'Gets credentials implicitly from the environment.
Checks environment in order of precedence:
- Google App Engine (production and testing)
- Environment variable GOOGLE_APPLICATION_CREDENTIALS pointing to
a file with stored credentials information.
- Stored "well known" file associated with `gcloud` command line tool.
- Google Compute Engine production environment.
Exceptions:
ApplicationDefaultCredentialsError: raised when the credentials fail
to be retrieved.'

@classmethod
def _get_implicit_credentials(cls):

    environ_checkers = [cls._implicit_credentials_from_gae, cls._implicit_credentials_from_files, cls._implicit_credentials_from_gce]
    for checker in environ_checkers:
        credentials = checker()
        if (credentials is not None):
            return credentials
    raise ApplicationDefaultCredentialsError(ADC_HELP_MSG)
