19.1.2. "email.parser": Parsing email messages
**********************************************

**Source code:** Lib/email/parser.py

======================================================================

Message object structures can be created in one of two ways: they can
be created from whole cloth by creating an "EmailMessage" object,
adding headers using the dictionary interface, and adding payload(s)
using "set_content()" and related methods, or they can be created by
parsing a serialized representation of the email message.

The "email" package provides a standard parser that understands most
email document structures, including MIME documents.  You can pass the
parser a bytes, string or file object, and the parser will return to
you the root "EmailMessage" instance of the object structure.  For
simple, non-MIME messages the payload of this root object will likely
be a string containing the text of the message.  For MIME messages,
the root object will return "True" from its "is_multipart()" method,
and the subparts can be accessed via the payload manipulation methods,
such as "get_body()", "iter_parts()", and "walk()".

There are actually two parser interfaces available for use, the
"Parser" API and the incremental "FeedParser" API.  The "Parser" API
is most useful if you have the entire text of the message in memory,
or if the entire message lives in a file on the file system.
"FeedParser" is more appropriate when you are reading the message from
a stream which might block waiting for more input (such as reading an
email message from a socket).  The "FeedParser" can consume and parse
the message incrementally, and only returns the root object when you
close the parser.

Note that the parser can be extended in limited ways, and of course
you can implement your own parser completely from scratch.  All of the
logic that connects the "email" package's bundled parser and the
"EmailMessage" class is embodied in the "policy" class, so a custom
parser can create message object trees any way it finds necessary by
implementing custom versions of the appropriate "policy" methods.


19.1.2.1. FeedParser API
========================

The "BytesFeedParser", imported from the "email.feedparser" module,
provides an API that is conducive to incremental parsing of email
messages, such as would be necessary when reading the text of an email
message from a source that can block (such as a socket).  The
"BytesFeedParser" can of course be used to parse an email message
fully contained in a *bytes-like object*, string, or file, but the
"BytesParser" API may be more convenient for such use cases.  The
semantics and results of the two parser APIs are identical.

The "BytesFeedParser"'s API is simple; you create an instance, feed it
a bunch of bytes until there's no more to feed it, then close the
parser to retrieve the root message object.  The "BytesFeedParser" is
extremely accurate when parsing standards-compliant messages, and it
does a very good job of parsing non-compliant messages, providing
information about how a message was deemed broken.  It will populate a
message object's "defects" attribute with a list of any problems it
found in a message.  See the "email.errors" module for the list of
defects that it can find.

Here is the API for the "BytesFeedParser":

class email.parser.BytesFeedParser(_factory=None, *, policy=policy.compat32)

   Create a "BytesFeedParser" instance.  Optional *_factory* is a no-
   argument callable; if not specified use the "message_factory" from
   the *policy*.  Call *_factory* whenever a new message object is
   needed.

   If *policy* is specified use the rules it specifies to update the
   representation of the message.  If *policy* is not set, use the
   "compat32" policy, which maintains backward compatibility with the
   Python 3.2 version of the email package and provides "Message" as
   the default factory.  All other policies provide "EmailMessage" as
   the default *_factory*. For more information on what else *policy*
   controls, see the "policy" documentation.

   Note: **The policy keyword should always be specified**; The
   default will change to "email.policy.default" in a future version
   of Python.

   3.2 新版功能.

   在 3.3 版更改: Added the *policy* keyword.

   在 3.6 版更改: *_factory* defaults to the policy "message_factory".

   feed(data)

      Feed the parser some more data.  *data* should be a *bytes-like
      object* containing one or more lines.  The lines can be partial
      and the parser will stitch such partial lines together properly.
      The lines can have any of the three common line endings:
      carriage return, newline, or carriage return and newline (they
      can even be mixed).

   close()

      Complete the parsing of all previously fed data and return the
      root message object.  It is undefined what happens if "feed()"
      is called after this method has been called.

class email.parser.FeedParser(_factory=None, *, policy=policy.compat32)

   Works like "BytesFeedParser" except that the input to the "feed()"
   method must be a string.  This is of limited utility, since the
   only way for such a message to be valid is for it to contain only
   ASCII text or, if "utf8" is "True", no binary attachments.

   在 3.3 版更改: Added the *policy* keyword.


19.1.2.2. Parser API
====================

The "BytesParser" class, imported from the "email.parser" module,
provides an API that can be used to parse a message when the complete
contents of the message are available in a *bytes-like object* or
file.  The "email.parser" module also provides "Parser" for parsing
strings, and header-only parsers, "BytesHeaderParser" and
"HeaderParser", which can be used if you're only interested in the
headers of the message.  "BytesHeaderParser" and "HeaderParser" can be
much faster in these situations, since they do not attempt to parse
the message body, instead setting the payload to the raw body.

class email.parser.BytesParser(_class=None, *, policy=policy.compat32)

   Create a "BytesParser" instance.  The *_class* and *policy*
   arguments have the same meaning and semantics as the *_factory* and
   *policy* arguments of "BytesFeedParser".

   Note: **The policy keyword should always be specified**; The
   default will change to "email.policy.default" in a future version
   of Python.

   在 3.3 版更改: Removed the *strict* argument that was deprecated in
   2.4.  Added the *policy* keyword.

   在 3.6 版更改: *_class* defaults to the policy "message_factory".

   parse(fp, headersonly=False)

      Read all the data from the binary file-like object *fp*, parse
      the resulting bytes, and return the message object.  *fp* must
      support both the "readline()" and the "read()" methods.

      The bytes contained in *fp* must be formatted as a block of
      **RFC 5322** (or, if "utf8" is "True", **RFC 6532**) style
      headers and header continuation lines, optionally preceded by an
      envelope header.  The header block is terminated either by the
      end of the data or by a blank line.  Following the header block
      is the body of the message (which may contain MIME-encoded
      subparts, including subparts with a *Content-Transfer-Encoding*
      of "8bit").

      Optional *headersonly* is a flag specifying whether to stop
      parsing after reading the headers or not.  The default is
      "False", meaning it parses the entire contents of the file.

   parsebytes(bytes, headersonly=False)

      Similar to the "parse()" method, except it takes a *bytes-like
      object* instead of a file-like object.  Calling this method on a
      *bytes-like object* is equivalent to wrapping *bytes* in a
      "BytesIO" instance first and calling "parse()".

      Optional *headersonly* is as with the "parse()" method.

   3.2 新版功能.

class email.parser.BytesHeaderParser(_class=None, *, policy=policy.compat32)

   Exactly like "BytesParser", except that *headersonly* defaults to
   "True".

   3.3 新版功能.

class email.parser.Parser(_class=None, *, policy=policy.compat32)

   This class is parallel to "BytesParser", but handles string input.

   在 3.3 版更改: Removed the *strict* argument.  Added the *policy*
   keyword.

   在 3.6 版更改: *_class* defaults to the policy "message_factory".

   parse(fp, headersonly=False)

      Read all the data from the text-mode file-like object *fp*,
      parse the resulting text, and return the root message object.
      *fp* must support both the "readline()" and the "read()" methods
      on file-like objects.

      Other than the text mode requirement, this method operates like
      "BytesParser.parse()".

   parsestr(text, headersonly=False)

      Similar to the "parse()" method, except it takes a string object
      instead of a file-like object.  Calling this method on a string
      is equivalent to wrapping *text* in a "StringIO" instance first
      and calling "parse()".

      Optional *headersonly* is as with the "parse()" method.

class email.parser.HeaderParser(_class=None, *, policy=policy.compat32)

   Exactly like "Parser", except that *headersonly* defaults to
   "True".

Since creating a message object structure from a string or a file
object is such a common task, four functions are provided as a
convenience.  They are available in the top-level "email" package
namespace.

email.message_from_bytes(s, _class=None, *, policy=policy.compat32)

   Return a message object structure from a *bytes-like object*.  This
   is equivalent to "BytesParser().parsebytes(s)".  Optional *_class*
   and *policy* are interpreted as with the "BytesParser" class
   constructor.

   3.2 新版功能.

   在 3.3 版更改: Removed the *strict* argument.  Added the *policy*
   keyword.

email.message_from_binary_file(fp, _class=None, *, policy=policy.compat32)

   Return a message object structure tree from an open binary *file
   object*.  This is equivalent to "BytesParser().parse(fp)".
   *_class* and *policy* are interpreted as with the "BytesParser"
   class constructor.

   3.2 新版功能.

   在 3.3 版更改: Removed the *strict* argument.  Added the *policy*
   keyword.

email.message_from_string(s, _class=None, *, policy=policy.compat32)

   Return a message object structure from a string.  This is
   equivalent to "Parser().parsestr(s)".  *_class* and *policy* are
   interpreted as with the "Parser" class constructor.

   在 3.3 版更改: Removed the *strict* argument.  Added the *policy*
   keyword.

email.message_from_file(fp, _class=None, *, policy=policy.compat32)

   Return a message object structure tree from an open *file object*.
   This is equivalent to "Parser().parse(fp)".  *_class* and *policy*
   are interpreted as with the "Parser" class constructor.

   在 3.3 版更改: Removed the *strict* argument.  Added the *policy*
   keyword.

   在 3.6 版更改: *_class* defaults to the policy "message_factory".

Here's an example of how you might use "message_from_bytes()" at an
interactive Python prompt:

   >>> import email
   >>> msg = email.message_from_bytes(myBytes)  


19.1.2.3. Additional notes
==========================

Here are some notes on the parsing semantics:

* Most non-*multipart* type messages are parsed as a single message
  object with a string payload.  These objects will return "False" for
  "is_multipart()", and "iter_parts()" will yield an empty list.

* All *multipart* type messages will be parsed as a container
  message object with a list of sub-message objects for their payload.
  The outer container message will return "True" for "is_multipart()",
  and "iter_parts()" will yield a list of subparts.

* Most messages with a content type of *message/** (such as *message
  /delivery-status* and *message/rfc822*) will also be parsed as
  container object containing a list payload of length 1.  Their
  "is_multipart()" method will return "True". The single element
  yielded by "iter_parts()" will be a sub-message object.

* Some non-standards-compliant messages may not be internally
  consistent about their *multipart*-edness.  Such messages may have a
  *Content-Type* header of type *multipart*, but their
  "is_multipart()" method may return "False". If such messages were
  parsed with the "FeedParser", they will have an instance of the
  "MultipartInvariantViolationDefect" class in their *defects*
  attribute list.  See "email.errors" for details.
