Make output placeholders like json.output index-able by name.

recipe_modules/raw_io/example.py

Index: recipe_modules/raw_io/example.py
diff --git a/recipe_modules/raw_io/example.py b/recipe_modules/raw_io/example.py
index 3bcbc44498a110f5e986981ab462c871fccdd222..ba8bb6d83ada6bdd4a6b94dd75250f9a8e599f94 100644
--- a/recipe_modules/raw_io/example.py
+++ b/recipe_modules/raw_io/example.py
@@ -4,6 +4,8 @@
 
 DEPS = [
   'path',
+  'properties',
+  'python',
   'raw_io',
   'step',
 ]
@@ -56,11 +58,37 @@ def RunSteps(api):
   api.step('leak dir', ['ls', api.raw_io.output_dir(
       leak_to=api.path['slave_build'].join('out'))])
 
+  # Example of overriding default mocked output for a single named output.
+  step_result = api.python.inline(
+      'override_default_mock',
+      """
+      import sys
+      with open(sys.argv[1], 'w') as f:
+        f.write(%r)
+      """ % api.properties.get('some_prop', 'good_value'),
+      args=[api.raw_io.output(name='test')],
+      step_test_data=(
+          lambda: api.raw_io.test_api.output('second_bad_value', name='test')))
+  assert step_result.raw_io.outputs['test'] == 'good_value'
+  assert step_result.raw_io.output == 'good_value'
+
 
 def GenTests(api):
+  # This test shows that you can override a specific placeholder, even with
+  # default `step_test_data`. However, since this recipe is ACTUALLY run in
+  # the presubmit, we need to do a trick with properties:
+  #   When run for real, "some_prop" will be "good_value" and pass.
+  #   When run for simulation, we override this property to provide a bad value,
+  #     AND the default step_test_data in RunSteps above ALSO provides another
+  #     bad value, the simulation passes ONLY because of the
+  #     'override_default_mock' below.
   yield (api.test('basic') +
+      api.properties(some_prop='bad_value') +
       api.step_data('echo',
           stdout=api.raw_io.output('Hello World\n'),
           stderr=api.raw_io.output('')) +
       api.step_data('cat',
-          stdout=api.raw_io.output('hello')))
+          stdout=api.raw_io.output('hello')) +
+      api.step_data('override_default_mock',
+          api.raw_io.output('good_value', name='test'))
+  )