Replace Python unit test iterables by iterators

Cast iterables into iterators for stream based compression, empty
message, and metadata tests.

src/python/grpcio_tests/tests/unit/_compression_test.py

@@ -125,7 +125,7 @@ class CompressionTest(unittest.TestCase):
     compressed_channel = grpc.insecure_channel('localhost:%d' % self._port,
         options=[('grpc.default_compression_algorithm', 1)])
     multi_callable = compressed_channel.stream_stream(_STREAM_STREAM)
-    call = multi_callable([request] * test_constants.STREAM_LENGTH)
+    call = multi_callable(iter([request] * test_constants.STREAM_LENGTH))
     for response in call:
       self.assertEqual(request, response)
 

src/python/grpcio_tests/tests/unit/_empty_message_test.py

@@ -123,12 +123,12 @@ class EmptyMessageTest(unittest.TestCase):
 
   def testStreamUnary(self):
     response = self._channel.stream_unary(_STREAM_UNARY)(
-        [_REQUEST] * test_constants.STREAM_LENGTH)
+        iter([_REQUEST] * test_constants.STREAM_LENGTH))
     self.assertEqual(_RESPONSE, response)
 
   def testStreamStream(self):
     response_iterator = self._channel.stream_stream(_STREAM_STREAM)(
-        [_REQUEST] * test_constants.STREAM_LENGTH)
+        iter([_REQUEST] * test_constants.STREAM_LENGTH))
     self.assertSequenceEqual(
         [_RESPONSE] * test_constants.STREAM_LENGTH, list(response_iterator))
 

src/python/grpcio_tests/tests/unit/_exit_scenarios.py

@@ -240,7 +240,7 @@ if __name__ == '__main__':
       multi_callable = channel.stream_unary(method)
       future = multi_callable.future(infinite_request_iterator())
       result, call = multi_callable.with_call(
-          [REQUEST] * test_constants.STREAM_LENGTH)
+          iter([REQUEST] * test_constants.STREAM_LENGTH))
     elif (args.scenario == IN_FLIGHT_STREAM_STREAM_CALL or
           args.scenario == IN_FLIGHT_PARTIAL_STREAM_STREAM_CALL):
       multi_callable = channel.stream_stream(method)

src/python/grpcio_tests/tests/unit/_metadata_test.py

@@ -193,7 +193,7 @@ class MetadataTest(unittest.TestCase):
   def testStreamUnary(self):
     multi_callable = self._channel.stream_unary(_STREAM_UNARY)
     unused_response, call = multi_callable.with_call(
-        [_REQUEST] * test_constants.STREAM_LENGTH,
+        iter([_REQUEST] * test_constants.STREAM_LENGTH),
         metadata=_CLIENT_METADATA)
     self.assertTrue(test_common.metadata_transmitted(
         _SERVER_INITIAL_METADATA, call.initial_metadata()))
@@ -202,7 +202,7 @@ class MetadataTest(unittest.TestCase):
 
   def testStreamStream(self):
     multi_callable = self._channel.stream_stream(_STREAM_STREAM)
-    call = multi_callable([_REQUEST] * test_constants.STREAM_LENGTH,
+    call = multi_callable(iter([_REQUEST] * test_constants.STREAM_LENGTH),
                           metadata=_CLIENT_METADATA)
     self.assertTrue(test_common.metadata_transmitted(
         _SERVER_INITIAL_METADATA, call.initial_metadata()))