step: expose get_context() to get the context object

recipe_modules/step/api.py

Index: recipe_modules/step/api.py
diff --git a/recipe_modules/step/api.py b/recipe_modules/step/api.py
index 8530f6ffdafcd8d71d8595732653e2a08a79fa7c..e26db2c4c8aabd382ea06ca0a30ad746ccebe16c 100644
--- a/recipe_modules/step/api.py
+++ b/recipe_modules/step/api.py
@@ -74,9 +74,23 @@ class StepApi(recipe_api.RecipeApiPlain):
 
   @property
   def context(self):
-    """ See recipe_api.py for docs. """
+    """Returns a context manager which can set values applying to all steps
+    within the block.
+
+    Example usage:
+      with api.step.context({'cwd': api.path['checkout']}):
+        api.step(...)
+    """
     return recipe_api.context
 
+  def get_from_context(self, key, default=None):
+    """Returns |key| from context if present, otherwise |default|."""

[iannucci, 2017/02/01 17:47:00]

Returns |key|'s value from context...

[Paweł Hajdan Jr., 2017/02/01 17:50:25]

Done.

+    return recipe_api._STEP_CONTEXT.get(key, default)
+
+  def combine_with_context(self, key, value):
+    """Combines |value| with the value for |key| in current context, if any."""

[iannucci, 2017/02/01 17:47:00]

... and returns the combined value.

It would be really good to include all the relevant keys and their 'combine'
functions in the docstring for the context property above. Probably just env and
cwd?

IIRC, cwd's combine is just replacement, but env does a dict.update?

[Paweł Hajdan Jr., 2017/02/01 17:50:25]

Done.
I'm worried about the docstring getting out of sync with reality.

There are more possible keys than just env and cwd.
Yup.

+    return recipe_api._STEP_CONTEXT.get_with_context(key, value)
+
   @contextlib.contextmanager
   def nest(self, name):
     """Nest is the high-level interface to annotated hierarchical steps.
@@ -171,9 +185,6 @@ class StepApi(recipe_api.RecipeApiPlain):
     kwargs['timeout'] = timeout
     kwargs['ok_ret'] = ok_ret
 
-    # Obtain information from composite step parent.
-    compositor = recipe_api._STEP_CONTEXT
-
     # Calculate our full step name. If a step already has that name, add an
     # index to the end of it.
     #
@@ -181,7 +192,7 @@ class StepApi(recipe_api.RecipeApiPlain):
     # by the user. If this happens, we'll continue appending indexes until we
     # have a unique step name.
     while True:
-      full_name = compositor.get_with_context('name', name)
+      full_name = self.combine_with_context('name', name)

[iannucci, 2017/02/01 17:47:00]

ew, I forgot about name. this is used to implement name prefixing, right?

[Paweł Hajdan Jr., 2017/02/01 17:50:25]

Yes, see api.step.nest .

       if full_name not in self._seen_steps:
         break
 
@@ -191,11 +202,11 @@ class StepApi(recipe_api.RecipeApiPlain):
     self._seen_steps.add(full_name)
 
     if 'cwd' not in kwargs:
-      kwargs['cwd'] = compositor.get('cwd')
-    kwargs['env'] = compositor.get_with_context('env', kwargs.get('env', {}))
-    kwargs['infra_step'] = compositor.get_with_context(
+      kwargs['cwd'] = self.get_from_context('cwd')
+    kwargs['env'] = self.combine_with_context('env', kwargs.get('env', {}))
+    kwargs['infra_step'] = self.combine_with_context(
         'infra_step', bool(infra_step))
-    kwargs['step_nest_level'] = compositor.get_with_context('nest_level', 0)
+    kwargs['step_nest_level'] = self.combine_with_context('nest_level', 0)
     kwargs['name'] = full_name
     kwargs['base_name'] = name