请教一个有关字符串format的用法

qxsw2016

两种方式处理字符串，输出结果不一致，是什么问题？
def writefile(elem,filename):
    filename.format(elem)

    return filename

elem = 't'
fn = r'E:\MeteoInfo\{0}'
writefile(elem,fn)
print fn
elem = 't'
fn = r'E:\MeteoInfo\{0}'
print fn.format(elem)