Memento Design Pattern
Videos
| Section | Video Links |
|---|---|
| Memento Overview |    |
| Memento Use Case | |
| Getters/Setters | |
Book
Overview
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Terminology
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Memento UML Diagram
Source Code
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Output
python ./memento/memento_concept.py Originator: Setting state to `State #1` Originator: Setting state to `State #2` CareTaker: Getting a copy of Originators current state Originator: Providing Memento of state to caretaker. Originator: Setting state to `State #3` CareTaker: Getting a copy of Originators current state Originator: Providing Memento of state to caretaker. Originator: Setting state to `State #4` State #4 CareTaker: Restoring Originators state from Memento Originator: State after restoring from Memento: `State #2` State #2 CareTaker: Restoring Originators state from Memento Originator: State after restoring from Memento: `State #3` State #3
Example Use Case
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Example UML Diagram
Output
python ./memento/client.py
Score: 200, Level: 0, Location: {'x': 0, 'y': 0, 'z': 2}
Inventory: {'rifle', 'sword'}
CareTaker: Game Save
Score: 500, Level: 1, Location: {'x': 0, 'y': 0, 'z': 13}
Inventory: {'motorbike', 'rifle', 'sword'}
CareTaker: Game Save
Score: 600, Level: 2, Location: {'x': 0, 'y': 0, 'z': 14}
Inventory: {'motorbike', 'rifle', 'sword'}
CareTaker: Restoring Characters attributes from Memento
Score: 200, Level: 0, Location: {'x': 0, 'y': 0, 'z': 2}
Inventory: {'rifle', 'sword'}New Coding Concepts
Python Getter/Setters
Often when coding attributes in classes, you may want to provide methods to allow external functions to read or modify a classes internal attributes.
A common approach would be to add two methods prefixed with get_ and set_,
class ExampleClass: def __init__(self): self._value = 123 def get_value(self): return self._value def set_value(self, value): self._value = value example = ExampleClass() print(example.get_value())
This makes perfect sense what the intentions are, but there is a more pythonic way of doing this and that is by using the inbuilt Python @property decorator.
class ExampleClass: def __init__(self): self._value = 123 @property def value(self): return self._value @value.setter def value(self, value): self._value = value example = ExampleClass() print(example.value)
Note that in the above example, there is an extra decorator named @value.setter . This is used for setting the _value attribute.
Along with the above two new getter/setter methods, there is also another method for deleting an attribute called deleter .
class ExampleClass: def __init__(self): self._value = 123 @property def value(self): return self._value @value.setter def value(self, value): self._value = value @value.deleter def value(self): print('Deleting _value') del self._value example = ExampleClass() print(example.value) del example.value print(example.value) # now raises an AttributeError
Summary
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