Python 电子邮件包：如何可靠地将多部分消息转换/解码为 str

Question

我试图用 Python 处理旧的、可能不合规的电子邮件。我可以毫无问题地阅读消息：

In [1]: m=email.message_from_binary_file(open('/path/to/problematic:2,S',mode='rb'))

但随后将其转换为字符串给出了 UnicodeEncodeError: 'gb2312' codec can't encode character '\ufffd' in position 1238: 非法多字节序列。此有问题的消息的（多）部分具有“Content-Type: text/plain; charset="gb2312" 和“Content-Transfer-Encoding: 8bit”。

In [2]: m.as_string()
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-26-919a3a20e7d8> in <module>()
----> 1 m.as_string()

~/tools/conda/envs/conda3.6/lib/python3.6/email/message.py in as_string(self, unixfrom, maxheaderlen, policy)
    156                       maxheaderlen=maxheaderlen,
    157                       policy=policy)
--> 158         g.flatten(self, unixfrom=unixfrom)
    159         return fp.getvalue()
    160

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in flatten(self, msg, unixfrom, linesep)
    114                     ufrom = 'From nobody ' + time.ctime(time.time())
    115                 self.write(ufrom + self._NL)
--> 116             self._write(msg)
    117         finally:
    118             self.policy = old_gen_policy

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in _write(self, msg)
    179             self._munge_cte = None
    180             self._fp = sfp = self._new_buffer()
--> 181             self._dispatch(msg)
    182         finally:
    183             self._fp = oldfp

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in _dispatch(self, msg)
    212             if meth is None:
    213                 meth = self._writeBody
--> 214         meth(msg)
    215
    216     #

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in _handle_multipart(self, msg)
    270             s = self._new_buffer()
    271             g = self.clone(s)
--> 272             g.flatten(part, unixfrom=False, linesep=self._NL)
    273             msgtexts.append(s.getvalue())
    274         # BAW: What about boundaries that are wrapped in double-quotes?

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in flatten(self, msg, unixfrom, linesep)
    114                     ufrom = 'From nobody ' + time.ctime(time.time())
    115                 self.write(ufrom + self._NL)
--> 116             self._write(msg)
    117         finally:
    118             self.policy = old_gen_policy

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in _write(self, msg)
    179             self._munge_cte = None
    180             self._fp = sfp = self._new_buffer()
--> 181             self._dispatch(msg)
    182         finally:
    183             self._fp = oldfp

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in _dispatch(self, msg)
    212             if meth is None:
    213                 meth = self._writeBody
--> 214         meth(msg)
    215
    216     #

~/tools/conda/envs/conda3.6/lib/python3.6/email/generator.py in _handle_text(self, msg)
    241                 msg = deepcopy(msg)
    242                 del msg['content-transfer-encoding']
--> 243                 msg.set_payload(payload, charset)
    244                 payload = msg.get_payload()
    245                 self._munge_cte = (msg['content-transfer-encoding'],

~/tools/conda/envs/conda3.6/lib/python3.6/email/message.py in set_payload(self, payload, charset)
    313             if not isinstance(charset, Charset):
    314                 charset = Charset(charset)
--> 315             payload = payload.encode(charset.output_charset)
    316         if hasattr(payload, 'decode'):
    317             self._payload = payload.decode('ascii', 'surrogateescape')

UnicodeEncodeError: 'gb2312' codec can't encode character '\ufffd' in position 1238: illegal multibyte sequence

我对电子邮件内部结构的特性并不十分熟悉，在网上搜索此类错误时，大多是在抓取网页时出现问题，并且基本上暗示了一些显而易见的问题：读入的原始字节包含 Unicode 字符，这些字符是无法使用目标编解码器进行编码。

我的问题是：可靠处理（可能不合规）电子邮件的正确方法是什么？

编辑

有趣的是m.get_payload(i=0).as_string() 会触发相同的异常，但m.get_payload(i=0).get_payload(decode=False) 给出了一个str，在我的终端上正确显示，而m.get_payload(i=0).get_payload(decode=True) 给出了一个bytes (b'\xd7\xaa...')，我可以' t 解码。但是，错误发生在 不同的字符上：

----> 1 m.get_payload(i=0).get_payload(decode=True).decode('gb2312')
UnicodeDecodeError: 'gb2312' codec can't decode byte 0xac in position 1995: illegal multibyte sequence

或

----> 1 m.get_payload(i=0).get_payload(decode=True).decode('gb18030')
UnicodeDecodeError: 'gb18030' codec can't decode byte 0xa3 in position 2033: illegal multibyte sequence

Answer 1

在您的bytes.decode 电话中，简短的回答通常是error handlers。但细节取决于很多事情。

首先，您要如何处理这些数据？通常你需要一些绝对可逆的东西，所以你可以保证在最坏的情况下你可以重新生成你输入的东西，在这种情况下你可能想要surrogate-escape。在其他情况下，您想要生成人类可读的东西，最好跳过不可能的 mojibake 而不是尝试呈现它，所以ignore 可能是正确的答案。以此类推。

其次，这是绝大多数消息正常但少数错误的情况，还是许多消息大部分正常但有一些错误的情况？

最后，在某些情况下（对于传统的中文编码尤其如此），实际问题只是有人指定了一个密切相关的字符集，而不是他们实际使用的字符集。如果这就是您所看到的，您可能想尝试编写显式后备代码：如果遇到异常，请在常见错误的字典中查找编码并尝试替代编码。如果它们都不起作用，则返回使用带有错误处理程序的特定编码。

Answer 2

显然，如果Content-Transfer-Encoding 是8bit，message.get_payload(decode=False) 仍会尝试对其进行解码以恢复原始字节。另一方面，message.get_payload(decode=True) 总是产生bytes，尽管只有当Content-Transfer-Encoding 存在并且是quoted-printable 或base64 时才会发生实际解码。

我最终得到了以下代码。不确定这是否是处理电子邮件的正确方法。

body = []
if m.preamble is not None:
    body.extend(m.preamble.splitlines(keepends=True))

for part in m.walk():
    if part.is_multipart():
        continue

    ctype = part.get_content_type()
    cte = part.get_params(header='Content-Transfer-Encoding')
    if (ctype is not None and not ctype.startswith('text')) or \
       (cte is not None and cte[0][0].lower() == '8bit'):
        part_body = part.get_payload(decode=False)
    else:
        charset = part.get_content_charset()
        if charset is None or len(charset) == 0:
            charsets = ['ascii', 'utf-8']
        else:
            charsets = [charset]

        part_body = part.get_payload(decode=True)
        for enc in charsets:
            try:
                part_body = part_body.decode(enc)
                break
            except UnicodeDecodeError as ex:
                continue
            except LookupError as ex:
                continue
        else:
            part_body = part.get_payload(decode=False)

    body.extend(part_body.splitlines(keepends=True))

if m.epilogue is not None:
    body.extend(m.epilogue.splitlines(keepends=True))