def _parse_config_file_impl(filename):
try:<EOL><INDENT>doc = yaml.load(file(filename).read())<EOL>project_id = doc["<STR_LIT>"]["<STR_LIT>"]<EOL>access_token = doc["<STR_LIT>"]["<STR_LIT>"]<EOL>api_domain = doc["<STR_LIT>"]["<STR_LIT>"]<EOL>return project_id, access_token, api_domain<EOL><DEDENT>except:<EOL><INDENT>return None, None, None<EOL><DEDENT>
Format for the file is:

     credentials:
         project_id: ...
         access_token: ...
         api_domain: ...

:param filename: The filename to parse
:return: A tuple with:
            - project_id
            - access_token
            - api_domain