def filter_notebook(expr,in_file,out_file=None):
c = Config()
c.FilterCellsPreprocessor.expression = expr
exporter = NotebookExporter(preprocessors=[FilterCellsPreprocessor], config=c)
body,resources = exporter.from_file(in_file)
if out_file:
write_nb(body,resources,out_file)
print 'written to: %s \n\n' %out_file
else:
return (body,resources)
