linux python2.7 编写程序读取yaml文件中的内容时报错

linux python2.7 编写程序读取yaml文件中的内容时，报yaml.constructor.ConstructorError:while constructing a mapping

程序内：

with open(yaml_file, 'r') as f:
    cfg = yaml.safe_load(f)
print cfg

执行时报：yaml.constructor.ConstructorError:while constructing a mapping

试试：cfg = yaml.safe_load(f.read())

with open(yaml_file, 'r') as f:
    cfg = yaml.safe_load(f.read())
print cfg