Use argparse.FileType in predict.

allennlp/commands/predict.py

@@ -11,9 +11,10 @@ def add_subparser(parser: argparse._SubParsersAction) -> argparse.ArgumentParser
     subparser = parser.add_parser(
             'predict', description=description, help='Use a trained model to make predictions.')
     subparser.add_argument('archive_file', type=str, help='the archived model to make predictions with')
-    subparser.add_argument('input_file', metavar='input-file', type=str, help='path to input file')
-    subparser.add_argument('--output-file', type=str, help='path to output file')
-    subparser.add_argument('--print', action='store_true', help='print results to string')
+    subparser.add_argument('input_file', metavar='input-file', type=argparse.FileType('r'),
+                           help='path to input file')
+    subparser.add_argument('--output-file', type=argparse.FileType('w'), help='path to output file')
+    subparser.add_argument('--print', action='store_true', help='print results to stdout')
 
     subparser.set_defaults(func=predict)
 
@@ -37,13 +38,12 @@ def run(predictor: Predictor, input_file: IO, output_file: Optional[IO], print_t
 
 def predict(args: argparse.Namespace) -> None:
     predictor = get_predictor(args)
+    output_file = None
 
     # ExitStack allows us to conditionally context-manage `output_file`, which may or may not exist
     with ExitStack() as stack:
-        input_file = stack.enter_context(open(args.input_file, 'r'))  # type: ignore
+        input_file = stack.enter_context(args.input_file)  # type: ignore
         if args.output_file:
-            output_file = stack.enter_context(open(args.output_file, 'w'))  # type: ignore
-        else:
-            output_file = None
+            output_file = stack.enter_context(args.output_file)  # type: ignore
 
         run(predictor, input_file, output_file, args.print)

tests/commands/predict_test.py

@@ -18,16 +18,14 @@ def test_add_predict_subparser(self):
 
         raw_args = ["predict",          # command
                     "/path/to/archive", # archive
-                    "/path/to/data",    # input_file
-                    "--output-file", "outfile",
+                    "/dev/null",    # input_file
+                    "--output-file", "/dev/null",
                     "--print"]
 
         args = parser.parse_args(raw_args)
 
         assert args.func == predict
         assert args.archive_file == "/path/to/archive"
-        assert args.input_file == "/path/to/data"
-        assert args.output_file == "outfile"
         assert args.print
 
     def test_works_with_known_model(self):