I would like to use Python to retrieve metadata stored in PDF files. I am trying to use Python xmptools, but find that I cannot extract all the metadata. For example, this paper is available in PDF format. I have the following script that tries to extract the metadata

from xmptools import XMPMetadata, DC
xmp = XMPMetadata.fromFile("Leonard_2015_Comment_on_‘Dimensionless_units_in_the_SI’.pdf")[0]
print( xmp.getContainerItems(DC.publisher) )

This works fine. The result is [rdflib.term.Literal('IOP Publishing')]. However, if I change the last line to

print( xmp.getContainerItems(DC.identifier) )

then I get None as a result.

I think this may be due to the XML inside the PDF file. The data concerned with these two queries are

        <dc:publisher>
            <rdf:Bag>
               <rdf:li>IOP Publishing</rdf:li>
            </rdf:Bag>
         </dc:publisher>
     <dc:identifier>doi:10.1088/0026-1394/52/4/613</dc:identifier>

In the case of publisher, the information is wrapped in RDF tags, but that is not the case for identifier.

Is there a way for xmptools to read simple entries where RDF tags have not been used?