Adding Chapter 3 files

Chapter3/classes/carexample6.py

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+#carexample6.py
+class Car:
+    __mileage_units = "Mi"
+    def __init__(self, col, mil):
+        self.__color = col
+        self.__mileage = mil
+
+    def __str__(self):
+        return f"car with color {self.get_color()} and mileage {self.get_mileage()}"
+
+    def get_color(self):
+        return self.__color
+
+    def get_mileage(self):
+        return self.__mileage
+
+    def set_mileage (self, new_mil):
+            self.__mileage = new_mil
+
+
+if __name__ == "__main__":
+    car = Car ("blue", 1000)
+
+    print (car)
+    print (car.get_color())
+    print(car.get_mileage())
+    car.set_mileage(2000)
+    print (car.get_color())
+    print(car.get_mileage())
+