<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_DOI(self):
 """ This method defines how the Article tries to detect the DOI. It attempts to determine the article DOI string by DTD-appropriate inspection of the article metadata. This method should be made as flexible as necessary to properly collect the DOI for any XML publishing specification. Returns ------- doi : str or None The full (publisher/article) DOI string for the article, or None on failure. """

        if self.dtd_name == 'JPTS':
            doi = self.root.xpath("./front/article-meta/article-id[@pub-id-type='doi']")
            if doi:
                return doi[0].text
            log.warning('Unable to locate DOI string for this article')
            return None
        else:
            log.warning('Unable to locate DOI string for this article')
            return None