Visitor Method – Python Design Patterns

Visitor Method is a Behavioral Design Pattern which allows us to separate the algorithm from an object structure on which it operates. It helps us to add new features to an existing class hierarchy dynamically without changing it. All the behavioral patterns proved as the best methods to handle the communication between the objects. Similarly, it is used when we have to perform an operation on a group of similar kinds of objects.
A Visitor Method consists of two parts: 

• method named as Visit() implemented by the visitor and used and called for every element of the data structure.
• Visitable classes providing Accept() methods that accept a visitor

Design Components

• Client: The Client class acts as the consumer of the classes of the visitor design pattern. It can access the data structure objects and can instruct them to accept a visitor for the future processing.
• Visitor: An Abstract class which is used to declare visit operations for all visitable classes.
• Concrete Visitor: Each Visitor will be responsible for different operations. For each type of visitor all the visit methods, declared in abstract visitor, must be implemented.
• Visitable: Accept operations is declared by this class. It also act as the entry point which enables an object to be visited by visitor.
• Concrete Visitable: These classes implement the Visitable class and defines the accept operation. The visitor object is passed to this object using the accept operation.

Problem without using Visitor Method

Imagine you are handling the Software management of GeeksforGeeks and they have started certain courses such as DSA, SDE, and STL which are definitely useful for students who are preparing for the product based companies. But how will you handle all the data of Courses, Instructors, students, classes, IDs in your database? If you go with a simple direct approach to handle such a situation, you will definitely end up with a mess only.
 

Visitor-problem-diagram

Solution using Visitor Method

Let’s look at the solution to the above-described problem. The Visitor method suggests adding a new behavior in a separate class called Visitor class instead of mixing it with the already existing classes. We will pass the original object to the visitor’s method as parameters such that the method will access all the necessary information.
 

""" The Courses hierarchy cannot be changed to add new 
   functionality dynamically. Abstract Crop class for
 Concrete Courses_At_GFG classes: methods defined in this class
 will be inherited by all Concrete Courses_At_GFG classes."""

class Courses_At_GFG:

    def accept(self, visitor):
        visitor.visit(self)

    def teaching(self, visitor):
        print(self, "Taught by ", visitor)

    def studying(self, visitor):
        print(self, "studied by ", visitor)


    def __str__(self):
        return self.__class__.__name__


"""Concrete Courses_At_GFG class: Classes being visited."""
class SDE(Courses_At_GFG): pass

class STL(Courses_At_GFG): pass

class DSA(Courses_At_GFG): pass


""" Abstract Visitor class for Concrete Visitor classes:
 method defined in this class will be inherited by all
 Concrete Visitor classes."""
class Visitor:

    def __str__(self):
        return self.__class__.__name__


""" Concrete Visitors: Classes visiting Concrete Course objects.
 These classes have a visit() method which is called by the
 accept() method of the Concrete Course_At_GFG classes."""
class Instructor(Visitor):
    def visit(self, crop):
        crop.teaching(self)


class Student(Visitor):
    def visit(self, crop):
        crop.studying(self)


"""creating objects for concrete classes"""
sde = SDE()
stl = STL()
dsa = DSA()

"""Creating Visitors"""
instructor = Instructor()
student = Student()

"""Visitors visiting courses"""
sde.accept(instructor)
sde.accept(student)

stl.accept(instructor)
stl.accept(student)

dsa.accept(instructor)
dsa.accept(student)

Output

SDE Taught by  Instructor
SDE studied by  Student
STL Taught by  Instructor
STL studied by  Student
DSA Taught by  Instructor
DSA studied by  Student

UML Diagram

Following is the UML diagram for Visitor Method

UML-diagram-visitor-method

Advantages

• Open/Closed principle: Introducing new behavior in class is easy which can work with objects of different classes without making changes in these classes.
• Single Responsibility Principle: Multiple versions of same behavior can be operated into the same class.
• Addition of entities: Adding an entity in Visitor Method is easy as we have to make changes in visitor class only and it will not affect the existing item.
• Updating Logic: If the logic of operation is updated, then we need to make change only in the visitor implementation rather than doing it in all the item classes.

Disadvantages

• Lots of Updates: We have to update each and every visitor whenever a class get added or removed from the primary hierarchy
• Hard to Extend: If there are too many visitor classes then it becomes really hard to extend the whole interface of the class.
• Lack of Access: Sometimes visitors might not have the access to private field of certain classes that they are supposed to work with.

Applicability 

• Recursive structures: Visitor Method works really well with recursive structures like directory trees or XML structures. The Visitor object can visit each node in the recursive structure
• Performing Operations: We can use the visitor method when we have to perform operations on all the elements of the complex object like Tree.

Further Read – Visitor Method in C++

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