未找到Turtle命令名

似乎在我进行函数调用之前,我必须不断地定义alex。例如,这是有效的:

 def polyline(t,n,length,angle):
    """Draws n line segments with the given length and 
    angle(in degrees) between them. t is a turtle.
    """
    for i in range(n):
        t.fd(length)
        t.lt(angle)


alex=turtle.Turtle() 
#Test polyline
polyline(alex,5,780,90)

失败了:

alex=turtle.Turtle() 
#insert some other functions


 #define polyline function

#call polyline