python文件讀取，刪除特定行數內容，然後將後面數據前移

# -- coding: utf-8 --
recipientAddrs="127.0.0.1"
subject=u"主題"
content=u"內容"
dirpath=u"路徑"
filename=u"name"
#寫入文件內容
with open("a.txt", "a")as f:
    f.writelines(recipientAddrs + "\n")
    f.writelines(subject + "\n")
    f.writelines(content + "\n")
    f.writelines(dirpath + "\n")
    f.writelines(filename)
    f.writelines("\n")
#讀取文件內容 獲取變量
with open("a.txt","r")as f:
    a=f.readlines()
    recipientAddrs=a[0][:-1]
    subject=a[1][:-1]
    content=a[2][:-1]
    dirpath=a[3][:-1]
    filename=a[4][:-1]
    print(filename)
    #清空文件內容前5行內容，重新寫入文件，
    with open("a.txt", 'r')as f:
        a = f.readlines()
    with open("a.txt", 'w')as f:
        b = ''.join(a[5:])
        f.write(b)