关于python：我如何保存到.txt文件而不覆盖其中已有的所有内容？

How can I save to a .txt file without overwriting everything that was already in there?

本问题已经有最佳答案，请猛点这里访问。

在我的空闲时间，我正在制作一个非常基本的操作系统。但是，我正在尝试这样做，以便您可以拥有尽可能多的用户，但是每次我创建一个新用户时，它都会删除旧的用户。到目前为止，我有：

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def typer():
print("Start typing to get started. Unfortunately, you cannot currently save your files.")
typerCMD = input(" ")
CMDLine()

def CMDLine():
print("Hello, and welcome to your new operating system. Type 'help' to get started.")
cmd = input("~$:")
if cmd == ("help"):
print("Use the 'leave' command to shut down the system. Use the 'type' command to start a text editor.")
cmdLvl2 = input("~$:")
if cmdLvl2 == ("leave"):
quit()
if cmdLvl2 == ("type"):
typer()

def redirect():
signIn()

def mUserRedirect():
makeUser()

def PwordSignIn():
rPword = input("Password:")
with open('passwords.txt', 'r') as f:
for line in f:
print(line)
if rPword == (line):
CMDLine()
else:
print("Incorrect password.")
signIn()

def signIn():
rUname = input("Username:")
with open('usernames.txt', 'r') as f:
for line in f:
print(line)
if rUname == (line):
PwordSignIn()
else:
print("Username not found.")
mUserRedirect()

def makeUser():
nUname = input("New username:")
nPword = input("Create a password for the user:")

with open('usernames.txt', 'w') as f:
f.write(nUname)
with open('passwords.txt', 'w') as f:
f.write(nPword)
signIn()

print("Create a new user? (Y/N)")
nUser = input("")
if nUser == ("N"):
signIn()
if nUser == ("n"):
signIn()
if nUser == ("Y"):
makeUser()
if nUser == ("y"):
makeUser()

那么，我如何才能在不删除文件中已有的所有内容的情况下写入该文件呢？