Mojo: Remove dangerous default argument in mojom_bindings_generatory.py: ProcessFile().

mojo/public/tools/bindings/mojom_bindings_generator.py

 #!/usr/bin/env python
 # Copyright 2013 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 frontend for the Mojo bindings system."""
 
 
 import argparse
 import imp
@@ skipping 55 matching lines @@
 
 
 def MakeImportStackMessage(imported_filename_stack):
   """Make a (human-readable) message listing a chain of imports. (Returned
   string begins with a newline (if nonempty) and does not end with one.)"""
   return ''.join(
       reversed(["\n  %s was imported by %s" % (a, b) for (a, b) in \
                     zip(imported_filename_stack[1:], imported_filename_stack)]))
 
 
-# Disable Check for dangerous default arguments (they're "private" keyword
-# arguments):
+# Disable check for dangerous default arguments (they're "private" keyword
+# arguments; note that we want |_processed_files| to memoize across invocations
+# of |ProcessFile()|):
 # pylint: disable=W0102
 def ProcessFile(args, remaining_args, generator_modules, filename,
-                _processed_files={}, _imported_filename_stack=[]):
+                _processed_files={}, _imported_filename_stack=None):
   # Memoized results.
   if filename in _processed_files:
     return _processed_files[filename]
 
+  if _imported_filename_stack is None:
+    _imported_filename_stack = []
+
   # Ensure we only visit each file once.
   if filename in _imported_filename_stack:
     print "%s: Error: Circular dependency" % filename + \
         MakeImportStackMessage(_imported_filename_stack + [filename])
     sys.exit(1)
 
   try:
     with open(filename) as f:
       source = f.read()
   except IOError as e:
@@ skipping 68 matching lines @@
     os.makedirs(args.output_dir)
 
   for filename in args.filename:
     ProcessFile(args, remaining_args, generator_modules, filename)
 
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())