Visitor Design Pattern

Videos

Section	Video Links
Visitor Overview
Visitor Use Case
hasattr() Method
expandtabs() Method

Book

Cover	Links
	https://www.amazon.com/dp/B08XLJ8Z2J    https://www.amazon.co.uk/dp/B08XLJ8Z2J    https://www.amazon.in/dp/B08Z282SBC    https://www.amazon.de/dp/B08XLJ8Z2J    https://www.amazon.fr/dp/B08XLJ8Z2J    https://www.amazon.es/dp/B08XLJ8Z2J    https://www.amazon.it/dp/B08XLJ8Z2J    https://www.amazon.co.jp/dp/B08XLJ8Z2J    https://www.amazon.ca/dp/B08XLJ8Z2J    https://www.amazon.com.au/dp/B08Z282SBC

Overview

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Terminology

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Visitor UML Diagram

Source Code

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Output

python ./visitor/visitor_concept.py  
D
B
C
A
561

Visitor Example Use Case

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Visitor Example UML Diagram

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 which 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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