Unrecognized attribute in target gives error that doesn't squelch stacktrace

[ericzundel]

I ran 'pants' the other day and got an exception that was difficult to read.

Exception message: __init__() got an unexpected keyword argument 'resources'

There are a few of problems I noticed:

• The stack trace was not suppressed as I expected it to be (I didn't have --print-exception-stacktrace enable)
• The name of the BUILD file with the error is buried in the middle of the exception
• The overall impression of the error makes this look like a bug in pants itself, not invalid BUILD file semantics.

The actual output looked something like this.

~/Src/Pants master * devpants goal compile src/python/pants
*** Running pants in dev mode from /Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py ***
Traceback (most recent call last):
  File "/Users/zundel/Src/pants/src/python/pants/base/build_graph.py", line 341, in target_addressable_to_target
    **addressable.kwargs)
  File "/Users/zundel/Src/pants/src/python/pants/backend/core/targets/dependencies.py", line 33, in __init__
    super(Dependencies, self).__init__(payload=EmptyPayload(), *args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'resources'
ERROR:pants.base.build_graph:Failed to instantiate Target with type <class 'pants.backend.core.targets.dependencies.Dependencies'> with name "pants" at address BuildFileAddress(/Users/zundel/Src/pants/src/python/pants/BUILD, pants)
Traceback (most recent call last):
  File "/Users/zundel/Src/pants/src/python/pants/base/build_graph.py", line 341, in target_addressable_to_target
    **addressable.kwargs)
  File "/Users/zundel/Src/pants/src/python/pants/backend/core/targets/dependencies.py", line 33, in __init__
    super(Dependencies, self).__init__(payload=EmptyPayload(), *args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'resources'

Exception message: __init__() got an unexpected keyword argument 'resources'

Here is a change that reproduces the problem:

 git diff
diff --git a/src/python/pants/BUILD b/src/python/pants/BUILD
index 325ae1f..e9eaef6 100644
--- a/src/python/pants/BUILD
+++ b/src/python/pants/BUILD
@@ -3,6 +3,7 @@

 target(
   name = 'pants',
+  resources = [],
   dependencies = [
     'src/python/pants/bin:pants',
   ],

I saw some code in jar_publish that surrounds Target.get_all_targets() with a catch of (TypeError, ImportError, SyntaxError) in jar_publish.py but I think this code is out of date (there is not Target.get_all_targets() method!) , and catching that exception might already obscure a real bug in pants, I wouldn't want to replicate it.

Maybe we could just update the exception handler in build_graph.py to handle TypeError() with extra care?

Another thought I had is that we could add **kwargs to Target.__init__() and then explicitly look for extra keywords and formulate a more helpful error message. To keep from obscuring a bug in Pants source code, maybe we could inspect the Target instance and to see if this target has attributes that will tell us it came from a BUILD file, then raise a different exception. That's not very pretty (but neither is the error message we get today.) Unfortunately, I didn't see anything obvious to tell us at that level.

(Pdb) p self.__class__
<class 'pants.backend.core.targets.dependencies.Dependencies'>
(Pdb) p dir(self)
['LANG_DISCRIMINATORS', '__class__', '__delattr__', '__dict__', '__doc__', '__eq__', '__format__', '__getattribute__', '__hash__', '__init__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_propagate_exclusives', '_propagate_exclusives_work', 'add_labels', 'add_to_exclusives', 'assert_list', 'closure', 'combine_ids', 'compute_invalidation_hash', 'concrete_derived_from', 'dependencies', 'dependents', 'derived_from', 'get_addressable_type', 'get_all_exclusives', 'has_label', 'has_resources', 'has_sources', 'id', 'identifier', 'identify', 'inject_dependency', 'invalidation_hash', 'is_android', 'is_apt', 'is_codegen', 'is_exported', 'is_jar', 'is_jar_library', 'is_java', 'is_java_agent', 'is_jvm', 'is_jvm_app', 'is_original', 'is_python', 'is_scala', 'is_scalac_plugin', 'is_synthetic', 'is_test', 'is_thrift', 'lang_discriminator', 'mark_extra_invalidation_hash_dirty', 'mark_extra_transitive_invalidation_hash_dirty', 'mark_invalidation_hash_dirty', 'mark_transitive_invalidation_hash_dirty', 'maybe_readable_combine_ids', 'maybe_readable_identify', 'remove_label', 'sources_relative_to_buildroot', 'sources_relative_to_source_root', 'target_base', 'transitive_invalidation_hash', 'traversable_dependency_specs', 'traversable_specs', 'walk', 'with_description']

[ericzundel]

The error message is improved now and includes the target name with the error

Exception message: Invalid target testprojects/tests/python/pants/build_parsing:test-nested-variable-access-in-function-call: Target received unknown arguments: 
  resources = []