Make it easier to parse the output assertion

grit/tool/build.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 '''The 'grit build' tool along with integration for this tool with the
 SCons build system.
 '''
 
 import filecmp
@@ skipping 345 matching lines @@
     Returns true if the asserted files are present. If they are not, returns
     False and prints the failure.
     '''
     # Compare the absolute path names, sorted.
     asserted = sorted([os.path.abspath(i) for i in assert_output_files])
     actual = sorted([
         os.path.abspath(os.path.join(self.output_directory, i.GetFilename()))
         for i in self.res.GetOutputFiles()])
 
     if asserted != actual:
-      print '''Asserted file list does not match.
+      missing = list(set(actual) - set(asserted))
+      extra = list(set(asserted) - set(actual))
+      error = '''Asserted file list does not match.
 
-Expected output files: %s
-
-Actual output files: %s
-''' % (asserted, actual)
+Expected output files:
+%s
+Actual output files:
+%s
+Missing output files:
+%s
+Extra output files:
+%s
+'''
+      print error % ('\n'.join(asserted), '\n'.join(actual), '\n'.join(missing),
+          '\n'.join(extra))
       return False
     return True
 
 
   def GenerateDepfile(self, depfile, depdir):
     '''Generate a depfile that contains the imlicit dependencies of the input
     grd. The depfile will be in the same format as a makefile, and will contain
     references to files relative to |depdir|. It will be put in |depfile|.
 
     For example, supposing we have three files in a directory src/
@@ skipping 34 matching lines @@
     self.MakeDirectoriesTo(depfile)
     outfile = self.fo_create(depfile, 'wb')
     outfile.writelines(depfile_contents)
 
   @staticmethod
   def MakeDirectoriesTo(file):
     '''Creates directories necessary to contain |file|.'''
     dir = os.path.split(file)[0]
     if not os.path.exists(dir):
       os.makedirs(dir)