rdflib SPARQL queries (involving subclasses) not behaving as expected - python-2.7

Question

I have just started to self-learn RDF and the Python rdflib library. But I have hit an issue with the following code. I expected the queries to return mark and nat as Persons and only natalie as a Professor. I can't see where I've gone wrong. (BTW, I know Professor should be a title rather than a kind of Person, but I'm just tinkering ATM.) Any help appreciated. Thanks.
rdflib 4, Python 2.7
>>> from rdflib import Graph, BNode, URIRef, Literal
INFO:rdflib:RDFLib Version: 4.2.
>>> from rdflib.namespace import RDF, RDFS, FOAF
>>> G = Graph()
>>> mark = BNode()
>>> nat = BNode()
>>> G.add((mark, RDF.type, FOAF.Person))
>>> G.add((mark, FOAF.firstName, Literal('mark')))
>>> G.add((URIRef('Professor'), RDF.type, RDFS.Class))
>>> G.add((URIRef('Professor'), RDFS.subClassOf, FOAF.Person))
>>> G.add((nat, RDF.type, URIRef('Professor')))
>>> G.add((nat, FOAF.firstName, Literal('natalie')))
>>> qres = G.query(
"""SELECT DISTINCT ?aname
WHERE {
?a rdf:type foaf:Person .
?a foaf:firstName ?aname .
}""", initNs = {"rdf": RDF,"foaf": FOAF})
>>> for row in qres:
print "%s is a person" % row
mark is a person
>>> qres = G.query(
"""SELECT DISTINCT ?aname
WHERE {
?a rdf:type ?prof .
?a foaf:firstName ?aname .
}""", initNs = {"rdf": RDF,"foaf": FOAF, "prof": URIRef('Professor')})
>>> for row in qres:
print "%s is a Prof" % row
natalie is a Prof
mark is a Prof
>>>

Answer 1

Your query is bringing back all types because the ?prof variable isn't bound to a value.
I think you want to use is the initBindings kwarg to pass in the URI for 'Professor' to your query. So changing your query to below retrieves just natalie.
qres = G.query(
"""SELECT DISTINCT ?aname
WHERE {
?a rdf:type ?prof .
?a foaf:firstName ?aname .
}""",
initNs ={"rdf": RDF,"foaf": FOAF},
initBindings={"prof": URIRef('Professor')})