# Working with Ontologies

To get started with Structured Machine Learning, the first thing
required is an [Ontology](https://www.w3.org/TR/owl2-overview/) with
[Named
Individuals](https://www.w3.org/TR/owl-syntax/#Named_Individuals). We
cannot provide such ontologies for you and depending on the use case,
it may be necessary to first map existing data into an ontology.

However, a sample ontology is included with OWL-APy so that you can
start using it right away. It is contained in the `KGs/father.owl`
file. It contains six persons (individuals), of which four are male
and two are female. We will use this tiny ontology in the following
example.

## Loading an Ontology

To load an ontology, use the following Python code:

```py
from owlapy import IRI
from owlapy.owlready2 import OWLOntologyManager_Owlready2

mgr = OWLOntologyManager_Owlready2()
onto = mgr.load_ontology(IRI.create("file://KGs/father.owl"))
```

First, we import the IRI class and a suitable OWLOntologyManager. To
load a file from our computer, we have to reference it with an
[IRI](https://tools.ietf.org/html/rfc3987). Secondly, we need an
Ontology Manager, which is a component that can manage ontologies for
us. Currently, OWL-APy contains one such manager: The
[OWLOntologyManager_Owlready2](owlapy.owlready2.OWLOntologyManager_Owlready2).

Now, we can already inspect the contents of the ontology. For example,
to list all individuals:

```py
for ind in onto.individuals_in_signature():
    print(ind)
```

Refer to the [OWLOntology](owlapy.model.OWLOntology) documentation for
more details.

## Attaching a reasoner

In order to validate facts about statements in the ontology (and thus
also for the Structured Machine Learning task), the help of a reasoner
component is required.

In our OWL-APy library, we provide several reasoners to choose
from. Currently, there are the the [fast instance
checker](owlapy.fast_instance_checker.OWLReasoner_FastInstanceChecker),
the [structural Owlready2
reasoner](owlapy.owlready2.OWLReasoner_Owlready2), and the [class
instantiation Owlready2
reasoner](owlapy.owlready2.temp_classes.OWLReasoner_Owlready2_TempClasses)
available to choose from.

To load any reasoner, follow this Python code:

```py
from owlapy.owlready2 import OWLReasoner_Owlready2
from owlapy.owlready2.temp_classes import OWLReasoner_Owlready2_TempClasses
from owlapy.fast_instance_checker import OWLReasoner_FastInstanceChecker

structural_reasoner = OWLReasoner_Owlready2(onto)
temp_classes_reasoner = OWLReasoner_Owlready2_TempClasses(onto)
fast_instance_checker = OWLReasoner_FastInstanceChecker(onto, temp_classes_reasoner)
```

The reasoner takes as its first argument the ontology to load. The
fast instance checker requires a base reasoner to which any reasoning
tasks not covered by the fast instance checking code are deferred to.