'Search for credentials in explicit environment variables.'

def load(self):

    if (self._mapping['access_key'] in self.environ):
        logger.info('Found    credentials    in    environment    variables.')
        fetcher = self._create_credentials_fetcher()
        credentials = fetcher(require_expiry=False)
        expiry_time = credentials['expiry_time']
        if (expiry_time is not None):
            expiry_time = parse(expiry_time)
            return RefreshableCredentials(credentials['access_key'], credentials['secret_key'], credentials['token'], expiry_time, refresh_using=fetcher, method=self.METHOD)
        return Credentials(credentials['access_key'], credentials['secret_key'], credentials['token'], method=self.METHOD)
    else:
        return None
