为什么我在每行的开头和结尾都有“b”?

我有一个使用pxsh的python脚本。剧本如下:

from pexpect import pxssh
import getpass
try:
    s = pxssh.pxssh()
    hostname = input('hostname: ')
    username = input('username: ')
    password = getpass.getpass('password: ')
    s.login(hostname, username, password)
    s.sendline('cat hiera/my.yaml')   # run a command
    s.prompt()             # match the prompt
    for line in s.before.splitlines()[1:]:
       print (line)
    s.logout()
except pxssh.ExceptionPxssh as e:
    print("pxssh failed on login.")
    print(e)

b'---'
b'settings_prod: |'
b'"""'
b"Generated by 'django-admin startproject' using Django 2.0.5."
b''
b'For more information on this file, see'
b'https://docs.djangoproject.com/en/2.0/topics/settings/'
b''
b'For the full list of settings and their values, see'
b'https://docs.djangoproject.com/en/2.0/ref/settings/'
b'"""'

这是怎么回事 b' 在每行和 '