Inheritance vs Composition

class: middle

# .eight[CSET 160:]
## .eight[Web Development II]

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class: middle

# Inheritance vs Composition

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# Agenda

1. [ ] [Implicit Inheritance](#implicit)
2. [ ] [Override Inheritance](#override)
3. [ ] [Alter Inheritance](#alter)
4. [ ] [Composition](#composition)

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class: middle

First,

# .eleven[Inheritance is Dangerous]

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# Inheritance

.eight[Inheritance gives features and functionality from a generic class to a more specialized one.]

- Keeps code DRY
- Modify or add to specialized class as needed

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# Inheritance

A .eight[sub class] inherits from a .eight[super] or .eight[base class].

Or .eight[child] and .eight[parent] classes.

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# Three Kinds of Inheritance

Actions on the child
1. **imply** an action on the parent.
2. **override** an action on the parent.
3. **alter** an action on the parent.

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name: implicit

# [Exercise 44A](https://github.com/ts-cset/cset-160/blob/master/python/ex44a.py)

- The .eight[pass] keyword allows you to create an empty block of code
- The sub class has no definition, but all the functionality

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name: override

# [Exercise 44B](https://github.com/ts-cset/cset-160/blob/master/python/ex44b.py)

- Both classes have the same method, which gets called?
- Useful if you need the same interface, but different functionality

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name: alter

# [Exercise 44C](https://github.com/ts-cset/cset-160/blob/master/python/ex44c.py)

- Again, both classes have the same interface
- Alter before or after the parent method

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# [Exercise 44D](https://github.com/ts-cset/cset-160/blob/master/python/ex44d.py)

Shows all three kinds together in the same classes

.eleven[Before you run it, try to guess what the printed output will be!]

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# Why .eight[super()]?

```
super(Child, self).method()
```

Go find the .eight[.method()] definition on any of the classes .eight[Child] inherits from and call it with .eight[self].

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name: composition

# Composition

Instead of mapping a special class to a generic class, just **use** two different classes.

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# [Exercise 44E](https://github.com/ts-cset/cset-160/blob/master/python/ex44e.py)

- In the last exercise, Child .eight[is-a] Parent
- Now, Compound .eight[has-a] Other

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# Which should you use?

1. Avoid multiple inheritance at all costs!
2. Use composition to package together unrelated things.
3. Use inheritance *only* when there's a clear relationship within a single concept.

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# Which should you use?

But as always,

## .fourteen[Don't be a slave to the rules]