Add the class name and scope name to the uninitialized subsystem erro…

…r message [IMPROVEMENT]

Add the class name and scope name to the uninitialized subsystem error message

Testing Done:
CI running at https://travis-ci.org/pantsbuild/pants/builds/77148097

Added new unit test

Bugs closed: 2064

Reviewed at https://rbcommons.com/s/twitter/r/2698/

src/python/pants/subsystem/subsystem.py

@@ -38,6 +38,12 @@ class Subsystem(SubsystemClientMixin, Optionable):
   """
   options_scope_category = ScopeInfo.SUBSYSTEM
 
+  class UninitializedSubsystemError(SubsystemError):
+    def __init__(self, class_name, scope):
+      super(Subsystem.UninitializedSubsystemError, self).__init__(
+        'Subsystem "{}" not initialized for scope "{}". '
+        'Is subsystem missing from subsystem_dependencies() in a task? '.format(class_name, scope))
+
   class CycleException(Exception):
     """Thrown when a circular dependency is detected."""
     def __init__(self, cycle):
@@ -128,7 +134,7 @@ def scoped_instance(cls, optionable):
   @classmethod
   def _instance_for_scope(cls, scope):
     if cls._options is None:
-      raise SubsystemError('Subsystem not initialized yet.')
+      raise cls.UninitializedSubsystemError(cls.__name__, scope)
     key = (cls, scope)
     if key not in cls._scoped_instances:
       cls._scoped_instances[key] = cls(scope, cls._options.for_scope(scope))

tests/python/pants_test/subsystem/test_subsystem.py

@@ -24,6 +24,10 @@ class DummyOptionable(Optionable):
   options_scope = 'foo'
 
 
+class UninitializedSubsystem(Subsystem):
+  options_scope = 'uninitialized-scope'
+
+
 class SubsystemTest(unittest.TestCase):
   def setUp(self):
     DummySubsystem._options = DummyOptions()
@@ -114,3 +118,18 @@ def subsystem_dependencies(cls):
     for root in SubsystemA, SubsystemB, SubsystemC:
       with self.assertRaises(Subsystem.CycleException):
         Subsystem.closure((root,))
+
+  def test_uninitialized_global(self):
+
+    with self.assertRaisesRegexp(Subsystem.UninitializedSubsystemError,
+                                 r'UninitializedSubsystem.*uninitialized-scope'):
+      UninitializedSubsystem.global_instance()
+
+  def test_uninitialized_scoped_instance(self):
+    class UninitializedOptional(Optionable):
+      options_scope = 'optional'
+
+    optional = UninitializedOptional()
+    with self.assertRaisesRegexp(Subsystem.UninitializedSubsystemError,
+                                 r'UninitializedSubsystem.*uninitialized-scope'):
+      UninitializedSubsystem.scoped_instance(optional)