Deprecate --tree flag for tasks list cli command (apache#41965)

This also deprecates the public helper functions that powered it. These
will all be removed in Airflow 3 in favor of `dags show`.

airflow/cli/cli_config.py

@@ -432,7 +432,11 @@ def string_lower_type(val):
 )
 
 # list_tasks
-ARG_TREE = Arg(("-t", "--tree"), help="Tree view", action="store_true")
+ARG_TREE = Arg(
+    ("-t", "--tree"),
+    help="Deprecated - use `dags show` instead. Display tasks in a tree. Note that generating the tree can be slow and the output very large for some DAGs.",
+    action="store_true",
+)
 
 # tasks_run
 # This is a hidden option -- not meant for users to set or know about

airflow/models/dag.py

@@ -2772,6 +2772,12 @@ def pickle(self, session=NEW_SESSION) -> DagPickle:
 
     def tree_view(self) -> None:
         """Print an ASCII tree representation of the DAG."""
+        warnings.warn(
+            "`tree_view` is deprecated and will be removed in Airflow 3.0.",
+            category=RemovedInAirflow3Warning,
+            stacklevel=2,
+        )
+
         for tmp in self._generate_tree_view():
             print(tmp)
 
@@ -2787,6 +2793,12 @@ def get_downstream(task, level=0) -> Generator[str, None, None]:
 
     def get_tree_view(self) -> str:
         """Return an ASCII tree representation of the DAG."""
+        warnings.warn(
+            "`get_tree_view` is deprecated and will be removed in Airflow 3.0.",
+            category=RemovedInAirflow3Warning,
+            stacklevel=2,
+        )
+
         rst = ""
         for tmp in self._generate_tree_view():
             rst += tmp + "\n"

tests/models/test_dag.py

@@ -1653,15 +1653,25 @@ def test_tree_view(self):
             op1_a >> op2 >> op3
 
             with redirect_stdout(StringIO()) as stdout:
-                dag.tree_view()
+                with pytest.warns(
+                    RemovedInAirflow3Warning,
+                    match="`tree_view` is deprecated and will be removed in Airflow 3.0",
+                ):
+                    dag.tree_view()
                 stdout = stdout.getvalue()
 
             stdout_lines = stdout.splitlines()
             assert "t1_a" in stdout_lines[0]
             assert "t2" in stdout_lines[1]
             assert "t3" in stdout_lines[2]
             assert "t1_b" in stdout_lines[3]
-            assert dag.get_tree_view() == (
+
+            with pytest.warns(
+                RemovedInAirflow3Warning,
+                match="`get_tree_view` is deprecated and will be removed in Airflow 3.0",
+            ):
+                get_tree_view = dag.get_tree_view()
+            assert get_tree_view == (
                 "<Task(EmptyOperator): t1_a>\n"
                 "    <Task(EmptyOperator): t2>\n"
                 "        <Task(EmptyOperator): t3>\n"