cli: handle malformed input files

Do not show a stacktrace on the cli tool when an invalid file is
specified. Instead, use the new Serializer class to abstract the load
process and handle the unified ValueError. Also handle FileNotFound
because that's a fairly normal user input scenario as well. Set return
code on sys.exit to indicate abnormal exit.

Fixes xlwings#71

Signed-off-by: Cory Todd <cory.todd@canonical.com>

jsondiff/cli.py

@@ -2,6 +2,16 @@
 import jsondiff
 import sys
 
+def load_file(serializer, file_path):
+    with open(file_path, "r") as f:
+        parsed = None
+        try:
+            parsed = serializer.deserialize_file(f)
+        except ValueError:
+            print(f"{file_path} is not a valid {serializer.file_format} file")
+        except FileNotFoundError:
+            print(f"{file_path} does not exist")
+    return parsed
 
 def main():
     parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
@@ -18,35 +28,33 @@ def main():
 
     args = parser.parse_args()
 
-    # pyyaml _can_ load json but is ~20 times slower and has known issues so use
-    # the json from stdlib when json is specified.
-    serializers = {
-        "json": (jsondiff.JsonLoader(), jsondiff.JsonDumper(indent=args.indent)),
-        "yaml": (jsondiff.YamlLoader(), jsondiff.YamlDumper(indent=args.indent)),
-    }
-    loader, dumper = serializers[args.format]
-
-    with open(args.first, "r") as f:
-        with open(args.second, "r") as g:
-            jf = loader(f)
-            jg = loader(g)
-            if args.patch:
-                x = jsondiff.patch(
-                    jf,
-                    jg,
-                    marshal=True,
-                    syntax=args.syntax
-                )
-            else:
-                x = jsondiff.diff(
-                    jf,
-                    jg,
-                    marshal=True,
-                    syntax=args.syntax
-                )
-
-            dumper(x, sys.stdout)
+    serializer = jsondiff.Serializer(args.format, args.indent)
 
+    parsed_first = load_file(serializer, args.first)
+    parsed_second = load_file(serializer, args.second)
+
+    if not (parsed_first and parsed_second):
+        return 1
+
+    if args.patch:
+        x = jsondiff.patch(
+            parsed_first,
+            parsed_second,
+            marshal=True,
+            syntax=args.syntax
+        )
+    else:
+        x = jsondiff.diff(
+            parsed_first,
+            parsed_second,
+            marshal=True,
+            syntax=args.syntax
+        )
+
+    serializer.serialize_data(x, sys.stdout)
+
+    return 0
 
 if __name__ == '__main__':
-    main()
+    ret = main()
+    sys.exit(ret)