18.1.8. "email.errors": Exception and Defect classes
****************************************************

The following exception classes are defined in the "email.errors"
module:

exception email.errors.MessageError

   This is the base class for all exceptions that the "email" package
   can raise.  It is derived from the standard "Exception" class and
   defines no additional methods.

exception email.errors.MessageParseError

   This is the base class for exceptions raised by the "Parser" class.
   It is derived from "MessageError".

exception email.errors.HeaderParseError

   Raised under some error conditions when parsing the **RFC 2822**
   headers of a message, this class is derived from
   "MessageParseError". It can be raised from the "Parser.parse" or
   "Parser.parsestr" methods.

   Situations where it can be raised include finding an envelope
   header after the first **RFC 2822** header of the message, finding
   a continuation line before the first **RFC 2822** header is found,
   or finding a line in the headers which is neither a header or a
   continuation line.

exception email.errors.BoundaryError

   Raised under some error conditions when parsing the **RFC 2822**
   headers of a message, this class is derived from
   "MessageParseError". It can be raised from the "Parser.parse" or
   "Parser.parsestr" methods.

   Situations where it can be raised include not being able to find
   the starting or terminating boundary in a *multipart/** message
   when strict parsing is used.

exception email.errors.MultipartConversionError

   Raised when a payload is added to a "Message" object using
   "add_payload()", but the payload is already a scalar and the
   message’s *Content-Type* main type is not either *multipart* or
   missing.  "MultipartConversionError" multiply inherits from
   "MessageError" and the built-in "TypeError".

   Since "Message.add_payload()" is deprecated, this exception is
   rarely raised in practice.  However the exception may also be
   raised if the "attach()" method is called on an instance of a class
   derived from "MIMENonMultipart" (e.g. "MIMEImage").

Here’s the list of the defects that the "FeedParser" can find while
parsing messages.  Note that the defects are added to the message
where the problem was found, so for example, if a message nested
inside a *multipart/alternative* had a malformed header, that nested
message object would have a defect, but the containing messages would
not.

All defect classes are subclassed from "email.errors.MessageDefect",
but this class is *not* an exception!

2.4 新版功能: All the defect classes were added.

* "NoBoundaryInMultipartDefect" – A message claimed to be a
  multipart, but had no *boundary* parameter.

* "StartBoundaryNotFoundDefect" – The start boundary claimed in the
  *Content-Type* header was never found.

* "FirstHeaderLineIsContinuationDefect" – The message had a
  continuation line as its first header line.

* "MisplacedEnvelopeHeaderDefect" - A “Unix From” header was found
  in the middle of a header block.

* "MalformedHeaderDefect" – A header was found that was missing a
  colon, or was otherwise malformed.

* "MultipartInvariantViolationDefect" – A message claimed to be a
  *multipart*, but no subparts were found.  Note that when a message
  has this defect, its "is_multipart()" method may return false even
  though its content type claims to be *multipart*.
