'Search for credentials in explicit environment variables.'

def load(self):

    if (self._mapping[ACCESS_KEY_ID] in self.environ):
        (access_key_id, private_key) = self._extract_creds_from_mapping(self.environ, self._mapping[ACCESS_KEY_ID], self._mapping[PRIVATE_KEY])
        LOG.info('Found    credentials    in    environment    variables.')
        if (not os.path.isfile(private_key)):
            LOG.debug(('Private    key    at    %s    does    not    exist!' % private_key))
            raise NoCredentialsError()
        pem = open(private_key).read()
        return Credentials(access_key_id, pem, method=self.METHOD)
    else:
        return None
