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‎slides/content/m09_classes.md‎

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# 🐍 Classes
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<!-- .element: class="headline" -->
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Python is object-oriented as well, of course, though, it follows a more lean approach towards classes.
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Let's have a look.
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Python is object-oriented, but follows a more lean approach towards classes.
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<div class="sidebyside">
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```java
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// ./java/M09_PassengersProgram.java#L3-L38
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// ./java/M09_PassengersProgram.java#L3-L40
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class Passenger {
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this.lastName = lastName;
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}
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void print() {
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void display() {
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System.out.printf("%s %s %n", firstName, lastName);
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}
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static Passenger fromInput() {
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Scanner scanner = new Scanner(System.in);
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System.out.print("Enter first name: ");
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String firstName = scanner.nextLine();
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System.out.print("Enter last name: ");
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String lastName = scanner.nextLine();
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scanner.close();
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Passenger lisa = new Passenger("Lisa", "Ha");
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Passenger user = Passenger.fromInput();
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lisa.print();
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user.print();
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lisa.display();
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user.display();
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}
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}
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```
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```py
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```py [|4-6|8-9|11-16|20-24|19|]
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# ./python/m09_passengers_program.py
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class Passenger:
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@staticmethod
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def from_input():
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first_name = input()
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last_name = input()
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first_name = input("Enter first name: ")
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last_name = input("Enter last name: ")
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return Passenger(first_name, last_name)
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lisa = Passenger("Lisa", "Ha")
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# user = Passenger.from_input()
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lisa.display()
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# user.display()
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if __name__ == "__main__":
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lisa = Passenger("Lisa", "Ha")
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user = Passenger.from_input()
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lisa.display()
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user.display()
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```
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### What do we notice here?
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- Python only ever has one constructor that is called `__init__` and must have the `self` parameter
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- Python has no method overloading
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- We don't explicitly define the class properties `first_name` and `last_name`
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- You create them on-the-fly in the constructor
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- What makes a function a method in Python is only governed by the explicit `self` parameter
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- And you always have to use `self.` if you want to access class properties
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- A static method in Python needs the `@staticmethod` annotation and omits the `self` parameter
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- A static method just means that it won't access any class properties
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- Creating a new object works similar in Python, but we omit the `new` keyword
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- There are no access modifiers and there also is no `final` in Python
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#### Constructors
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---
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- There is only one **constructor** that is called `__init__` with a mandatory `self` parameter
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- Python does not support constructor overloading or **method overloading**
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- We don't explicitly define the **class properties** `first_name` and `last_name`,
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but you create them on-the-fly in the constructor or elsewhere
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#### Functions and methods
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- What makes a function a method is only governed by the explicit `self` parameter
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- You must always use `self.` if you want to access class properties
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- There are no access modifiers and there is also no `final` in Python
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#### Static methods
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- A **static method** needs the `@staticmethod` annotation and omits the `self` parameter
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- A static method just means that it won't access any class properties
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## Interfaces
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#### Instantiation
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What you might know from interfaces in Java can be achieved a little bit different with a dynamically-typed language like Python.
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- Creating a new object works similar, but you omit the `new` keyword
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- The `main` method in Python is actually a condition - we will learn more in the next modules
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---
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## Informal Interfaces
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<div class="sidebyside">
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class ShapesProgram {
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public static void main(String[] args) {
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Square square = new Square(10);
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Circle circle = new Circle(5);
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Shape[] shapes = {square, circle};
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Shape[] shapes = { new Circle(5), new Square(10) };
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for (Shape shape : shapes) {
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System.out.println(shape.area());
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}
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```
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```py
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```py [|3-5|8-13|16-21|24-26|]
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# ./python/m09_shapes_program.py
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class Circle:
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class Shape:
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def area(self):
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pass
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class Circle(Shape):
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def __init__(self, radius):
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self.radius = radius
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def area(self):
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return 3.14159 * self.radius * self.radius
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class Square:
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class Square(Shape):
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def __init__(self, length):
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self.length = length
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def area(self):
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return self.length * self.length
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shapes = [Circle(5), Square(10)]
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for shape in shapes:
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print(shape.area())
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</div>
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### What do we notice here?
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- There is no `interface` type in Python, instead we use a normal `class` and inhert from it
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- Methods are overridden without an explicit annotation - they are always replaced
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- No type checks are enforced by Python, try removing `Shape` or renaming the `area()` method ...
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---
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### What do we notice here?
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## Duck-Typing
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Since there is NO type-checking at compile-time, we may also solely rely on [**duck-typing**](https://docs.python.org/3/glossary.html#term-duck-typing).
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An `AttributeError` is thrown at runtime, if the method you are calling wouldn't exist. *
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```py [|3-16|19-21|21|]
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# ./python/m09_shapes_duck_typing.py
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class Circle:
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def __init__(self, radius):
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self.radius = radius
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def area(self):
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return 3.14159 * self.radius * self.radius
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class Square:
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def __init__(self, length):
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self.length = length
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def area(self):
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return self.length * self.length
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shapes = [Circle(5), Square(10)]
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for shape in shapes:
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print(shape.area())
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```
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### But, how can I have a "real" interfaces and inheritance in Python?
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- Use abstract classes via the [**`abc` module**](https://docs.python.org/3/library/abc.html)
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- Learn more about formal interfaces at [realpython.com/python-interface](https://realpython.com/python-interface/)
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- Because there is no hard type checking at compile-time, we can rely on duck-typing here
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- However, for a better code structures, Python offers the following
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- abstract classes (see https://docs.python.org/3/library/abc.html)
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- protocols (see https://www.python.org/dev/peps/pep-0544/)
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<small>* Look into [**type hints**](https://docs.python.org/3/library/typing.html) to have better type-checking at runtime and even before you run your code.</small>

‎snippets/java/M09_PassengersProgram.java‎

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@@ -10,14 +10,16 @@ class Passenger {
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this.lastName = lastName;
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}
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void print() {
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void display() {
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System.out.printf("%s %s %n", firstName, lastName);
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}
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static Passenger fromInput() {
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Scanner scanner = new Scanner(System.in);
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System.out.print("Enter first name: ");
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String firstName = scanner.nextLine();
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System.out.print("Enter last name: ");
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String lastName = scanner.nextLine();
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scanner.close();
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Passenger lisa = new Passenger("Lisa", "Ha");
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Passenger user = Passenger.fromInput();
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lisa.print();
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user.print();
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lisa.display();
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user.display();
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}
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}

‎snippets/java/M09_ShapesProgram.java‎

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class ShapesProgram {
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public static void main(String[] args) {
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Square square = new Square(10);
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Circle circle = new Circle(5);
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Shape[] shapes = {square, circle};
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Shape[] shapes = { new Circle(5), new Square(10) };
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for (Shape shape : shapes) {
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System.out.println(shape.area());
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}

‎snippets/python/m09_passengers_program.py‎

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@staticmethod
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def from_input():
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first_name = input()
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last_name = input()
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first_name = input("Enter first name: ")
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last_name = input("Enter last name: ")
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return Passenger(first_name, last_name)
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lisa = Passenger("Lisa", "Ha")
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# user = Passenger.from_input()
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lisa.display()
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# user.display()
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if __name__ == "__main__":
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lisa = Passenger("Lisa", "Ha")
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user = Passenger.from_input()
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lisa.display()
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user.display()

‎snippets/python/m09_shapes_duck_typing.py‎

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class Circle:
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def __init__(self, radius):
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self.radius = radius
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def area(self):
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return 3.14159 * self.radius * self.radius
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class Square:
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def __init__(self, length):
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self.length = length
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def area(self):
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return self.length * self.length
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shapes = [Circle(5), Square(10)]
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for shape in shapes:
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print(shape.area())

‎snippets/python/m09_shapes_program.py‎

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class Circle:
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class Shape:
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def area(self):
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pass
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class Circle(Shape):
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def __init__(self, radius):
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self.radius = radius
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def area(self):
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return 3.14159 * self.radius * self.radius
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class Square:
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class Square(Shape):
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def __init__(self, length):
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self.length = length
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def area(self):
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return self.length * self.length
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shapes = [Circle(5), Square(10)]
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for shape in shapes:
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print(shape.area())

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