creds = result.find('Credentials')
            return dict(
                access_key=creds.get_child_text('AccessKeyId'),
                secret_key=creds.get_child_text('SecretAccessKey'),
                token=creds.get_child_text('SessionToken'),
                expiry_time=parse(creds.get_child_text('Expiration')).isoformat())

        """Similar to ElementTree.Element.find()

        """
        elt = self.element.find('sts:{}'.format(name), _STS_NS)
        return STSElement(self.root_name, elt) if elt is not None else None

    def get_child_text(self, name, strict=True):
        """Extract text of a child element. If strict, and child element is
        not present, raises InvalidXMLError and otherwise returns
        None.

        """
        if strict: