Decorator Design Pattern

Videos

Section	Video Links
Decorator Overview
Decorator Use Case
_str_ Dunder Method
Python getattr() Method

Book

Cover	Links
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Overview

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Terminology

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Builder UML Diagram

Output

python ./decorator/decorator_concept.py
Component Method
Decorator Method(Component Method)

Example Use Case

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Example UML Diagram

Output

python ./decorator/client.py
3
101
4
6
-1
106
113
114
1
2
5

New Coding Concepts

Python getattr() Function

Syntax: getattr(object, attribute, default)

In the Sub and Add classes, I use the getattr() method like a ternary operator.

When initializing the Add or Sub classes, you have the option of providing an integer or an existing instance of the Value, Sub or Add classes.

So, for example, the line in the Sub class,

val1 = getattr(val1, 'value', val1)

is saying, if the val1 just passed into the function already has an attribute value, then val1 must be an object of Value, Sub or Add . Otherwise, the val1 that was passed in is a new integer and it will use that instead to calculate the final value of the instance on the next few lines of code. This behavior allows the Sub and Add classes to be used recursively.

E.g.,

A = Value(2)
Add(Sub(Add(200, 15), A), 100)

Dunder __str__ method

When you print() an object, it will print out the objects type and memory location in hex.

class ExampleClass:
    abc = 123

print(ExampleClass())

Outputs

<__main__.ExampleClass object at 0x00000283038B1D00>

You can change this default output by implementing the __str__ dunder method in your class. Dunder is short for saying double underscore.

Dunder methods are predefined methods in python that you can override with your own implementations.

class ExampleClass:
    abc = 123

    def __str__(self):
        return "Something different"

print(ExampleClass())

Now outputs

In all the classes in the above use case example that implement the IValue interface, the __str__ method is overridden to return a string version of the integer value. This allows to print the numerical value of any object that implements the IValue interface rather than printing a string that resembles something like below.

<__main__.ValueClass object at 0x00000283038B1D00>

The __str__ dunder was also overridden in the /prototype/prototype_concept.py concept code.

Summary

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