[Python] Deprecate --enable_streaming_engine flag since it is always enabled.

sdks/python/apache_beam/options/pipeline_options.py

@@ -1021,9 +1021,10 @@ def _add_argparse_args(cls, parser):
         'updating-a-pipeline')
     parser.add_argument(
         '--enable_streaming_engine',
-        default=False,
+        default=True,
         action='store_true',
-        help='Enable Windmill Service for this Dataflow job. ')
+        help='Deprecated. All Python streaming pipelines on Dataflow'
+        'use Streaming Engine.')
     parser.add_argument(
         '--dataflow_kms_key',
         default=None,

sdks/python/apache_beam/runners/dataflow/dataflow_runner.py

@@ -633,23 +633,8 @@ def _check_and_add_missing_streaming_options(options):
   # Runner v2 only supports using streaming engine (aka windmill service)
   if options.view_as(StandardOptions).streaming:
     debug_options = options.view_as(DebugOptions)
-    google_cloud_options = options.view_as(GoogleCloudOptions)
-    if (not google_cloud_options.enable_streaming_engine and
-        (debug_options.lookup_experiment("enable_windmill_service") or
-         debug_options.lookup_experiment("enable_streaming_engine"))):
-      raise ValueError(
-          """Streaming engine both disabled and enabled:
-          --enable_streaming_engine flag is not set, but
-          enable_windmill_service and/or enable_streaming_engine experiments
-          are present. It is recommended you only set the
-          --enable_streaming_engine flag.""")
-
-    # Ensure that if we detected a streaming pipeline that streaming specific
-    # options and experiments.
-    options.view_as(StandardOptions).streaming = True
-    google_cloud_options.enable_streaming_engine = True
-    debug_options.add_experiment("enable_streaming_engine")
-    debug_options.add_experiment("enable_windmill_service")
+    debug_options.add_experiment('enable_streaming_engine')
+    debug_options.add_experiment('enable_windmill_service')
 
 
 def _is_runner_v2_disabled(options):

sdks/python/apache_beam/runners/dataflow/dataflow_runner_test.py

@@ -421,10 +421,9 @@ def test_min_cpu_platform_flag_is_propagated_to_experiments(self):
         'min_cpu_platform=Intel Haswell',
         remote_runner.job.options.view_as(DebugOptions).experiments)
 
-  def test_streaming_engine_flag_adds_windmill_experiments(self):
+  def test_streaming_adds_windmill_experiments(self):
     remote_runner = DataflowRunner()
     self.default_properties.append('--streaming')
-    self.default_properties.append('--enable_streaming_engine')
     self.default_properties.append('--experiment=some_other_experiment')
 
     with Pipeline(remote_runner, PipelineOptions(self.default_properties)) as p: