Visitor Design Pattern
Video Lecture
Overview
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Terminology
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Visitor UML Diagram
Source Code
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./visitor/visitor_concept.py
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75 | # pylint: disable=too-few-public-methods
"The Visitor Pattern Concept"
from abc import ABCMeta, abstractmethod
class IVisitor(metaclass=ABCMeta):
"An interface that custom Visitors should implement"
@staticmethod
@abstractmethod
def visit(element):
"Visitors visit Elements/Objects within the application"
class IVisitable(metaclass=ABCMeta):
"""
An interface the concrete objects should implement that allows
the visitor to traverse a hierarchical structure of objects
"""
@staticmethod
@abstractmethod
def accept(visitor):
"""
The Visitor traverses and accesses each object through this
method
"""
class Element(IVisitable):
"An Object that can be part of any hierarchy"
def __init__(self, name, value, parent=None):
self.name = name
self.value = value
self.elements = set()
if parent:
parent.elements.add(self)
def accept(self, visitor):
"required by the Visitor that will traverse"
for element in self.elements:
element.accept(visitor)
visitor.visit(self)
# The Client
# Creating an example object hierarchy.
Element_A = Element("A", 101)
Element_B = Element("B", 305, Element_A)
Element_C = Element("C", 185, Element_A)
Element_D = Element("D", -30, Element_B)
# Now Rather than changing the Element class to support custom
# operations, we can utilise the accept method that was
# implemented in the Element class because of the addition of
# the IVisitable interface
class PrintElementNamesVisitor(IVisitor):
"Create a visitor that prints the Element names"
@staticmethod
def visit(element):
print(element.name)
# Using the PrintElementNamesVisitor to traverse the object hierarchy
Element_A.accept(PrintElementNamesVisitor)
class CalculateElementTotalsVisitor(IVisitor):
"Create a visitor that totals the Element values"
total_value = 0
@classmethod
def visit(cls, element):
cls.total_value += element.value
return cls.total_value
# Using the CalculateElementTotalsVisitor to traverse the
# object hierarchy
TOTAL = CalculateElementTotalsVisitor()
Element_A.accept(CalculateElementTotalsVisitor)
print(TOTAL.total_value)
|
Output
| python ./visitor/visitor_concept.py
D
B
C
A
561
|
Visitor Example Use Case
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Visitor Example UML Diagram
Source Code
./visitor/client.py
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132 | # pylint: disable=too-few-public-methods
"The Visitor Pattern Use Case Example"
from abc import ABCMeta, abstractmethod
class IVisitor(metaclass=ABCMeta):
"An interface that custom Visitors should implement"
@staticmethod
@abstractmethod
def visit(element):
"Visitors visit Elements/Objects within the application"
class IVisitable(metaclass=ABCMeta):
"""
An interface that concrete objects should implement that allows
the visitor to traverse a hierarchical structure of objects
"""
@staticmethod
@abstractmethod
def accept(visitor):
"""
The Visitor traverses and accesses each object through this
method
"""
class AbstractCarPart():
"The Abstract Car Part"
@property
def name(self):
"a name for the part"
return self._name
@name.setter
def name(self, value):
self._name = value
@property
def sku(self):
"The Stock Keeping Unit (sku)"
return self._sku
@sku.setter
def sku(self, value):
self._sku = value
@property
def price(self):
"The price per unit"
return self._price
@price.setter
def price(self, value):
self._price = value
class Body(AbstractCarPart, IVisitable):
"A part of the car"
def __init__(self, name, sku, price):
self.name = name
self.sku = sku
self.price = price
def accept(self, visitor):
visitor.visit(self)
class Engine(AbstractCarPart, IVisitable):
"A part of the car"
def __init__(self, name, sku, price):
self.name = name
self.sku = sku
self.price = price
def accept(self, visitor):
visitor.visit(self)
class Wheel(AbstractCarPart, IVisitable):
"A part of the car"
def __init__(self, name, sku, price):
self.name = name
self.sku = sku
self.price = price
def accept(self, visitor):
visitor.visit(self)
class Car(AbstractCarPart, IVisitable):
"A Car with parts"
def __init__(self, name):
self.name = name
self._parts = [
Body("Utility", "ABC-123-21", 1001),
Engine("V8 engine", "DEF-456-21", 2555),
Wheel("FrontLeft", "GHI-789FL-21", 136),
Wheel("FrontRight", "GHI-789FR-21", 136),
Wheel("BackLeft", "GHI-789BL-21", 152),
Wheel("BackRight", "GHI-789BR-21", 152),
]
def accept(self, visitor):
for parts in self._parts:
parts.accept(visitor)
visitor.visit(self)
class PrintPartsVisitor(IVisitor):
"Print out the part name and sku"
@staticmethod
def visit(element):
if hasattr(element, 'sku'):
print(f"{element.name}\t:{element.sku}".expandtabs(6))
class TotalPriceVisitor(IVisitor):
"Print out the total cost of the parts in the car"
total_price = 0
@classmethod
def visit(cls, element):
if hasattr(element, 'price'):
cls.total_price += element.price
return cls.total_price
# The Client
CAR = Car("DeLorean")
# Print out the part name and sku using the PrintPartsVisitor
CAR.accept(PrintPartsVisitor())
# Calculate the total prince of the parts using the TotalPriceVisitor
TOTAL_PRICE_VISITOR = TotalPriceVisitor()
CAR.accept(TOTAL_PRICE_VISITOR)
print(f"Total Price = {TOTAL_PRICE_VISITOR.total_price}")
|
Output
| python ./visitor/client.py
Utility :ABC-123-21
V8 engine :DEF-456-21
FrontLeft :GHI-789FL-21
FrontRight :GHI-789FR-21
BackLeft :GHI-789BL-21
BackRight :GHI-789BR-21
Total Price = 4132
|
New Coding Concepts
Instance hasattr()
In the Visitor objects in the example use case above, I test if the elements have a certain attribute during the visit operation.
| def visit(cls, element):
if hasattr(element, 'price'):
...
|
The hasattr() method can be used to test if an instantiated object has an attribute of a particular name.
| class ClassA():
name = "abc"
value = 123
CLASS_A = ClassA()
print(hasattr(CLASS_A, "name"))
print(hasattr(CLASS_A, "value"))
print(hasattr(CLASS_A, "date"))
|
Outputs
String expandtabs()
When printing strings to the console, you can include special characters \t that print a series of extra spaces called tabs. The tabs help present multiline text in a more tabular form that appears to be neater to look at.
| abc 123
defg 456
hi 78910
|
The number of spaces added depends on the size of the word before the \t character in the string. By default, a tab makes up 8 spaces.
Now, not all words separated by a tab will line up the same on the next line.
| abcdef 123
cdefghij 4563
ghi 789
jklmn 1011
|
The problem occurs usually when a word is already 8 or more characters long.
To help solve the spacing issue, you can use the expandtabs() method on a string to set how many characters a tab will use.
| print("abcdef\t123".expandtabs(10))
print("cdefghij\t4563".expandtabs(10))
print("ghi\t789".expandtabs(10))
print("jklmn\t1011".expandtabs(10))
|
Now outputs
| abcdef 123
cdefghij 4563
ghi 789
jklmn 1011
|
Summary
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