Using Visitors

The Visitor Pattern is a well known and commonly used pattern for performing an operation on the elements of an object structure. There are many online resources for learning about the visitor pattern.

The implementation of the Visitor Pattern in pySBOL3 is very simple. When a pySBOL3 object’s accept method is called, a visitor is passed as the only argument. The accept method, in turn, invokes visit_type on the visitor, passing the pySBOL3 object as the only argument.

Traversal of the pySBOL3 object graph is left to the visitor itself. When a visit_type method is called, the visitor must then invoke accept on any child objects it might want to visit. See the code sample below for an example of this traversal.

Resources

• https://sourcemaking.com/design_patterns/visitor

• https://refactoring.guru/design-patterns/visitor

• https://www.informit.com/store/design-patterns-elements-of-reusable-object-oriented-9780201633610

Example Code

The program below will visit each top level object in the document as well as visiting any features on the top level components. Note that the visitor must direct the traversal of the features, as discussed above.

import sbol3


class MyVisitor:

    def visit_component(self, c: sbol3.Component):
        print(f'Component {c.identity}')
        for f in c.features:
            f.accept(self)

    def visit_sequence(self, s: sbol3.Component):
        print(f'Sequence {s.identity}')

    def visit_sub_component(self, sc: sbol3.Component):
        print(f'SubComponent {sc.identity}')


doc = sbol3.Document()
doc.read('test/SBOLTestSuite/SBOL3/BBa_F2620_PoPSReceiver/BBa_F2620_PoPSReceiver.ttl')
visitor = MyVisitor()
for obj in doc.objects:
    obj.accept(visitor)

Visit Methods

The table below lists each class that has an accept method and the corresponding method that is invoked on the visitor passed to the accept method.

Class	Visit Method
Activity	visit_activity
Agent	visit_agent
Association	visit_association
Attachment	visit_attachment
BinaryPrefix	visit_binary_prefix
Collection	visit_collection
CombinatorialDerivation	visit_combinatorial_derivation
Component	visit_component
ComponentReference	visit_component_reference
Constraint	visit_constraint
Cut	visit_cut
Document	visit_document(self)
EntireSequence	visit_entire_sequence
Experiment	visit_experiment
ExperimentalData	visit_experimental_data
ExternallyDefined	visit_externally_defined
Implementation	visit_implementation
Interaction	visit_interaction
Interface	visit_interface
LocalSubComponent	visit_local_sub_component
Measure	visit_measure
Model	visit_model
Participation	visit_participation
Plan	visit_plan
PrefixedUnit	visit_prefixed_unit
Range	visit_range
SIPrefix	visit_si_prefix
Sequence	visit_sequence
SequenceFeature	visit_sequence_feature
SingularUnit	visit_singular_unit
SubComponent	visit_sub_component
UnitDivision	visit_unit_division
UnitExponentiation	visit_unit_exponentiation
UnitMultiplication	visit_unit_multiplication
Usage	visit_usage
VariableFeature	visit_variable_feature