point format "{self.db_point_fmt}"'
-            )
+            ) from None
         return cycle
 
     @staticmethod
diff --git a/cylc/flow/graph_parser.py b/cylc/flow/graph_parser.py
index efbce16fb36..3fbfd8c1754 100644
--- a/cylc/flow/graph_parser.py
+++ b/cylc/flow/graph_parser.py
@@ -346,7 +346,7 @@ def parse_graph(self, graph_string: str) -> None:
                         raise GraphParseError(
                             f"Dangling {seq}:"
                             f"{this_line}"
-                        )
+                        ) from None
             part_lines.append(this_line)
 
             # Check that a continuation sequence doesn't end this line and
@@ -638,7 +638,8 @@ def _proc_dep_pair(
                     except KeyError:
                         # "FAM:bad => foo" in LHS (includes "FAM => bar" too).
                         raise GraphParseError(
-                            f"Illegal family trigger in {expr}")
+                            f"Illegal family trigger in {expr}"
+                        ) from None
                 else:
                     # Not a family.
                     if trig in self.__class__.fam_to_mem_trigger_map:
@@ -911,7 +912,8 @@ def _compute_triggers(
                 except KeyError:
                     # Illegal family trigger on RHS of a pair.
                     raise GraphParseError(
-                        f"Illegal family trigger: {name}:{output}")
+                        f"Illegal family trigger: {name}:{output}"
+                    ) from None
             else:
                 fam = False
                 if not output:
diff --git a/cylc/flow/host_select.py b/cylc/flow/host_select.py
index 0eb34d088ca..69e32c68a71 100644
--- a/cylc/flow/host_select.py
+++ b/cylc/flow/host_select.py
@@ -373,7 +373,7 @@ def _filter_by_ranking(hosts, rankings, results, data=None):
                     f'\n    Expression: {item}'
                     f'\n    Configuration: {GLBL_CFG_STR}'
                     f'\n    Error: {exc}'
-                )
+                ) from None
             if isinstance(result, bool):
                 host_rankings[item] = result
                 data[host][item] = result
diff --git a/cylc/flow/id.py b/cylc/flow/id.py
index 58fff7fa7bc..f2c8b05b4a1 100644
--- a/cylc/flow/id.py
+++ b/cylc/flow/id.py
@@ -128,7 +128,7 @@ def __getitem__(self, key):
             return dict.__getitem__(self, key)
         except KeyError:
             if key not in self._KEYS:
-                raise ValueError(f'Invalid token: {key}')
+                raise ValueError(f'Invalid token: {key}') from None
             return None
 
     def __str__(self):
diff --git a/cylc/flow/id_cli.py b/cylc/flow/id_cli.py
index 9c7493fd612..35afbf80d5a 100644
--- a/cylc/flow/id_cli.py
+++ b/cylc/flow/id_cli.py
@@ -167,9 +167,9 @@ def _parse_cli(*ids: str) -> List[Tokens]:
                     # this ID is invalid with or without the trailing slash
                     tokens = cli_tokenise(id_[:-1])
                 except ValueError:
-                    raise InputError(f'Invalid ID: {id_}')
+                    raise InputError(f'Invalid ID: {id_}') from None
             else:
-                raise InputError(f'Invalid ID: {id_}')
+                raise InputError(f'Invalid ID: {id_}') from None
         is_partial = tokens.get('workflow') and not tokens.get('cycle')
         is_relative = not tokens.get('workflow')
 
@@ -347,7 +347,7 @@ async def parse_ids_async(
     if src:
         if not flow_file_path:
             # get the workflow file path from the run dir
-            flow_file_path = get_flow_file(list(workflows)[0])
+            flow_file_path = get_flow_file(next(iter(workflows)))
         return workflows, flow_file_path
     return workflows, multi_mode
 
@@ -375,7 +375,7 @@ async def parse_id_async(
             'max_tasks': 1,
         },
     )
-    workflow_id = list(workflows)[0]
+    workflow_id = next(iter(workflows))
     tokens_list = workflows[workflow_id]
     tokens: Optional[Tokens]
     if tokens_list:
diff --git a/cylc/flow/install.py b/cylc/flow/install.py
index 27810f72e97..2e94943d9d6 100644
--- a/cylc/flow/install.py
+++ b/cylc/flow/install.py
@@ -328,7 +328,7 @@ def install_workflow(
         # This occurs when the file exists but is _not_ a directory.
         raise WorkflowFilesError(
             f"Cannot install as there is an existing file at {rundir}."
-        )
+        ) from None
     if relink:
         link_runN(rundir)
     rsync_cmd = get_rsync_rund_cmd(source, rundir)
@@ -529,7 +529,7 @@ def parse_cli_sym_dirs(symlink_dirs: str) -> Dict[str, Dict[str, Any]]:
                 'There is an error in --symlink-dirs option:'
                 f' {pair}. Try entering option in the form '
                 '--symlink-dirs=\'log=$DIR, share=$DIR2, ...\''
-            )
+            ) from None
         if key not in possible_symlink_dirs:
             dirs = ', '.join(possible_symlink_dirs)
             raise InputError(
diff --git a/cylc/flow/install_plugins/log_vc_info.py b/cylc/flow/install_plugins/log_vc_info.py
index 29d861f7654..41479d6f1ed 100644
--- a/cylc/flow/install_plugins/log_vc_info.py
+++ b/cylc/flow/install_plugins/log_vc_info.py
@@ -253,7 +253,7 @@ def _run_cmd(
     except FileNotFoundError as exc:
         # This will only be raised if the VCS command is not installed,
         # otherwise Popen() will succeed with a non-zero return code
-        raise VCSNotInstalledError(vcs, exc)
+        raise VCSNotInstalledError(vcs, exc) from None
     if stdout == PIPE:
         out, err = pipe_poller(proc, proc.stdout, proc.stderr)
     else:
diff --git a/cylc/flow/loggingutil.py b/cylc/flow/loggingutil.py
index 3d77bdcb037..a6f1fbee558 100644
--- a/cylc/flow/loggingutil.py
+++ b/cylc/flow/loggingutil.py
@@ -206,7 +206,7 @@ def should_rollover(self, record: logging.LogRecord) -> bool:
             self.stream.seek(0, 2)
         except ValueError as exc:
             # intended to catch - ValueError: I/O operation on closed file
-            raise SystemExit(exc)
+            raise SystemExit(exc) from None
         return self.stream.tell() + len(msg.encode('utf8')) >= max_bytes
 
     @property
diff --git a/cylc/flow/main_loop/__init__.py b/cylc/flow/main_loop/__init__.py
index 2350153842c..e9f9f35f5da 100644
--- a/cylc/flow/main_loop/__init__.py
+++ b/cylc/flow/main_loop/__init__.py
@@ -329,14 +329,14 @@ def load(config, additional_plugins=None):
                 f'No main-loop plugin: "{plugin_name}"\n'
                 + '    Available plugins:\n'
                 + indent('\n'.join(sorted(entry_points)), '        ')
-            )
+            ) from None
         # load plugin
         try:
             module = entry_point.load()
         except Exception as exc:
             raise PluginError(
                 'cylc.main_loop', entry_point.name, exc
-            )
+            ) from None
         # load coroutines
         log = []
         for coro_name, coro in getmembers(module):
diff --git a/cylc/flow/main_loop/health_check.py b/cylc/flow/main_loop/health_check.py
index b488c878c0e..a470fd3cb88 100644
--- a/cylc/flow/main_loop/health_check.py
+++ b/cylc/flow/main_loop/health_check.py
@@ -56,4 +56,4 @@ def _check_contact_file(scheduler):
         raise CylcError(
             '%s: contact file corrupted/modified and may be left'
             % workflow_files.get_contact_file_path(scheduler.workflow)
-        )
+        ) from None
diff --git a/cylc/flow/network/__init__.py b/cylc/flow/network/__init__.py
index 916b129e244..315a57a23ba 100644
--- a/cylc/flow/network/__init__.py
+++ b/cylc/flow/network/__init__.py
@@ -78,7 +78,7 @@ def get_location(workflow: str) -> Tuple[str, int, int]:
         contact = load_contact_file(workflow)
     except (IOError, ValueError, ServiceFileError):
         # Contact file does not exist or corrupted, workflow should be dead
-        raise WorkflowStopped(workflow)
+        raise WorkflowStopped(workflow) from None
 
     host = contact[ContactFileFields.HOST]
     host = get_fqdn_by_host(host)
@@ -179,13 +179,13 @@ def _socket_bind(self, min_port, max_port, srv_prv_key_loc=None):
                 f"Failed to find server's public "
                 f"key in "
                 f"{srv_prv_key_info.full_key_path}."
-            )
+            ) from None
         except OSError:
             raise ServiceFileError(
                 f"IO error opening server's private "
                 f"key from "
                 f"{srv_prv_key_info.full_key_path}."
-            )
+            ) from None
         if server_private_key is None:  # this can't be caught by exception
             raise ServiceFileError(
                 f"Failed to find server's private "
@@ -204,7 +204,9 @@ def _socket_bind(self, min_port, max_port, srv_prv_key_loc=None):
                 self.port = self.socket.bind_to_random_port(
                     'tcp://*', min_port, max_port)
         except (zmq.error.ZMQError, zmq.error.ZMQBindError) as exc:
-            raise CylcError(f'could not start Cylc ZMQ server: {exc}')
+            raise CylcError(
+                f'could not start Cylc ZMQ server: {exc}'
+            ) from None
 
     # Keeping srv_public_key_loc as optional arg so as to not break interface
     def _socket_connect(self, host, port, srv_public_key_loc=None):
@@ -237,7 +239,7 @@ def _socket_connect(self, host, port, srv_public_key_loc=None):
             client_public_key, client_priv_key = zmq.auth.load_certificate(
                 client_priv_key_info.full_key_path)
         except (OSError, ValueError):
-            raise ClientError(error_msg)
+            raise ClientError(error_msg) from None
         if client_priv_key is None:  # this can't be caught by exception
             raise ClientError(error_msg)
         self.socket.curve_publickey = client_public_key
@@ -256,7 +258,8 @@ def _socket_connect(self, host, port, srv_public_key_loc=None):
             self.socket.curve_serverkey = server_public_key
         except (OSError, ValueError):  # ValueError raised w/ no public key
             raise ClientError(
-                "Failed to load the workflow's public key, so cannot connect.")
+                "Failed to load the workflow's public key, so cannot connect."
+            ) from None
 
         self.socket.connect(f'tcp://{host}:{port}')
 
diff --git a/cylc/flow/network/client.py b/cylc/flow/network/client.py
index e7e26954d56..099ef8bc0ff 100644
--- a/cylc/flow/network/client.py
+++ b/cylc/flow/network/client.py
@@ -326,7 +326,7 @@ async def async_request(
             raise ClientError(
                 error.get('message'),  # type: ignore
                 error.get('traceback'),  # type: ignore
-            )
+            ) from None
 
     def get_header(self) -> dict:
         """Return "header" data to attach to each request for traceability.
diff --git a/cylc/flow/network/multi.py b/cylc/flow/network/multi.py
index 2b9ea418976..9c190f68799 100644
--- a/cylc/flow/network/multi.py
+++ b/cylc/flow/network/multi.py
@@ -235,7 +235,7 @@ def _report(
     """
     try:
         ret: List[Tuple[Optional[str], Optional[str], bool]] = []
-        for _mutation_name, mutation_response in response.items():
+        for mutation_response in response.values():
             # extract the result of each mutation result in the response
             success, msg = mutation_response['result'][0]['response']
             out = None
diff --git a/cylc/flow/network/resolvers.py b/cylc/flow/network/resolvers.py
index 11eafd8bea5..fc9b67eeef5 100644
--- a/cylc/flow/network/resolvers.py
+++ b/cylc/flow/network/resolvers.py
@@ -750,7 +750,7 @@ async def _mutation_mapper(
         try:
             meth = COMMANDS[command]
         except KeyError:
-            raise ValueError(f"Command '{command}' not found")
+            raise ValueError(f"Command '{command}' not found") from None
 
         try:
             # Initiate the command. Validation may be performed at this point,
diff --git a/cylc/flow/network/schema.py b/cylc/flow/network/schema.py
index 70e40232c1d..b962b582b70 100644
--- a/cylc/flow/network/schema.py
+++ b/cylc/flow/network/schema.py
@@ -271,7 +271,7 @@ def field_name_from_type(
     try:
         return NODE_MAP[named_type.name]
     except KeyError:
-        raise ValueError(f"'{named_type.name}' is not a node type")
+        raise ValueError(f"'{named_type.name}' is not a node type") from None
 
 
 def get_resolvers(info: 'ResolveInfo') -> 'BaseResolvers':
diff --git a/cylc/flow/network/ssh_client.py b/cylc/flow/network/ssh_client.py
index d1dd4fb6da4..d2ed0dd33e9 100644
--- a/cylc/flow/network/ssh_client.py
+++ b/cylc/flow/network/ssh_client.py
@@ -90,7 +90,7 @@ async def async_request(
                 f"Command exceeded the timeout {timeout}s. "
                 "This could be due to network problems. "
                 "Check the workflow log."
-            )
+            ) from None
 
     def prepare_command(
         self, command: str, args: Optional[dict], timeout: Union[float, str]
diff --git a/cylc/flow/param_expand.py b/cylc/flow/param_expand.py
index 0707a46e1a3..6ab104404e4 100644
--- a/cylc/flow/param_expand.py
+++ b/cylc/flow/param_expand.py
@@ -195,8 +195,9 @@ def _expand_name(self, results, tmpl, params, spec_vals=None):
             try:
                 results.append((tmpl % current_values, current_values))
             except KeyError as exc:
-                raise ParamExpandError('parameter %s is not '
-                                       'defined.' % str(exc.args[0]))
+                raise ParamExpandError(
+                    'parameter %s is not ' 'defined.' % str(exc.args[0])
+                ) from None
         else:
             for param_val in params[0][1]:
                 spec_vals[params[0][0]] = param_val
@@ -306,8 +307,8 @@ def expand_parent_params(self, parent, param_values, origin):
                     used[item] = param_values[item]
                 except KeyError:
                     raise ParamExpandError(
-                        "parameter '%s' undefined in '%s'" % (
-                            item, origin))
+                        "parameter '%s' undefined in '%s'" % (item, origin)
+                    ) from None
 
         # For each parameter substitute the param_tmpl_cfg.
         tmpl = tmpl.format(**self.param_tmpl_cfg)
@@ -425,8 +426,9 @@ def _expand_graph(self, line, all_params,
                 try:
                     repl = tmpl % param_values
                 except KeyError as exc:
-                    raise ParamExpandError('parameter %s is not '
-                                           'defined.' % str(exc.args[0]))
+                    raise ParamExpandError(
+                        'parameter %s is not ' 'defined.' % str(exc.args[0])
+                    ) from None
                 line = line.replace('<' + p_group + '>', repl)
             if line:
                 line_set.add(line)
diff --git a/cylc/flow/parsec/config.py b/cylc/flow/parsec/config.py
index 19f937d8e5b..29944c03b30 100644
--- a/cylc/flow/parsec/config.py
+++ b/cylc/flow/parsec/config.py
@@ -150,10 +150,12 @@ def get(self, keys: Optional[Iterable[str]] = None, sparse: bool = False):
                         # setting not present in __MANY__ section:
                         key in self.spec.get(*parents)
                     ):
-                        raise ItemNotFoundError(itemstr(parents, key))
+                        raise ItemNotFoundError(
+                            itemstr(parents, key)
+                        ) from None
                     raise InvalidConfigError(
                         itemstr(parents, key), self.spec.name
-                    )
+                    ) from None
                 else:
                     parents.append(key)
 
diff --git a/cylc/flow/parsec/empysupport.py b/cylc/flow/parsec/empysupport.py
index b4164894e0f..e3dc5e28df4 100644
--- a/cylc/flow/parsec/empysupport.py
+++ b/cylc/flow/parsec/empysupport.py
@@ -66,7 +66,7 @@ def empyprocess(
         raise EmPyError(
             str(exc),
             lines={'<template>': flines[max(lineno - 4, 0): lineno]},
-        )
+        ) from None
     finally:
         interpreter.shutdown()
         xworkflow = xtempl.getvalue()
diff --git a/cylc/flow/parsec/fileparse.py b/cylc/flow/parsec/fileparse.py
index 57b3449561a..f4de85aee3c 100644
--- a/cylc/flow/parsec/fileparse.py
+++ b/cylc/flow/parsec/fileparse.py
@@ -493,8 +493,10 @@ def read_and_proc(
             try:
                 from cylc.flow.parsec.empysupport import empyprocess
             except ImportError:
-                raise ParsecError('EmPy Python package must be installed '
-                                  'to process file: ' + fpath)
+                raise ParsecError(
+                    'EmPy Python package must be installed '
+                    'to process file: ' + fpath
+                ) from None
             flines = empyprocess(
                 fpath, flines, fdir, template_vars
             )
@@ -513,8 +515,10 @@ def read_and_proc(
             try:
                 from cylc.flow.parsec.jinja2support import jinja2process
             except ImportError:
-                raise ParsecError('Jinja2 Python package must be installed '
-                                  'to process file: ' + fpath)
+                raise ParsecError(
+                    'Jinja2 Python package must be installed '
+                    'to process file: ' + fpath
+                ) from None
             flines = jinja2process(
                 fpath, flines, fdir, template_vars
             )
diff --git a/cylc/flow/parsec/jinja2support.py b/cylc/flow/parsec/jinja2support.py
index 34f92c11519..74335358421 100644
--- a/cylc/flow/parsec/jinja2support.py
+++ b/cylc/flow/parsec/jinja2support.py
@@ -75,7 +75,7 @@ def load(
         try:
             mdict = __import__(name, fromlist=['*']).__dict__
         except ImportError:
-            raise TemplateNotFound(name)
+            raise TemplateNotFound(name) from None
 
         # inject module dict into the context of an empty template
         def root_render_func(context, *args, **kwargs):
@@ -297,12 +297,12 @@ def jinja2process(
             exc,
             lines=get_error_lines(fpath, flines),
             filename=filename
-        )
+        ) from None
     except Exception as exc:
         raise Jinja2Error(
             exc,
             lines=get_error_lines(fpath, flines),
-        )
+        ) from None
 
     # Ignore blank lines (lone Jinja2 statements leave blank lines behind)
     return [line for line in lines if line.strip()]
diff --git a/cylc/flow/parsec/upgrade.py b/cylc/flow/parsec/upgrade.py
index 0f75732f6d5..3ef53b006f6 100644
--- a/cylc/flow/parsec/upgrade.py
+++ b/cylc/flow/parsec/upgrade.py
@@ -110,7 +110,7 @@ def get_item(self, keys):
                 raise UpgradeError(
                     f'{self.show_keys(keys[:-1], True)}'
                     f' ("{keys[-2]}" should be a [section] not a setting)'
-                )
+                ) from None
         return item
 
     def put_item(self, keys, val):
diff --git a/cylc/flow/parsec/validate.py b/cylc/flow/parsec/validate.py
index 18c19596a63..12e2163a894 100644
--- a/cylc/flow/parsec/validate.py
+++ b/cylc/flow/parsec/validate.py
@@ -280,7 +280,7 @@ def coerce_float(cls, value, keys):
         try:
             return float(value)
         except ValueError as exc:
-            raise IllegalValueError('float', keys, value, exc=exc)
+            raise IllegalValueError('float', keys, value, exc=exc) from None
 
     @classmethod
     def coerce_float_list(cls, value, keys):
@@ -309,7 +309,7 @@ def coerce_int(cls, value, keys):
         try:
             return int(value)
         except ValueError as exc:
-            raise IllegalValueError('int', keys, value, exc=exc)
+            raise IllegalValueError('int', keys, value, exc=exc) from None
 
     @classmethod
     def coerce_int_list(cls, value, keys):
@@ -480,13 +480,17 @@ def expand_list(cls, values, keys, type_):
                 try:
                     lvalues.append(type_(item))
                 except ValueError as exc:
-                    raise IllegalValueError('list', keys, item, exc=exc)
+                    raise IllegalValueError(
+                        'list', keys, item, exc=exc
+                    ) from None
             else:
                 # mult * val
                 try:
                     lvalues += int(mult) * [type_(val)]
                 except ValueError as exc:
-                    raise IllegalValueError('list', keys, item, exc=exc)
+                    raise IllegalValueError(
+                        'list', keys, item, exc=exc
+                    ) from None
         return lvalues
 
     @classmethod
@@ -832,7 +836,9 @@ def coerce_cycle_point(cls, value, keys):
                     parser.parse(value)
                 return value
             except IsodatetimeError as exc:
-                raise IllegalValueError('cycle point', keys, value, exc=exc)
+                raise IllegalValueError(
+                    'cycle point', keys, value, exc=exc
+                ) from None
         try:
             TimePointParser().parse(value)
         except IsodatetimeError as exc:
@@ -841,7 +847,9 @@ def coerce_cycle_point(cls, value, keys):
                 details = {'msg': "Invalid cycle point"}
             else:
                 details = {'exc': exc}
-            raise IllegalValueError('cycle point', keys, value, **details)
+            raise IllegalValueError(
+                'cycle point', keys, value, **details
+            ) from None
         return value
 
     @classmethod
@@ -883,7 +891,7 @@ def coerce_cycle_point_format(cls, value, keys):
             except IsodatetimeError as exc:
                 raise IllegalValueError(
                     'cycle point format', keys, value, exc=exc
-                )
+                ) from None
             return value
         if 'X' in value:
             for i in range(1, 101):
@@ -898,7 +906,9 @@ def coerce_cycle_point_format(cls, value, keys):
         try:
             dumper.dump(test_timepoint, value)
         except IsodatetimeError as exc:
-            raise IllegalValueError('cycle point format', keys, value, exc=exc)
+            raise IllegalValueError(
+                'cycle point format', keys, value, exc=exc
+            ) from None
         return value
 
     @classmethod
@@ -932,7 +942,7 @@ def coerce_cycle_point_time_zone(cls, value, keys):
         except ValueError as exc:  # not IsodatetimeError as too specific
             raise IllegalValueError(
                 'cycle point time zone format', keys, value, exc=exc
-            )
+            ) from None
         return value
 
     @classmethod
@@ -951,7 +961,9 @@ def coerce_interval(cls, value, keys):
         try:
             interval = DurationParser().parse(value)
         except IsodatetimeError as exc:
-            raise IllegalValueError("ISO 8601 interval", keys, value, exc=exc)
+            raise IllegalValueError(
+                "ISO 8601 interval", keys, value, exc=exc
+            ) from None
         return DurationFloat(interval.get_seconds())
 
     @classmethod
@@ -1012,8 +1024,11 @@ def coerce_parameter_list(cls, value, keys):
                 except ValueError:
                     if can_only_be == int:
                         raise IllegalValueError(
-                            'parameter', keys, value,
-                            'mixing int range and str')
+                            'parameter',
+                            keys,
+                            value,
+                            'mixing int range and str',
+                        ) from None
                     can_only_be = str
                 items.append(item)
             else:
@@ -1248,7 +1263,8 @@ def coerce_interval(cls, value, keys):
                 interval = DurationParser().parse(str(DurationFloat(value)))
             except IsodatetimeError as exc:
                 raise IllegalValueError(
-                    "ISO 8601 interval", keys, value, exc=exc)
+                    "ISO 8601 interval", keys, value, exc=exc
+                ) from None
         return DurationFloat(interval.get_seconds())
 
 
diff --git a/cylc/flow/pathutil.py b/cylc/flow/pathutil.py
index aa097a2e680..da3d6c93cde 100644
--- a/cylc/flow/pathutil.py
+++ b/cylc/flow/pathutil.py
@@ -272,7 +272,7 @@ def make_symlink_dir(path: Union[Path, str], target: Union[Path, str]) -> bool:
         except OSError as exc:
             raise WorkflowFilesError(
                 f"Failed to remove broken symlink {path}\n{exc}"
-            )
+            ) from None
     try:
         target.mkdir(parents=True, exist_ok=False)
     except FileExistsError:
@@ -280,7 +280,7 @@ def make_symlink_dir(path: Union[Path, str], target: Union[Path, str]) -> bool:
             f"Symlink dir target already exists: ({path} ->) {target}\n"
             "Tip: in future, use 'cylc clean' instead of manually deleting "
             "workflow run dirs."
-        )
+        ) from None
 
     # This is needed in case share and share/cycle have the same symlink dir:
     if path.exists():
@@ -291,7 +291,7 @@ def make_symlink_dir(path: Union[Path, str], target: Union[Path, str]) -> bool:
         path.symlink_to(target)
         return True
     except OSError as exc:
-        raise WorkflowFilesError(f"Error when symlinking\n{exc}")
+        raise WorkflowFilesError(f"Error when symlinking\n{exc}") from None
 
 
 def remove_dir_and_target(path: Union[Path, str]) -> None:
diff --git a/cylc/flow/prerequisite.py b/cylc/flow/prerequisite.py
index b388934e2de..449a5009656 100644
--- a/cylc/flow/prerequisite.py
+++ b/cylc/flow/prerequisite.py
@@ -195,7 +195,8 @@ def _conditional_is_satisfied(self):
                 err_msg += (
                     " (could be unmatched parentheses in the graph string?)")
             raise TriggerExpressionError(
-                '"%s":\n%s' % (self.get_raw_conditional_expression(), err_msg))
+                '"%s":\n%s' % (self.get_raw_conditional_expression(), err_msg)
+            ) from None
         return res
 
     def satisfy_me(self, outputs: Iterable['Tokens']) -> 'Set[Tokens]':
diff --git a/cylc/flow/resources.py b/cylc/flow/resources.py
index d81b7f7211b..956ec27e8aa 100644
--- a/cylc/flow/resources.py
+++ b/cylc/flow/resources.py
@@ -18,7 +18,7 @@
 
 from contextlib import suppress
 from pathlib import Path
-from random import shuffle
+from random import choice
 import shutil
 import sys
 from typing import Optional
@@ -205,12 +205,9 @@ def get_api_key() -> str:
     load over a larger number of keys to prevent hitting the cap with group
     sessions.
     """
-    keys = []
     with open((TUTORIAL_DIR / 'api-keys'), 'r') as api_keys:
-        for api_key in api_keys:
-            keys.append(api_key)
-    shuffle(keys)
-    return keys[0].strip()
+        return choice(list(api_keys)).strip()  # nosec
+        # (the randomness of this choice is not a security concern)
 
 
 def set_api_key(tgt):
diff --git a/cylc/flow/scheduler.py b/cylc/flow/scheduler.py
index b3bbbd23d7d..92702b0b55e 100644
--- a/cylc/flow/scheduler.py
+++ b/cylc/flow/scheduler.py
@@ -917,7 +917,7 @@ def process_queued_task_messages(self) -> None:
         # Remaining unprocessed messages have no corresponding task proxy.
         # For example, if I manually set a running task to succeeded, the
         # proxy can be removed, but the orphaned job still sends messages.
-        for _id, tms in messages.items():
+        for tms in messages.values():
             warn = "Undeliverable task messages received and ignored:"
             for _, msg in tms:
                 warn += f'\n  {msg.job_id}: {msg.severity} - "{msg.message}"'
diff --git a/cylc/flow/scripts/broadcast.py b/cylc/flow/scripts/broadcast.py
index c7e5d2a4f3b..8cfda85bdd8 100755
--- a/cylc/flow/scripts/broadcast.py
+++ b/cylc/flow/scripts/broadcast.py
@@ -360,7 +360,7 @@ async def run(options: 'Values', workflow_id):
                 # TODO validate showtask?
                 raise InputError(
                     'TASK_ID_GLOB must be in the format: cycle/task'
-                )
+                ) from None
         result = await pclient.async_request('graphql', query_kwargs)
         for wflow in result['workflows']:
             settings = wflow['broadcasts']
diff --git a/cylc/flow/scripts/cat_log.py b/cylc/flow/scripts/cat_log.py
index 688f51ddd35..e53524cc648 100755
--- a/cylc/flow/scripts/cat_log.py
+++ b/cylc/flow/scripts/cat_log.py
@@ -495,7 +495,7 @@ def _main(
                     raise InputError(
                         f"--rotation {rotation_number} invalid "
                         f"(max value is {len(logs) - 1})"
-                    )
+                    ) from None
             else:
                 raise InputError('Log file not found.')
         else:
diff --git a/cylc/flow/scripts/clean.py b/cylc/flow/scripts/clean.py
index 2166a98f055..fffd27752d7 100644
--- a/cylc/flow/scripts/clean.py
+++ b/cylc/flow/scripts/clean.py
@@ -157,7 +157,7 @@ def parse_timeout(opts: 'Values') -> None:
                 raise InputError(
                     f"Invalid timeout: {opts.remote_timeout}. Must be "
                     "an ISO 8601 duration or number of seconds."
-                )
+                ) from None
         opts.remote_timeout = str(timeout)
 
 
diff --git a/cylc/flow/scripts/cylc.py b/cylc/flow/scripts/cylc.py
index 4ca411f8e9b..209512264a9 100644
--- a/cylc/flow/scripts/cylc.py
+++ b/cylc/flow/scripts/cylc.py
@@ -612,7 +612,7 @@ def pycoverage(cmd_args):  # pragma: no cover
             'Could not initiate coverage, likely because Cylc was not '
             'installed in editable mode.'
             '\n\n*****************************\n\n'
-        )
+        ) from None
 
     # start the coverage running
     cov.start()
diff --git a/cylc/flow/scripts/dump.py b/cylc/flow/scripts/dump.py
index f891c943d77..f47ee8d2df8 100755
--- a/cylc/flow/scripts/dump.py
+++ b/cylc/flow/scripts/dump.py
@@ -296,4 +296,5 @@ async def dump(workflow_id, options, write=print):
                         write(', '.join(values))
     except Exception as exc:
         raise CylcError(
-            json.dumps(workflows, indent=4) + '\n' + str(exc) + '\n')
+            json.dumps(workflows, indent=4) + '\n' + str(exc) + '\n'
+        ) from None
diff --git a/cylc/flow/scripts/ext_trigger.py b/cylc/flow/scripts/ext_trigger.py
index c76bbb86712..775d6eaa0da 100755
--- a/cylc/flow/scripts/ext_trigger.py
+++ b/cylc/flow/scripts/ext_trigger.py
@@ -145,7 +145,7 @@ def main(
             LOG.exception(exc)
             LOG.info(MSG_SEND_FAILED, i_try + 1, max_n_tries)
             if i_try == max_n_tries - 1:  # final attempt
-                raise CylcError('send failed')
+                raise CylcError('send failed') from None
             LOG.info(MSG_SEND_RETRY, retry_intvl_secs, options.comms_timeout)
             sleep(retry_intvl_secs)
         else:
diff --git a/cylc/flow/scripts/get_workflow_contact.py b/cylc/flow/scripts/get_workflow_contact.py
index 9903aec6abb..66194cce732 100755
--- a/cylc/flow/scripts/get_workflow_contact.py
+++ b/cylc/flow/scripts/get_workflow_contact.py
@@ -49,10 +49,10 @@ def main(parser: COP, options: 'Values', workflow_id: str) -> None:
     )
     try:
         data = load_contact_file(workflow_id)
-    except ServiceFileError:
+    except ServiceFileError as exc:
         raise CylcError(
             f"{workflow_id}: cannot get contact info, workflow not running?"
-        )
+        ) from exc
     else:
         for key, value in sorted(data.items()):
             print("%s=%s" % (key, value))
diff --git a/cylc/flow/scripts/lint.py b/cylc/flow/scripts/lint.py
index 1561ad743f3..c8aa7279590 100755
--- a/cylc/flow/scripts/lint.py
+++ b/cylc/flow/scripts/lint.py
@@ -860,7 +860,7 @@ def get_pyproject_toml(dir_: Path) -> Dict[str, Any]:
         try:
             loadeddata = toml_loads(tomlfile.read_text())
         except TOMLDecodeError as exc:
-            raise CylcError(f'pyproject.toml did not load: {exc}')
+            raise CylcError(f'pyproject.toml did not load: {exc}') from None
 
         _tool, _cylc, _lint = LINT_TABLE
         try:
diff --git a/cylc/flow/scripts/report_timings.py b/cylc/flow/scripts/report_timings.py
index b6d796ad732..362f40953c2 100755
--- a/cylc/flow/scripts/report_timings.py
+++ b/cylc/flow/scripts/report_timings.py
@@ -257,7 +257,7 @@ def _check_imports(self):
             raise CylcError(
                 'Cannot import pandas - summary unavailable.'
                 ' try: pip install cylc-flow[report-timings]'
-            )
+            ) from None
         else:
             del pandas
 
@@ -402,5 +402,5 @@ def _check_imports(self):
         except ImportError:
             raise CylcError(
                 'Cannot import matplotlib - HTML summary unavailable.'
-            )
+            ) from None
         super(HTMLTimingSummary, self)._check_imports()
diff --git a/cylc/flow/scripts/show.py b/cylc/flow/scripts/show.py
index b146f339e10..2fdfaf7fde4 100755
--- a/cylc/flow/scripts/show.py
+++ b/cylc/flow/scripts/show.py
@@ -490,7 +490,7 @@ def main(_, options: 'Values', *ids) -> None:
         constraint='mixed',
         max_workflows=1,
     )
-    workflow_id = list(workflow_args)[0]
+    workflow_id = next(iter(workflow_args))
     tokens_list = workflow_args[workflow_id]
 
     if tokens_list and options.task_defs:
diff --git a/cylc/flow/scripts/validate.py b/cylc/flow/scripts/validate.py
index a224302c1b0..f3a77087069 100755
--- a/cylc/flow/scripts/validate.py
+++ b/cylc/flow/scripts/validate.py
@@ -181,7 +181,8 @@ async def run(
             continue
         except Exception as exc:
             raise WorkflowConfigError(
-                'failed to instantiate task %s: %s' % (name, exc))
+                'failed to instantiate task %s: %s' % (name, exc)
+            ) from None
 
         # force trigger evaluation now
         try:
@@ -189,17 +190,21 @@ async def run(
         except TriggerExpressionError as exc:
             err = str(exc)
             if '@' in err:
-                print(f"ERROR, {name}: xtriggers can't be in conditional"
-                      f" expressions: {err}",
-                      file=sys.stderr)
+                print(
+                    f"ERROR, {name}: xtriggers can't be in conditional"
+                    f" expressions: {err}",
+                    file=sys.stderr,
+                )
             else:
-                print('ERROR, %s: bad trigger: %s' % (name, err),
-                      file=sys.stderr)
-            raise WorkflowConfigError("ERROR: bad trigger")
+                print(
+                    'ERROR, %s: bad trigger: %s' % (name, err), file=sys.stderr
+                )
+            raise WorkflowConfigError("ERROR: bad trigger") from None
         except Exception as exc:
             print(str(exc), file=sys.stderr)
             raise WorkflowConfigError(
-                '%s: failed to evaluate triggers.' % name)
+                '%s: failed to evaluate triggers.' % name
+            ) from None
         if cylc.flow.flags.verbosity > 0:
             print('  + %s ok' % itask.identity)
 
diff --git a/cylc/flow/scripts/workflow_state.py b/cylc/flow/scripts/workflow_state.py
index f6350110223..cea5ab906ab 100755
--- a/cylc/flow/scripts/workflow_state.py
+++ b/cylc/flow/scripts/workflow_state.py
@@ -179,7 +179,7 @@ def __init__(
         try:
             tokens = Tokens(self.id_)
         except ValueError as exc:
-            raise InputError(exc)
+            raise InputError(exc) from None
 
         self.workflow_id_raw = tokens.workflow_id
         self.selector = (
@@ -404,7 +404,8 @@ def main(parser: COP, options: 'Values', *ids: str) -> None:
                 options.depr_point = os.environ["CYLC_TASK_CYCLE_POINT"]
             except KeyError:
                 raise InputError(
-                    "--task-point: $CYLC_TASK_CYCLE_POINT is not defined")
+                    "--task-point: $CYLC_TASK_CYCLE_POINT is not defined"
+                ) from None
 
         if options.depr_point is not None:
             id_ += f"//{options.depr_point}"
diff --git a/cylc/flow/task_id.py b/cylc/flow/task_id.py
index dbe003222b6..50075f92b9b 100644
--- a/cylc/flow/task_id.py
+++ b/cylc/flow/task_id.py
@@ -92,7 +92,8 @@ def get_standardised_point_string(cls, point_string):
         except PointParsingError as exc:
             # (This is only needed to raise a clearer error message).
             raise ValueError(
-                "Invalid cycle point: %s (%s)" % (point_string, exc))
+                "Invalid cycle point: %s (%s)" % (point_string, exc)
+            ) from None
         return point_string
 
     @classmethod
diff --git a/cylc/flow/task_pool.py b/cylc/flow/task_pool.py
index f1a6942f0ab..e7922e94d2f 100644
--- a/cylc/flow/task_pool.py
+++ b/cylc/flow/task_pool.py
@@ -873,8 +873,8 @@ def get_tasks(self) -> List[TaskProxy]:
         if self.active_tasks_changed:
             self.active_tasks_changed = False
             self._active_tasks_list = []
-            for _, itask_id_map in self.active_tasks.items():
-                for __, itask in itask_id_map.items():
+            for itask_id_map in self.active_tasks.values():
+                for itask in itask_id_map.values():
                     self._active_tasks_list.append(itask)
         return self._active_tasks_list
 
diff --git a/cylc/flow/time_parser.py b/cylc/flow/time_parser.py
index 932a62677aa..c3d242d6883 100644
--- a/cylc/flow/time_parser.py
+++ b/cylc/flow/time_parser.py
@@ -255,7 +255,7 @@ def parse_recurrence(
             except CylcMissingContextPointError:
                 raise CylcMissingFinalCyclePointError(
                     "This workflow requires a final cycle point."
-                )
+                ) from None
 
             exclusion_points = []
             # Convert the exclusion strings to ISO8601 points
diff --git a/cylc/flow/tui/data.py b/cylc/flow/tui/data.py
index 90344e2462b..948d1906fe9 100644
--- a/cylc/flow/tui/data.py
+++ b/cylc/flow/tui/data.py
@@ -408,11 +408,11 @@ def online_mutate(mutation, selection):
     except WorkflowStopped:
         raise Exception(
             f'Cannot peform command {mutation} on a stopped workflow'
-        )
+        ) from None
     except (ClientError, ClientTimeout) as exc:
         raise Exception(
             f'Error connecting to workflow: {exc}'
-        )
+        ) from None
 
     request_string = generate_mutation(mutation, variables)
     client(
diff --git a/cylc/flow/util.py b/cylc/flow/util.py
index 74a11d0505a..5b7d5df06f8 100644
--- a/cylc/flow/util.py
+++ b/cylc/flow/util.py
@@ -281,7 +281,7 @@ def _eval(expr, **variables):
                 error_class,
                 f'{exc.msg}: {exc.text}',
                 {'expr': expr}
-            )
+            ) from None
 
         # check against whitelisted types
         try:
@@ -302,7 +302,7 @@ def _eval(expr, **variables):
                     'error_node': error_node,
                     'error_type': error_node.__class__.__name__,
                 },
-            )
+            ) from None
 
         # run the expresion
         # Note: this may raise runtime errors
diff --git a/cylc/flow/workflow_db_mgr.py b/cylc/flow/workflow_db_mgr.py
index 8e57d19292b..c3f3ea9a263 100644
--- a/cylc/flow/workflow_db_mgr.py
+++ b/cylc/flow/workflow_db_mgr.py
@@ -701,7 +701,9 @@ def _get_last_run_version(cls, pri_dao: CylcWorkflowDAO) -> Version:
                 [cls.KEY_CYLC_VERSION]
             ).fetchone()[0]
         except (TypeError, OperationalError):
-            raise ServiceFileError(f"{INCOMPAT_MSG}, or is corrupted.")
+            raise ServiceFileError(
+                f"{INCOMPAT_MSG}, or is corrupted."
+            ) from None
         return parse_version(last_run_ver)
 
     @classmethod
diff --git a/cylc/flow/workflow_files.py b/cylc/flow/workflow_files.py
index 023d481324c..ab390ec10aa 100644
--- a/cylc/flow/workflow_files.py
+++ b/cylc/flow/workflow_files.py
@@ -426,7 +426,7 @@ def _is_process_running(
         raise CylcError(
             f'Attempt to determine whether workflow is running on {host}'
             ' timed out after 10 seconds.'
-        )
+        ) from None
 
     if proc.returncode == 2:
         # the psutil call failed to gather metrics on the process
@@ -505,7 +505,7 @@ def detect_old_contact_file(
         # this can happen if contact file is from an outdated version of Cylc
         raise ServiceFileError(
             f'Found contact file with incomplete data:\n{exc}.'
-        )
+        ) from None
 
     # check if the workflow process is running ...
     # NOTE: can raise CylcError
@@ -631,7 +631,7 @@ def load_contact_file(id_: str, run_dir=None) -> Dict[str, str]:
         with open(path) as f:
             file_content = f.read()
     except IOError:
-        raise ServiceFileError("Couldn't load contact file")
+        raise ServiceFileError("Couldn't load contact file") from None
     data: Dict[str, str] = {}
     for line in file_content.splitlines():
         key, value = [item.strip() for item in line.split("=", 1)]
@@ -914,7 +914,7 @@ def infer_latest_run(
     try:
         id_ = str(path.relative_to(cylc_run_dir))
     except ValueError:
-        raise ValueError(f"{path} is not in the cylc-run directory")
+        raise ValueError(f"{path} is not in the cylc-run directory") from None
 
     if not path.exists():
         raise InputError(
diff --git a/cylc/flow/xtrigger_mgr.py b/cylc/flow/xtrigger_mgr.py
index 5ba90738a01..525a19cf2d3 100644
--- a/cylc/flow/xtrigger_mgr.py
+++ b/cylc/flow/xtrigger_mgr.py
@@ -250,12 +250,12 @@ def _validate(
         try:
             func = get_xtrig_func(fctx.mod_name, fctx.func_name, fdir)
         except (ImportError, AttributeError) as exc:
-            raise XtriggerConfigError(label, sig_str, exc)
+            raise XtriggerConfigError(label, sig_str, exc) from None
         try:
             sig = signature(func)
         except TypeError as exc:
             # not callable
-            raise XtriggerConfigError(label, sig_str, exc)
+            raise XtriggerConfigError(label, sig_str, exc) from None
 
         sig = cls._handle_sequential_kwarg(label, fctx, sig)
 
@@ -269,7 +269,7 @@ def _validate(
                     label, fctx, err
                 )
             else:
-                raise err
+                raise err from None
 
         # Specific xtrigger.validate(), if available.
         # Note arg string templating has not been done at this point.
@@ -293,7 +293,7 @@ def _validate(
                     raise XtriggerConfigError(
                         label, sig_str,
                         f"Illegal template in xtrigger: {match}",
-                    )
+                    ) from None
 
         # check for deprecated template variables
         deprecated_variables = template_vars & {
@@ -364,7 +364,7 @@ def _try_xtrig_validate_func(
         except Exception as exc:  # Note: catch all errors
             if not isinstance(exc, WorkflowConfigError):
                 LOG.exception(exc)
-            raise XtriggerConfigError(label, signature_str, exc)
+            raise XtriggerConfigError(label, signature_str, exc) from None
 
     # BACK COMPAT: workflow_state_backcompat
     # from: 8.0.0
@@ -390,7 +390,7 @@ def _try_workflow_state_backcompat(
             bound_args = sig.bind(*fctx.func_args, **fctx.func_kwargs)
         except TypeError:
             # failed signature check for backcompat function
-            raise err  # original signature check error
+            raise
 
         old_sig_str = fctx.get_signature()
         upg_sig_str = "workflow_state({})".format(
diff --git a/cylc/flow/xtriggers/wall_clock.py b/cylc/flow/xtriggers/wall_clock.py
index f000319d1ff..cea8d0b3026 100644
--- a/cylc/flow/xtriggers/wall_clock.py
+++ b/cylc/flow/xtriggers/wall_clock.py
@@ -75,4 +75,6 @@ def validate(args: Dict[str, Any]):
     try:
         interval_parse(args["offset"])
     except (ValueError, AttributeError):
-        raise WorkflowConfigError(f"Invalid offset: {args['offset']}")
+        raise WorkflowConfigError(
+            f"Invalid offset: {args['offset']}"
+        ) from None