format {}占位符 {:s}

import ospath = '/home/user/project'
filename = 'example'
annopath = os.path.join(path, "Annotations", "{}.xml")xml_path = annopath.format(filename)
print(xml_path)

结果： 

/home/user/project\Annotations\example.xml

例2：

x="{:s}xxx"
y=x.format('888')
print(y)

 结果

888xxx