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

recipe_engine/util.py

Index: recipe_engine/util.py
diff --git a/recipe_engine/util.py b/recipe_engine/util.py
index 75cd5f9441f5b9eed881ef37bdf4ccedabcaa6e1..49526329d470c1ed4dad722fa26f5b55abbb680f 100644
--- a/recipe_engine/util.py
+++ b/recipe_engine/util.py
@@ -34,9 +34,12 @@ class ModuleInjectionSite(object):
 
 
 class Placeholder(object):
+  DEFAULT_ID = object()
+
   """Base class for json placeholders. Do not use directly."""
-  def __init__(self):
+  def __init__(self, id=None):

[iannucci, 2016/03/10 03:17:42]

I think you can do

  id=DEFAULT_ID

or maybe
  
  id=Placeholder.DEFAULT_ID

?

[stgao, 2016/03/10 20:34:23]

No. We can't do that, because subclasses will call this with id=None and
id=DEFAULT_ID here won't work in that case.

     self.name_pieces = None
+    self.id = id or Placeholder.DEFAULT_ID
 
   @property
   def backing_file(self):  # pragma: no cover
@@ -68,6 +71,12 @@ class Placeholder(object):
     assert self.name_pieces
     return "%s.%s" % self.name_pieces
 
+  @property
+  def id_name(self):
+    if self.id == self.DEFAULT_ID:

[iannucci, 2016/03/10 03:17:42]

use the `is` keyword for comparison (compares pointer)

[stgao, 2016/03/10 20:34:23]

Done.

+      return self.name
+    else:
+      return "%s.%s" % (self.id, self.name)
 
 def static_wraps(func):
   wrapped_fn = func