python基础问题求解

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

import turtle as t
color = ['green','red','blue']
r = ['20','50','60']
for i in range(3):
t.pu()
t.goto(0,-r[i])
t.pd()
t.pencolor(color[i])
t.circle(r[i])
t.done()

运行结果及报错内容

File "C:\pycharm\武球王.py", line 7, in
t.goto(0,-r[i])
TypeError: bad operand type for unary -: 'str'

我的解答思路和尝试过的方法

我尝试过将减号去掉，但是没用，依旧报错

我想要达到的结果

正确运行

import turtle as t
color = ['green','red','blue']
r = ['20','50','60']
for i in range(3):
    t.pu()
    t.goto(0,-eval(r[i]))
    t.pd()
    t.pencolor(color[i])
    t.circle(r[i])
t.done()

或者：

import turtle as t
color = ['green','red','blue']
r = [20,50,60]
for i in range(3):
    t.pu()
    t.goto(0,-r[i])
    t.pd()
    t.pencolor(color[i])
    t.circle(r[i])
t.done()