Add assert to make sure the files argument type is valid.

presubmit_support.py

 #!/usr/bin/env python
 # Copyright (c) 2011 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.
 
 """Enables directory-specific presubmit checks to run at upload and/or commit.
 """
 
 __version__ = '1.6.1'
 
@@ skipping 672 matching lines @@
       m = self._TAG_LINE_RE.match(line)
       if m:
         self.tags[m.group('key')] = m.group('value')
       else:
         description_without_tags.append(line)
 
     # Change back to text and remove whitespace at end.
     self._description_without_tags = (
         '\n'.join(description_without_tags).rstrip())
 
+    assert all(
+        (isinstance(f, (list, tuple)) and len(f) == 2) for f in files), files
+
     self._affected_files = [
         self._AFFECTED_FILES(info[1], info[0].strip(), self._local_root)
         for info in files
     ]
 
   def Name(self):
     """Returns the change name."""
     return self._name
 
   def DescriptionText(self):
@@ skipping 549 matching lines @@
   except PresubmitFailure, e:
     print >> sys.stderr, e
     print >> sys.stderr, 'Maybe your depot_tools is out of date?'
     print >> sys.stderr, 'If all fails, contact maruel@'
     return 2
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   sys.exit(Main(None))