parameters: (self, option)
docstring: Ensure that 'option' is the name of an existing file.
docstring_summary: Ensure that 'option' is the name of an existing file.
docstring_tokens: Ensure that option is the name of an existing file .
function: def ensure_filename(self, option):
          """Ensure that 'option' is the name of an existing file."""
          self._ensure_tested_string(option, os.path.isfile,
                                     "filename",
                                     "'%s' does not exist or is not a file")
function_tokens: def ensure_filename ( self , option ) : self . _ensure_tested_string ( option , os . path . isfile , "filename" , "'%s' does not exist or is not a file" )