code: def get_default_config_filename():
      
      global _CONFIG_FN
      if _CONFIG_FN is not None:
          return _CONFIG_FN
      with _CONFIG_FN_LOCK:
          if _CONFIG_FN is not None:
              return _CONFIG_FN
          if 'PEYOTL_CONFIG_FILE' in os.environ:
              cfn = os.path.abspath(os.environ['PEYOTL_CONFIG_FILE'])
          else:
              cfn = os.path.expanduser("~/.peyotl/config")
          if not os.path.isfile(cfn):
              # noinspection PyProtectedMember
              if 'PEYOTL_CONFIG_FILE' in os.environ:
                  from peyotl.utility.get_logger import warn_from_util_logger
                  msg = 'Filepath "{}" specified via PEYOTL_CONFIG_FILE={} was not found'.format(cfn, os.environ[
                      'PEYOTL_CONFIG_FILE'])
                  warn_from_util_logger(msg)
              from pkg_resources import Requirement, resource_filename
              pr = Requirement.parse('peyotl')
              cfn = resource_filename(pr, 'peyotl/default.conf')
          if not os.path.isfile(cfn):
              raise RuntimeError('The peyotl configuration file cascade failed looking for "{}"'.format(cfn))
          _CONFIG_FN = os.path.abspath(cfn)
      return _CONFIG_FN
docs: Returns the configuration filepath.
  
      If PEYOTL_CONFIG_FILE is in the env that is the preferred choice; otherwise ~/.peyotl/config is preferred.
      If the preferred file does not exist, then the packaged peyotl/default.conf from the installation of peyotl is
      used.
      A RuntimeError is raised if that fails.