测试代码报错，求解答

可以看看这段测试代码为什么报错吗，照着书上敲的，书上都能运行的，实在想不通呀

错误处：

可以试试这样写：

#population可选参数
def city_country(city, country, population=-1):
    if population!=-1:
        return city.title() + ", " + country.title() + " - population " + str(population);
    return city.title() + ", " + country.title();

#test_cities
import unittest
from city_functions2 import city_country

class TestCities(unittest.TestCase):      
    def test_city_country(self):  
        format_ans=city_country('Santiago','chile')  
        self.assertEqual(format_ans,'Santiago, Chile')  
unittest.main() 

希望对题主有所帮助！可以的话，帮忙点个采纳！