'Parses the dos.yaml file.
Args:
basepath: the directory of the application.
appyaml: The app.yaml, if present.
Returns:
A DosInfoExternal object or None if the file does not exist.'

def _ParseDosYaml(self, basepath, appyaml=None):

    dos_yaml = self._ParseYamlFile(basepath, 'dos', dosinfo.LoadSingleDos)
    if (not dos_yaml):
        return None
    self._SetApplication(dos_yaml, 'dos', appyaml)
    return dos_yaml
