Python 3 UnboundLocalError：在try语句中赋值之前引用的局部变量

Question

我正在尝试制作一个程序来处理 RSA 加密和解密，以便使用 rsa 库发送消息。 目前我正在实施它以仅使用 1 个密钥对，但稍后会更改。 我在检查存储密钥的文件是否存在的逻辑上遇到了一些问题。 这些函数的逻辑如下所示：

import rsa

################################################
keySize = 256
################################################


def genKeys():
    pubkey, privkey = rsa.newkeys(keySize)

    with open("public.pem", "w") as pub:
        pub.write(str(pubkey))

    with open("private.pem", "w") as priv:
        priv.write(str(privkey))

    return pubkey, privkey

def readKeys():

      with open("private.pem", "r") as priv:
          keydata = priv.read()
          privkey = rsa.PrivateKey.load_pkcs1(keydata)
          print(privkey)

      with open("public.pem", "r") as pub:
          keydata = pub.read()
          pubkey = rsa.PublicKey.load_pkcs1(keydata)
          print(pubkey)
      return pubkey, privkey




def send(message, pubkey):
    message = message.encode('utf-8')
    crypto = rsa.encrypt(message, pubkey)
    return crypto

def recv(crypto, privkey):
    message = rsa.decrypt(crypto, privkey)
    message = message.decode('utf-8')
    return message

def main():
    global pubkey, privkey
    try:
        if (open("private.pem", "r") and open("public.pem", "r")) == True:

            pubkey, privkey = readKeys()

    except:
        (pubkey, privkey) = genKeys()

    cryptMessage = send('hello world', pubkey)
    print(cryptMessage)
    print(recv(cryptMessage, privkey))

if __name__ == "__main__":
    main()

main() function，特别是try: except:语句是我的问题所在。 我得到的错误是builtins.NameError: name 'pubkey' is not defined 。 我尝试将变量声明为全局变量，这是其他答案所建议的，但这对我不起作用，或者我做错了。

谢谢您的帮助。 对不起菜鸟问题。

编辑：所以，这解决了我的第一个问题。 但是，现在当我使用try/except语句时，它会尝试调用值为None的变量，即使我使用函数为它们分配了值。 堆栈跟踪是

File "", line 61, in <module>
  main()
File "", line 56, in main
  cryptMessage = send('hello world', pubkey)
File "", line 37, in send
  crypto = rsa.encrypt(message, pubkey)
File "/usr/local/lib/python3.6/dist-packages/rsa/pkcs1.py", line 169, in encrypt
  keylength = common.byte_size(pub_key.n)

builtins.AttributeError: 'NoneType' object has no attribute 'n'

Answer 1

即使您将变量声明为函数内部的全局变量，仍然需要在全局范围内或在声明变量时对其进行初始化。

import rsa

################################################
keySize = 256
################################################

# DECLARATION IN GLOBAL SCOPE
pubkey = None
privkey = None

def genKeys():
    pubkey, privkey = rsa.newkeys(keySize)

    with open("public.pem", "w") as pub:
        pub.write(str(pubkey))

    with open("private.pem", "w") as priv:
        priv.write(str(privkey))

    return pubkey, privkey

def readKeys():

      with open("private.pem", "r") as priv:
          keydata = priv.read()
          privkey = rsa.PrivateKey.load_pkcs1(keydata)
          print(privkey)

      with open("public.pem", "r") as pub:
          keydata = pub.read()
          pubkey = rsa.PublicKey.load_pkcs1(keydata)
          print(pubkey)
      return pubkey, privkey




def send(message, pubkey):
    message = message.encode('utf-8')
    crypto = rsa.encrypt(message, pubkey)
    return crypto

def recv(crypto, privkey):
    message = rsa.decrypt(crypto, privkey)
    message = message.decode('utf-8')
    return message

def main():
    # DECLARATION INSIDE THE FUNCTION
    global pubkey, privkey; pubkey = privkey = None
    try:
        if (open("private.pem", "r") and open("public.pem", "r")) == True:

            pubkey, privkey = readKeys()

    except:
        (pubkey, privkey) = genKeys()

    cryptMessage = send('hello world', pubkey)
    print(cryptMessage)
    print(recv(cryptMessage, privkey))

if __name__ == "__main__":
    main()

Answer 2

您应该在genKeys()函数外部声明变量pubkey和privkey，然后错误应消失。 这是我的第一篇文章，希望对您有所帮助。 问候。

Answer 3

您的问题在这里：

if (open("private.pem", "r") and open("public.pem", "r")) == True:

Python逻辑运算符返回最后检查的值，而不是True或False。 该值是来自open命令的file对象，不等于True。 您可以使用os.path.isfile检查文件是否存在，并完全跳过try / except块。 或者您可以尝试在try块中实际读取，就像这样

（为演示而精简）

# dummys for test...

def readKeys():
    print('try read')
    return open('public.pem').read(), open('private.pem').read()

def genKeys():
    print('try write')
    open('public.pem','w').write('foo')
    open('private.pem','w').write('bar')
    return 'foo', 'bar'


def main():
    global pubkey, privkey
    try:
        pubkey, privkey = readKeys()
    except:
        (pubkey, privkey) = genKeys()


if __name__ == "__main__":
    main()

Answer 4

复制另一个答案的代码并进行更改以检查globals()而不是在每次 function 调用时覆盖None变量。

import rsa

################################################
keySize = 256
################################################

# DECLARATION IN GLOBAL SCOPE
pubkey = None
privkey = None

def genKeys():
    pubkey, privkey = rsa.newkeys(keySize)

    with open("public.pem", "w") as pub:
        pub.write(str(pubkey))

    with open("private.pem", "w") as priv:
        priv.write(str(privkey))

    return pubkey, privkey

def readKeys():

      with open("private.pem", "r") as priv:
          keydata = priv.read()
          privkey = rsa.PrivateKey.load_pkcs1(keydata)
          print(privkey)

      with open("public.pem", "r") as pub:
          keydata = pub.read()
          pubkey = rsa.PublicKey.load_pkcs1(keydata)
          print(pubkey)
      return pubkey, privkey




def send(message, pubkey):
    message = message.encode('utf-8')
    crypto = rsa.encrypt(message, pubkey)
    return crypto

def recv(crypto, privkey):
    message = rsa.decrypt(crypto, privkey)
    message = message.decode('utf-8')
    return message

def main():
    # DECLARATION INSIDE THE FUNCTION
    global pubkey, privkey;
    if 'pubkey' not in globals() or 'privkey' not in in globals():
        pubkey = privkey = None
    try:
        if (open("private.pem", "r") and open("public.pem", "r")) == True:
            if 'pubkey' in globals() and 'privkey' in globals():
                pubkey, privkey = readKeys()
            
    except:
        if 'pubkey' in globals() and 'privkey' in globals():
            (pubkey, privkey) = genKeys()


    if 'pubkey' in globals() and 'privkey' in globals():
        cryptMessage = send('hello world', pubkey)
        print(cryptMessage)
        print(recv(cryptMessage, privkey))

if __name__ == "__main__":
    main()

这是未经测试的。 我对局部变量执行此操作，而不是在 finally 块中关闭了本地连接 object。 为此，我什至不必在任何时候将对象设置为None ，因为我在关闭它之前检查了连接 object 是否在globals()中。 错误消失了。

然而，在这段代码中，您不仅关闭了对象，还覆盖了它们，无论是在try块中还是在except块中。 因此，我只是猜测您需要在第一次加载时创建None对象。 如果这个技巧也适用于全局变量，那么这将是唯一不会在每次调用 function 时用None覆盖全局对象的代码，但前提是第一次接触到变量。