Dump empty json to output when no shard json file is given.

build/android/pylib/results/presentation/test_results_presentation.py

Index: build/android/pylib/results/presentation/test_results_presentation.py
diff --git a/build/android/pylib/results/presentation/test_results_presentation.py b/build/android/pylib/results/presentation/test_results_presentation.py
index d7e2b5a300ae13e292647270d54562990dd91462..ef5ab558b31dd66a1d94b118c4dee0283edfaa6a 100755
--- a/build/android/pylib/results/presentation/test_results_presentation.py
+++ b/build/android/pylib/results/presentation/test_results_presentation.py
@@ -306,6 +306,7 @@ def upload_to_google_bucket(html, test_name, builder_name, build_number,
 
   return '%s/%s/%s' % (server_url, bucket, dest)
 
+
 def main():
   parser = argparse.ArgumentParser()
   parser.add_argument('--json-file', help='Path of json file.')
@@ -347,16 +348,22 @@ def main():
   if ((args.build_properties is None) ==
          (args.build_number is None or args.builder_name is None)):
     raise parser.error('Exactly one of build_perperties or '
-                    '(build_number or builder_names) should be given.')
+                       '(build_number or builder_name) should be given.')
 
   if (args.build_number is None) != (args.builder_name is None):
     raise parser.error('args.build_number and args.builder_name '
-                    'has to be be given together'
-                    'or not given at all.')
+                       'has to be be given together'
+                       'or not given at all.')
 
   if (len(args.positional) == 0) == (args.json_file is None):
-    raise parser.error('Exactly one of args.positional and '
-                    'args.json_file should be given.')
+    if args.json_file:

[jbudorick, 2017/04/27 18:40:37]

Given that you're handling the two cases differently, this might be a bit more
readable if we just split it into two conditions at the top level, e.g.

  if len(args.positional) == 0 and args.json_file is None:
    if args.output_json:
      ...
  elif len(args.positional) != 0 and args.json_file:
    raise parser.error(...)

[BigBossZhiling, 2017/04/27 20:41:49]

Done.

+      raise parser.error('Exactly one of args.positional and '
+                         'args.json_file should be given.')
+    else:
+      if args.output_json:
+        with open(args.output_json, 'w') as f:
+          f.write('{}')

[jbudorick, 2017/04/27 18:40:37]

nit: json.dump({}, f)

I know the result is the same, but I think it's a bit more clear.

[BigBossZhiling, 2017/04/27 20:41:49]

Done.

+      return
 
   if args.build_properties:
     build_properties = json.loads(args.build_properties)