Make Python not support "all or none" for two-phase data pipe read/write.

mojo/public/dart/tools/fetch_dart_packages.py

 #!/usr/bin/env python
 #
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Utility for updating third_party packages used in the Mojo Dart SDK"""
 
 import argparse
 import errno
@@ skipping 37 matching lines @@
     print('Copying %s to %s' % (package_path, destinaton_path))
     shutil.copytree(package_path, destinaton_path)
 
 def cleanup(base_path):
   shutil.rmtree(os.path.join(base_path, 'packages'), True)
   shutil.rmtree(os.path.join(base_path, '.pub'), True)
   os.remove(os.path.join(base_path, '.packages'))
 
 def main():
   parser = argparse.ArgumentParser(description='Update third_party packages')
-  parser.add_argument('--pub-exe',
-                      action='store',
-                      metavar='pub_exe',
-                      help='Path to the pub executable',
-                      default='../../../../third_party/dart-sdk/dart-sdk/bin/pub')
+  parser.add_argument(
+      '--pub-exe',
+      action='store',
+      metavar='pub_exe',
+      help='Path to the pub executable',
+      default='../../../../third_party/dart-sdk/dart-sdk/bin/pub')
   parser.add_argument('--directory',
                       action='store',
                       metavar='directory',
                       help='Directory containing pubspec.yaml',
                       default='.')
   parser.add_argument('--list',
                       help='List mode',
                       action='store_true',
                       default=False)
   args = parser.parse_args()
   args.directory = os.path.abspath(args.directory)
 
   if not check_for_pubspec_yaml(args.directory):
     print('Error could not find pubspec.yaml')
     return 1
 
   if args.list:
     list_packages(args.directory)
   else:
     remove_existing_packages(args.directory)
     change_directory(args.directory)
     print('Running pub get')
     pub_get(args.pub_exe)
     copy_packages(args.directory)
     print('Cleaning up')
     cleanup(args.directory)
 
 if __name__ == '__main__':
   sys.exit(main())