[autoroll] add tests for autoroll subcommand parsing.

recipe_engine/autoroll.py

 # Copyright 2016 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 copy
 import json
 import logging
 import os
 import subprocess
 import sys
 import time
 
 from collections import namedtuple
 
 from . import package
 from . import package_io
 from .autoroll_impl.candidate_algorithm import get_roll_candidates
 
+from . import env
+
+import argparse  # this is vendored
+
 
 LOGGER = logging.getLogger(__name__)
 
 ROOT_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), '..')
 
 
 # This is the path within the recipes-py repo to the per-repo recipes.py script.
 # Ideally we'd read this somehow from each candidate engine repo version, but
 # for now assume it lives in a fixed location within the engine.
 RECIPES_PY_REL_PATH = ('doc', 'recipes.py')
@@ skipping 235 matching lines @@
       package_io.dump_obj(c.package_pb) for c in rejected_candidates]
   return ret
 
 
 def add_subparser(parser):
   helpstr = 'Roll dependencies of a recipe package forward.'
   autoroll_p = parser.add_parser(
     'autoroll', help=helpstr, description=helpstr)
   autoroll_p.add_argument(
     '--output-json',
-    type=os.path.abspath,
+    type=argparse.FileType('w'),
     help='A json file to output information about the roll to.')
   autoroll_p.add_argument(
     '--verbose-json',
     action='store_true',
     help=('Emit even more data in the output-json file. '
           'Requires --output-json.'))
 
   def postprocess_func(parser, args):
     if args.no_fetch:
       parser.error('autoroll with --no-fetch does not make sense.')
@@ skipping 24 matching lines @@
     results = test_rolls(
       repo_cfg_block, config_file, context, package_spec, args.verbose_json)
   finally:
     if not results.get('success'):
       # Restore initial state. Since we could be running simulation tests
       # on other revisions, re-run them now as well.
       write_spec_to_disk(context, repo_cfg_block, config_file, package_pb)
       run_simulation_test(repo_root, package_spec.recipes_path, ['train'])
 
   if args.output_json:
-    with open(args.output_json, 'w') as f:
-      json.dump(results, f, sort_keys=True, indent=2)
+    with args.output_json:
+      json.dump(results, args.output_json, sort_keys=True, indent=2)
 
   return 0