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Explain encapsulation, abstraction, inheritance and polymorphism with a short example.

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class Shape:
    def area(self):
        raise NotImplementedError

class Circle(Shape):
    def __init__(self, r):
        self._r = r            # encapsulated state
    def area(self):
        return round(3.14159 * self._r ** 2, 1)

class Square(Shape):
    def __init__(self, side):
        self._side = side
    def area(self):
        return self._side ** 2

for s in [Circle(1), Square(3)]:   # polymorphism: same call, different behaviour
    print(type(s).__name__, s.area())

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