Should open file with 'w' to write a file.

tools/determinism/compare_build_artifacts.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 """Compare the artifacts from two builds."""
 
 import difflib
 import json
 import optparse
@@ skipping 653 matching lines @@
   all_diffs = expected_diffs + unexpected_diffs
   diffs_to_investigate = sorted(set(all_diffs).difference(missing_files))
   compare_deps(first_dir, second_dir, diffs_to_investigate)
 
   if json_output:
     try:
       out = {
           'expected_diffs': expected_diffs,
           'unexpected_diffs': unexpected_diffs,
       }
-      with open(json_output) as f:
+      with open(json_output, 'w') as f:
         json.dump(out, f)
     except Exception as e:
       print('failed to write json output: %s' % e)
 
   return int(bool(unexpected_diffs))
 
 
 def main():
   parser = optparse.OptionParser(usage='%prog [options]')
   parser.add_option(
@@ skipping 19 matching lines @@
 
   return compare_build_artifacts(os.path.abspath(options.first_build_dir),
                                  os.path.abspath(options.second_build_dir),
                                  options.target_platform,
                                  options.json_output,
                                  options.recursive)
 
 
 if __name__ == '__main__':
   sys.exit(main())