Avoid clobbering generated files if the content hasn't changed.

chrome/browser/resources/chromeos/chromevox/tools/generate_deps.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.
 
 ''' Generates a deps.js file based on an input list of javascript files using
 Closure style provide/require calls.
 '''
 
 import optparse
 import os
 import sys
 
 from jsbundler import PathRewriter
 
 _SCRIPT_DIR = os.path.realpath(os.path.dirname(__file__))
 _CHROME_SOURCE = os.path.realpath(
     os.path.join(_SCRIPT_DIR, *[os.path.pardir] * 6))
 sys.path.insert(0, os.path.join(
     _CHROME_SOURCE, ('chrome/third_party/chromevox/third_party/' +
                      'closure-library/closure/bin/build')))
 import source
 
 
+def _HasSameContent(filename, content):
+  '''Returns true if the given file is readable and has the given content.'''
+  try:
+    with open(filename) as file:
+      return file.read() == content
+  except:
+    # Ignore all errors and fall back on a safe bet.
+    return False
+
+
 def main():
   parser = optparse.OptionParser(description=__doc__)
   parser.add_option('-w', '--rewrite_prefix', action='append', default=[],
                     dest='prefix_map', metavar='SPEC',
                     help=('Two path prefixes, separated by colons ' +
                           'specifying that a file whose (relative) path ' +
                           'name starts with the first prefix should have ' +
                           'that prefix replaced by the second prefix to ' +
                           'form a path relative to the output directory. ' +
                           'The resulting path is used in the deps mapping ' +
                           'file path to a list of provided and required ' +
                           'namespaces.'))
   parser.add_option('-o', '--output_file', action='store', default=[],
                     metavar='SPEC',
                     help=('Where to output the generated deps file.'))
   options, args = parser.parse_args()
 
   path_rewriter = PathRewriter(options.prefix_map)
 
+  content = ''
+  for path in args:
+    js_deps = source.Source(source.GetFileContents(path))
+    path = path_rewriter.RewritePath(path)
+    content += 'goog.addDependency(\'%s\', %s, %s);\n' % (
+        path, sorted(js_deps.provides), sorted(js_deps.requires))
+  if _HasSameContent(options.output_file, content):
+    return
   # Write the generated deps file.
   with open(options.output_file, 'w') as output:
-    for path in args:
-      js_deps = source.Source(source.GetFileContents(path))
-      path = path_rewriter.RewritePath(path)
-      line = 'goog.addDependency(\'%s\', %s, %s);\n' % (
-          path, sorted(js_deps.provides), sorted(js_deps.requires))
-      output.write(line)
-
+    output.write(content)
 
 if __name__ == '__main__':
   main()