执行时为什么下面的代码说NameError：名称'n'没有定义？

from swampy.TurtleWorld import *

world = TurtleWorld
bob = Turtle()

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):
        fd(t, length)
        lt(t, 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)

polygon(bob, n=7, length=70)
from swampy.TurtleWorld import *

world = Turtl