在Unicode文本文件上接收App Engine Python中的电子邮件附件错误
问题内容：
我有一些代码可以解析电子邮件并找到附件，然后将其作为db.BlobProperties存储到数据存储中（以后可以将其更改为Blobstore）。问题是，当我发送UTF8编码的文本文件时，它将生成错误。
该代码基本上保存了文件并返回一个键，该键被转换为字符串，然后存储在父电子邮件实体中。如您所见，我先解码文件，然后将其存储为Blob。我已经发送了许多附件，并且此代码适用于除使用Unicode编码的文本以外的所有内容。有一个更好的方法吗？如何处理Unicode文本附件？
代码嗅探
my_file = []
my_list = []
if hasattr(mail_message, 'attachments'):
file_name = ""
file_blob = ""
for filename, filecontents in mail_message.attachments:
file_name = filename
file_blob = filecontents.decode()
my_file.append(file_name)
my_list.append(str(store_file(self, file_name, file_blob)))
store_file
def store_file(self, file_name, file_blob):
new_file = myblob(file_name = file_name,
file_blob = file_blob)
return new_file.put()
我尝试file_blob = str(file_blob)在上面使用无济于事。这只会破坏代码，文件永远不会被存储。
Unicode文本文件的日志1
Property file_blob must be convertible to a Blob instance (Blob() argument should be str instance, not unicode)
Traceback (most recent call last):
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 1530, in __call__
rv = self.router.dispatch(request, response)
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 1278, in default_dispatcher
return route.handler_adapter(request, response)
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 1102, in __call__
return handler.dispatch()
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 572, in dispatch
return self.handle_exception(e, self.app.debug)
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 570, in dispatch
return method(*args, **kwargs)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/webapp/mail_handlers.py", line 65, in post
self.receive(mail.InboundEmailMessage(self.request.body))
File "/base/data/home/apps/s~ae-baseapp/1.359073377819595139/controllers/InboundHandler.py", line 51, in receive
file_list.append(str(store_file(self, file_name, file_blob)))
File "/base/data/home/apps/s~ae-baseapp/1.359073377819595139/models/MyModel.py", line 63, in store_file
file_blob = file_blob)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/db/__init__.py", line 974, in __init__
prop.__set__(self, value)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/db/__init__.py", line 614, in __set__
value = self.validate(value)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/db/__init__.py", line 2780, in validate
(self.name, self.data_type.__name__, err))
BadValueError: Property file_blob must be convertible to a Blob instance (Blob() argument should be str instance, not unicode)
记录2，删除filecontents.decode（）并将其替换为filecontents。
Property file_blob must be convertible to a Blob instance (Blob() argument should be str instance, not EncodedPayload)
Traceback (most recent call last):
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 1530, in __call__
rv = self.router.dispatch(request, response)
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 1278, in default_dispatcher
return route.handler_adapter(request, response)
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 1102, in __call__
return handler.dispatch()
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 572, in dispatch
return self.handle_exception(e, self.app.debug)
File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.5.1/webapp2.py", line 570, in dispatch
return method(*args, **kwargs)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/webapp/mail_handlers.py", line 65, in post
self.receive(mail.InboundEmailMessage(self.request.body))
File "/base/data/home/apps/s~ae-baseapp/1.359097282640216691/controllers/InboundHandler.py", line 57, in receive
file_list.append(str(store_file(self, file_name, file_blob)))
File "/base/data/home/apps/s~ae-baseapp/1.359097282640216691/models/MyModel.py", line 64, in store_file
file_blob = file_blob)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/db/__init__.py", line 974, in __init__
prop.__set__(self, value)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/db/__init__.py", line 614, in __set__
value = self.validate(value)
File "/base/python27_runtime/python27_lib/versions/1/google/appengine/ext/db/__init__.py", line 2780, in validate
(self.name, self.data_type.__name__, err))
BadValueError: Property file_blob must be convertible to a Blob instance (Blob() argument should be str instance, not EncodedPayload)
问题答案：
附件有效负载是EncodedPayload类的实例。附件具有编码和可选字符集。前者是指传输编码，例如base64；后者用于字符编码，例如UTF-8（此处的字符集有点过时和误导性的术语）。该EncodedPayload.decode()方法对传输编码和文本编码都进行解码，正如您所注意到的，如果您只想获取用户附加到其邮件中的原始字节，则该方法不是很有用。
您可以在此处采用多种方法，但是我建议您复制EncodedPayload的逻辑来解码传输编码，如下所示：
if filecontents.encoding and filecontents.encoding.lower() != '7bit':
try:
payload = filecontents.payload.decode(filecontents.encoding)
except LookupError:
raise UnknownEncodingError('Unknown decoding %s.' % filecontents.encoding)
except (Exception, Error), e:
raise PayloadEncodingError('Could not decode payload: %s' % e)
else:
payload = filecontents.payload
请注意，如果附件 是 文本，则在存储附件 时 需要包括字符编码，否则当您将附件发送回用户时将无法解释它-原始文本可能已使用任何字符编码进行了编码。
同样，如果可以的话，您还应该保存附件的模仿类型，但这在API中的任何地方都没有公开。您可能要考虑完全避免使用IncomingMessage类，而是使用Python的mime消息模块对POST请求的主体进行解码。
