# Restrictions

Restrictions are special type of Classes in ontology.

## Restrictions on a Property

    >>> from owlready import *

    >>> onto = Ontology("http://test.org/onto.owl")

    >>> class Drug(Thing):
    ...     ontology = onto

    >>> class ActivePrinciple(Thing):
    ...     ontology = onto

    >>> class has_for_active_principle(Property):
    ...     ontology = onto
    ...     domain   = [Drug]
    ...     range    = [ActivePrinciple]

For example, a Placebo is a Drug with no Active Principle:

    >>> class Placebo(Drug):
    ...     equivalent_to = [Drug & NOT(restriction(has_for_active_principle, SOME, ActivePrinciple))]

In the example above, ‘restriction(has_for_active_principle, SOME, ActivePrinciple)’ is the Class of all objects that have at least one Active Principle. The NOT() function returns the negation of a Class. The & operator returns the intersection of two Classes.

Another example: an Association Drug is a Drug that associates two or more Active Principle:

    >>> class DrugAssociation(Drug):
    ...     equivalent_to = [Drug & restriction(has_for_active_principle, MIN, 2, ActivePrinciple)]

Owlready provides the following types of restrictions (they have the same names than in Protégé):

> - some : restriction(Property Class, SOME, Range Class)
> - only : restriction(Property Class, ONLY, Range Class)
> - min : restriction(Property Class, MIN, cardinality, Range Class)
> - max : restriction(Property Class, MAX, cardinality, Range Class)
> - exactly : restriction(Property Class, EXACTLY, cardinality, Range Class)
> - value : restriction(Property Class, VALUE, Range Instance)

## Class operators

Owlready provides the following operators between Classes (normal Classes but also restrictions):

> - & : and operator (intersection). For example: Class1 & Class2

> - : or operator (union). For example: Class1 | Class2

> - NOT() : not operator (negation). For example: NOT(Class1)

## One Of restrictions

In ontologies, a ‘One Of’ statement is used for defining a Class by extension, i.e. by listing its Instances rather than by defining its properties.

    >>> class DrugForm(Thing):
    ...     ontology = onto

    >>> tablet     = DrugForm()
    >>> capsule    = DrugForm()
    >>> injectable = DrugForm()
    >>> pomade     = DrugForm()

    # Assert that there is only four drug forms possible
    >>> DrugForm.is_a.append(one_of(tablet, capsule, injectable, pomade))