import math

import turtle

def square(t, length):

    """Draws a square with sides of the given length.

    Returns the Turtle to the starting position and location.

    """

    for i in range(4):

        t.fd(length)

        t.lt(90)

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

    n = int(arc_length / 4) + 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

    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)

# the following condition checks whether we are

# running as a script, in which case run the test code,

# or being imported, in which case don't.

if __name__ == '__main__':

    bob = turtle.Turtle()

    # draw a circle centered on the origin

    radius = 100

    bob.pu()

    bob.fd(radius)

    bob.lt(90)

    bob.pd()

    circle(bob, radius)

    # wait for the user to close the window

    turtle.mainloop()

#你从命令指示符里面运行一下试试