Split Placeholder into InputPlaceholder and OutputPlaceholder.

recipe_engine/util.py

Index: recipe_engine/util.py
diff --git a/recipe_engine/util.py b/recipe_engine/util.py
index 75cd5f9441f5b9eed881ef37bdf4ccedabcaa6e1..1bc3f4e54cfd6935288c60c188ce1e99d58e7575 100644
--- a/recipe_engine/util.py
+++ b/recipe_engine/util.py
@@ -34,7 +34,7 @@ class ModuleInjectionSite(object):
 
 
 class Placeholder(object):
-  """Base class for json placeholders. Do not use directly."""
+  """Base class for command line argument placeholders. Do not use directly."""
   def __init__(self):
     self.name_pieces = None
 
@@ -57,8 +57,6 @@ class Placeholder(object):
       presentation (StepPresentation) - for the current step.
       test (PlaceholderTestData) - test data for this placeholder.
 
-    Returns value to add to step result.
-
     May optionally modify presentation as a side-effect.
     """
     pass
@@ -69,6 +67,22 @@ class Placeholder(object):
     return "%s.%s" % self.name_pieces
 
 
+class InputPlaceholder(Placeholder):
+  """Base class for json/raw_io input placeholders. Do not use directly."""
+  def result(self, presentation, test):
+    """Returned value will be discarded."""
+    pass
+  result.__doc__ = Placeholder.result.__doc__
+
+
+class OutputPlaceholder(Placeholder):
+  """Base class for json/raw_io output placeholders. Do not use directly."""
+  def result(self, presentation, test):
+    """Returned value will be added to the step result."""
+    pass
+  result.__doc__ = Placeholder.result.__doc__
+
+
 def static_wraps(func):
   wrapped_fn = func
   if isinstance(func, staticmethod):