Code

import owlready2
owlready2.JAVA_EXE = r"my-path-to-java-exe"

# new.owl is a non-existing file and therefore onto has no pre-defined classes
# if you know of any nicer way to define an ontology, I'd appreciate it

onto = get_ontology("new.owl")

with onto:
    class Drug(Thing): pass
    class number_of_tablets(Drug >> int, FunctionalProperty): pass          # Creating some properties
    class price(Drug >> float, FunctionalProperty): pass
    class price_per_tablet(Drug >> float, FunctionalProperty): pass

    rule = Imp()

    # Rule: "Drug instance ?d     AND     price of ?d is ?p     AND     drug ?d has number_of_tablets = ?n
    #        AND     ?r = ?p/?n     ->      Drug ?d has price_per_tablet = ?r"

    rule.set_as_rule("""Drug(?d), price(?d,?p), number_of_tablets(?d,?n), divide(?r, ?p, ?n) -> price_per_tablet(?d, ?r)""")

    # Create an instance "drug" with properties defined in brackets
    drug = Drug(number_of_tablets = 10, price = 25.0)
    #print(drug.iri)

    # Syncing the reasoner infers new info
    sync_reasoner_pellet(infer_property_values = True, infer_data_property_values = True)

    # New property price_per_tablet is now added to drug and we can use it normally:
    print(drug.price_per_tablet)

# Save this ontology with rules in the same folder, filename: test
onto.save(file = "test", format = "rdfxml")