Remove duplicate log message in output when using pre-allocation (#981)

* Fixes #963 where source stages are printed twice in log output
* Removes unused post-build method in `FileSourceStage` that was no longer being used (unrelated but I noticed it).
* Misc pylint/flake8 fixes

Authors:
  - David Gardner (https://github.com/dagardner-nv)

Approvers:
  - Michael Demoret (https://github.com/mdemoret-nv)

URL: #981

morpheus/pipeline/preallocator_mixin.py

@@ -89,7 +89,6 @@ def _preallocate_multi(self, msg: MultiMessage) -> MultiMessage:
     def _post_build_single(self, builder: mrc.Builder, out_pair: StreamPair) -> StreamPair:
         (out_stream, out_type) = out_pair
         pretty_type = pretty_print_type_name(out_type)
-        logger.info("Added source: {}\n  └─> {}".format(str(self), pretty_type))
 
         if len(self._needed_columns) > 0:
             node_name = f"{self.unique_name}-preallocate"
@@ -111,8 +110,8 @@ def _post_build_single(self, builder: mrc.Builder, out_pair: StreamPair) -> Stre
             elif issubclass(out_type, (cudf.DataFrame, pd.DataFrame)):
                 stream = builder.make_node(node_name, ops.map(self._preallocate_df))
             else:
-                msg = ("Additional columns were requested to be inserted into the Dataframe, but the output type {}"
-                       " isn't a supported type".format(pretty_type))
+                msg = ("Additional columns were requested to be inserted into the Dataframe, but the output type "
+                       f"{pretty_type} isn't a supported type")
                 raise RuntimeError(msg)
 
             builder.make_edge(out_stream, stream)

morpheus/stages/input/file_source_stage.py

@@ -11,14 +11,13 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+"""File source stage."""
 
 import logging
 import pathlib
 import typing
 
 import mrc
-import typing_utils
-from mrc.core import operators as ops
 
 from morpheus.cli import register_stage
 from morpheus.common import FileTypes
@@ -86,14 +85,16 @@ def __init__(self,
 
     @property
     def name(self) -> str:
+        """Return the name of the stage"""
         return "from-file"
 
     @property
     def input_count(self) -> int:
         """Return None for no max intput count"""
         return self._input_count
 
-    def supports_cpp_node(self):
+    def supports_cpp_node(self) -> bool:
+        """Indicates whether or not this stage supports a C++ node"""
         return True
 
     def _build_source(self, builder: mrc.Builder) -> StreamPair:
@@ -108,21 +109,7 @@ def _build_source(self, builder: mrc.Builder) -> StreamPair:
 
         return out_stream, out_type
 
-    def _post_build_single(self, builder: mrc.Builder, out_pair: StreamPair) -> StreamPair:
-
-        out_stream = out_pair[0]
-        out_type = out_pair[1]
-
-        # Convert our list of dataframes into the desired type. Flatten if necessary
-        if (typing_utils.issubtype(out_type, typing.List)):
-            flattened = builder.make_node(self.unique_name + "-post", ops.flatten())
-            builder.make_edge(out_stream, flattened)
-            out_stream = flattened
-            out_type = typing.get_args(out_type)[0]
-
-        return super()._post_build_single(builder, (out_stream, out_type))
-
-    def _generate_frames(self):
+    def _generate_frames(self) -> typing.Iterable[MessageMeta]:
 
         df = read_file_to_df(
             self._filename,