Fix recipes.py test train when on-disk expectation files are invalid

recipe_engine/test.py

 # Copyright 2017 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 from __future__ import print_function
 
 import argparse
 import bdb
 import cStringIO
 import contextlib
@@ skipping 189 matching lines @@
     print('Uncaught exception. Entering post mortem debugging')
     print('Running \'cont\' or \'step\' will restart the program')
     t = sys.exc_info()[2]
     debugger.interaction(None, t)
 
 
 def run_test(test_description, debug=False, train=False):
   """Runs a test. Returns TestResults object."""
   expected = None
   if os.path.exists(test_description.expectation_path):
-    with open(test_description.expectation_path) as f:
-      # TODO(phajdan.jr): why do we need to re-encode golden data files?
-      expected = re_encode(json.load(f))
+    try:
+      with open(test_description.expectation_path) as f:
+        # TODO(phajdan.jr): why do we need to re-encode golden data files?
+          expected = re_encode(json.load(f))
+    except Exception:
+      if train:
+        # Ignore errors when training; we're going to overwrite the file anyway.
+        expected = None
+      else:
+        raise
 
   break_funcs = [
     _UNIVERSE_VIEW.load_recipe(test_description.recipe_name).run_steps,
   ]
 
   with maybe_debug(break_funcs, debug):
     actual, failed_checks, coverage_data = run_recipe(
         test_description.recipe_name, test_description.test_name,
         test_description.covers,
         enable_coverage=(not debug))
@@ skipping 680 matching lines @@
   Returns:
     Exit code
   """
   global _UNIVERSE_VIEW
   _UNIVERSE_VIEW = universe_view
   global _ENGINE_FLAGS
   _ENGINE_FLAGS = engine_flags
 
   args = parse_args(raw_args)
   return args.func(args)