PyPro #91 Visitor Design Pattern

Visitor Design Pattern in Python

Visitor Design Pattern in Python

Write a Python program to implement Visitor Design Pattern.

# The Crops hierarchy cannot be changed to add new functionality dynamically.
# Abstract Crop class for Concrete Crop classes: methods defined in this class will be inherited by all Concrete Crop classes.
class Crop:
    def accept(self, visitor):
        visitor.visit(self)
    def sow(self, visitor):
        print(self, "Sown by", visitor)
    def irrigate(self, visitor):
        print(self, "Irrigated by", visitor)
    def harvest(self, visitor):
        print(self, "Harvested by", visitor)
    def __str__(self):
        return self.__class__.__name__
 
# Concrete Crops: Classes being visited.
class Wheat(Crop): pass
class Corn(Crop): pass
class Tomato(Crop): 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 Crop objects. These classes have a visit() method which is called by the accept() method of the Concrete Crop classes.
class GardenerOne(Visitor):
    def visit(self, crop):
        crop.sow(self)
 
class GardenerTwo(Visitor):
    def visit(self, crop):
        crop.irrigate(self)
 
class GardenerThree(Visitor):
    def visit(self, crop):
        crop.harvest(self)
 
 
### USING THE ABOVE SETUP ###
# Creating Plants
wheatPatch = Wheat()
cornPatch = Corn()
tomatoPatch = Tomato()
 
# Creating Visitors
gardenerOne = GardenerOne()
gardenerTwo = GardenerTwo()
gardenerThree = GardenerThree()
 
# Visitors visiting Plants
wheatPatch.accept(gardenerOne)
wheatPatch.accept(gardenerTwo)
wheatPatch.accept(gardenerThree)
 
cornPatch.accept(gardenerOne)
cornPatch.accept(gardenerTwo)
cornPatch.accept(gardenerThree)
 
tomatoPatch.accept(gardenerOne)
tomatoPatch.accept(gardenerTwo)
tomatoPatch.accept(gardenerThree)
 
 
### OUTPUT ###
Wheat Sown by GardenerOne
Wheat Irrigated by GardenerTwo
Wheat Harvested by GardenerThree
Corn Sown by GardenerOne
Corn Irrigated by GardenerTwo
Corn Harvested by GardenerThree
Tomato Sown by GardenerOne
Tomato Irrigated by GardenerTwo
Tomato Harvested by GardenerThree

See also:

• Python 101
• Object Oriented Python
• Design Patterns in Python
• 50+ Handy Standard Library Modules
• 60+ Handy Third Library Modules
• 50+ Tips & Tricks for Python Developers
• 50+ Know-How(s) Every Pythonista Must Know