[context] Add path prefix/suffix manipulation.

recipe_engine/step_runner.py

Index: recipe_engine/step_runner.py
diff --git a/recipe_engine/step_runner.py b/recipe_engine/step_runner.py
index f0e02fa698e64b7c01c358a972018fd17456e5cf..421072e31e9884a3867486abd3e55116fb6b6e4e 100644
--- a/recipe_engine/step_runner.py
+++ b/recipe_engine/step_runner.py
@@ -425,6 +425,9 @@ class fakeEnviron(object):
   def __getitem__(self, key):
     return '<%s>' % key
 
+  def keys(self):
+    return self.data.keys()
+
   def __delitem__(self, key):
     self.data[key] = None
 
@@ -695,6 +698,8 @@ def _merge_envs(original, override):
   opposed to overwriting, variables like PATH).
   """
   result = original.copy()
+  subst = (original if isinstance(original, fakeEnviron)
+           else collections.defaultdict(lambda: '', **original))
   if not override:
     return result
   for k, v in override.items():
@@ -702,7 +707,7 @@ def _merge_envs(original, override):
       if k in result:
         del result[k]
     else:
-      result[str(k)] = str(v) % original
+      result[str(k)] = str(v) % subst

[dnj, 2017/06/03 15:37:33]

This fixes a bug where an environment variable that is referenced (e.g.,
"IDONTEXIST") will actually cause a KeyError instead of just being substituted
as an empty string for real runs. I think we're just lucky so far that most
referenced environment variables are "PATH" / "PYTHONPATH", and that both are
always defined.

[iannucci, 2017/06/05 19:23:04]

Yeah, and with this CL I'd like to eventually eliminate the substitution
entirely (so that it's a straight assign/delete, and the pre/suffix path stuff).

[dnj, 2017/06/07 02:59:28]

ACK, but I think for now we still need it for backwards compatibility. Would be
nice to remove, make env either full append or full prescribe. The substitution
is def one of the most obscure and ugly part of this API.

OTOH referencing environment might be necessary, e.g.,

LOGDOG_STREAM_BASE=foo/bar

"Oh I want my step to log to a substream!"

env={'LOGDOG_STREAM_BASE': '%(LOGDOG_STREAM_BASE)s/baz'}

   return result