Think Python Exercise 15.2

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import turtle

class Point:
    """表示一个二维的点
    """

class Rectangle:
    """Represents a rectangle.
    attributes: width, height, corner."""
    

def draw_rect(t,rect):
    t.pu() #在移动到矩形左下角的过程中，不画出痕迹（提笔，put up）
    t.goto(rect.corner.x, rect.corner.y)
    t.pd() #落笔（put down）
    for i in range(2):
        t.fd(box.width)
        t.lt(90)
        t.fd(box.height)
        t.lt(90)

bob = turtle.Turtle()

box = Rectangle()
box.width = 100.0
box.height = 200.0
box.corner = Point()
box.corner.x = 100
box.corner.y = 90

draw_rect(bob,box)

turtle.mainloop()

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import turtle, math

class Point:
    """表示一个二维的点
    """
    
class Circle:
    """属性：圆心center、半径radius
    圆心是一个Point类
    半径是一个数字
    """

#下面几个函数，用了4.1题画圆的代码
def polyline(t, n, length, angle):
    """Draws n line segments.
    t: Turtle object
    n: number of line segments
    length: length of each segment
    angle: degrees between segments
    """
    for i in range(n):
        t.fd(length)
        t.lt(angle)

def polygon(t, n, length):
    """Draws a polygon with n sides.
    t: Turtle
    n: number of sides
    length: length of each side.
    """
    angle = 360.0/n
    polyline(t, n, length, angle)

def arc(t, r, angle):
    """Draws an arc with the given radius and angle.
    t: Turtle
    r: radius
    angle: angle subtended by the arc, in degrees
    """
    arc_length = 2 * math.pi * r * abs(angle) / 360 #角度变为绝对值
    n = int(arc_length / 4) + 3 #之前是int(arc_length / 3) + 1
    step_length = arc_length / n
    step_angle = float(angle) / n

    # making a slight left turn before starting reduces
    # the error caused by the linear approximation of the arc
    # 在开始前稍微左转可以减少由直线逼近弧线所引起的误差
    t.lt(step_angle/2)
    polyline(t, n, step_length, step_angle)
    t.rt(step_angle/2)


def circle(t, r):
    """Draws a circle with the given radius.
    t: Turtle
    r: radius
    """
    arc(t, r, 360)
    
def draw_circle(t,circ):
    t.pu() #在移动到圆心的过程中，不画出痕迹（提笔，put up）
    t.goto(circ.center.x, circ.center.y)
    t.pd() #落笔（put down）
    circle(t, circ.radius)

bob = turtle.Turtle()

tom = Circle()
tom.center = Point()
tom.center.x = 150
tom.center.y = 100
tom.radius = 75

draw_circle(bob,tom)

turtle.mainloop()