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15.5: Objects are mutable

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  • You can change the state of an object by making an assignment to one of its attributes. For example, to change the size of a rectangle without changing its position, you can modify the values of width and height:

    box.width = box.width + 50
    box.height = box.height + 100

    You can also write functions that modify objects. For example, grow_rectangle takes a Rectangle object and two numbers, dwidth and dheight, and adds the numbers to the width and height of the rectangle:

    def grow_rectangle(rect, dwidth, dheight):
        rect.width += dwidth
        rect.height += dheight

    Here is an example that demonstrates the effect:

    >>> box.width, box.height
    (150.0, 300.0)
    >>> grow_rectangle(box, 50, 100)
    >>> box.width, box.height
    (200.0, 400.0)

    Inside the function, rect is an alias for box, so when the function modifies rect, box changes.

    As an exercise, write a function named move_rectangle that takes a Rectangle and two numbers named dx and dy. It should change the location of the rectangle by adding dx to the x coordinate of corner and adding dy to the y coordinate of corner.

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