cpython/Doc/library/email.parser.rst

292 lines
13 KiB
ReStructuredText

:mod:`email`: Parsing email messages
------------------------------------
.. module:: email.parser
:synopsis: Parse flat text email messages to produce a message object structure.
Message object structures can be created in one of two ways: they can be created
from whole cloth by instantiating :class:`~email.message.Message` objects and
stringing them together via :meth:`attach` and :meth:`set_payload` calls, or they
can be created by parsing a flat text representation of the email message.
The :mod:`email` package provides a standard parser that understands most email
document structures, including MIME documents. You can pass the parser a string
or a file object, and the parser will return to you the root
:class:`~email.message.Message` 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 :meth:`is_multipart` method, and the subparts can be
accessed via the :meth:`get_payload` and :meth:`walk` methods.
There are actually two parser interfaces available for use, the classic
:class:`Parser` API and the incremental :class:`FeedParser` API. The classic
:class:`Parser` API is fine if you have the entire text of the message in memory
as a string, or if the entire message lives in a file on the file system.
:class:`FeedParser` is more appropriate for when you're reading the message from
a stream which might block waiting for more input (e.g. reading an email message
from a socket). The :class:`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. There is no magical
connection between the :mod:`email` package's bundled parser and the
:class:`~email.message.Message` class, so your custom parser can create message
object trees any way it finds necessary.
FeedParser API
^^^^^^^^^^^^^^
The :class:`FeedParser`, imported from the :mod:`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 (e.g. a socket). The :class:`FeedParser` can of course be used
to parse an email message fully contained in a string or a file, but the classic
:class:`Parser` API may be more convenient for such use cases. The semantics
and results of the two parser APIs are identical.
The :class:`FeedParser`'s API is simple; you create an instance, feed it a bunch
of text until there's no more to feed it, then close the parser to retrieve the
root message object. The :class:`FeedParser` 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 :mod:`email.errors` module for the
list of defects that it can find.
Here is the API for the :class:`FeedParser`:
.. class:: FeedParser(_factory=email.message.Message, *, policy=policy.default)
Create a :class:`FeedParser` instance. Optional *_factory* is a no-argument
callable that will be called whenever a new message object is needed. It
defaults to the :class:`email.message.Message` class.
The *policy* keyword specifies a :mod:`~email.policy` object that controls a
number of aspects of the parser's operation. The default policy maintains
backward compatibility.
.. versionchanged:: 3.3 Added the *policy* keyword.
.. method:: feed(data)
Feed the :class:`FeedParser` some more data. *data* should be a string
containing one or more lines. The lines can be partial and the
:class:`FeedParser` will stitch such partial lines together properly. The
lines in the string can have any of the common three line endings,
carriage return, newline, or carriage return and newline (they can even be
mixed).
.. method:: close()
Closing a :class:`FeedParser` completes the parsing of all previously fed
data, and returns the root message object. It is undefined what happens
if you feed more data to a closed :class:`FeedParser`.
.. class:: BytesFeedParser(_factory=email.message.Message)
Works exactly like :class:`FeedParser` except that the input to the
:meth:`~FeedParser.feed` method must be bytes and not string.
.. versionadded:: 3.2
Parser class API
^^^^^^^^^^^^^^^^
The :class:`Parser` class, imported from the :mod:`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 string or file. The :mod:`email.parser`
module also provides header-only parsers, called :class:`HeaderParser` and
:class:`BytesHeaderParser`, which can be used if you're only interested in the
headers of the message. :class:`HeaderParser` and :class:`BytesHeaderParser`
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 as a string. They
have the same API as the :class:`Parser` and :class:`BytesParser` classes.
.. versionadded:: 3.3 BytesHeaderParser
.. class:: Parser(_class=email.message.Message, *, policy=policy.default)
The constructor for the :class:`Parser` class takes an optional argument
*_class*. This must be a callable factory (such as a function or a class), and
it is used whenever a sub-message object needs to be created. It defaults to
:class:`~email.message.Message` (see :mod:`email.message`). The factory will
be called without arguments.
The *policy* keyword specifies a :mod:`~email.policy` object that controls a
number of aspects of the parser's operation. The default policy maintains
backward compatibility.
.. versionchanged:: 3.3
Removed the *strict* argument that was deprecated in 2.4. Added the
*policy* keyword.
The other public :class:`Parser` methods are:
.. method:: parse(fp, headersonly=False)
Read all the data from the file-like object *fp*, parse the resulting
text, and return the root message object. *fp* must support both the
:meth:`readline` and the :meth:`read` methods on file-like objects.
The text contained in *fp* must be formatted as a block of :rfc:`2822`
style headers and header continuation lines, optionally preceded by a
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).
Optional *headersonly* is as with the :meth:`parse` method.
.. method:: parsestr(text, headersonly=False)
Similar to the :meth:`parse` method, except it takes a string object
instead of a file-like object. Calling this method on a string is exactly
equivalent to wrapping *text* in a :class:`~io.StringIO` instance first and
calling :meth:`parse`.
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.
.. class:: BytesParser(_class=email.message.Message, *, policy=policy.default)
This class is exactly parallel to :class:`Parser`, but handles bytes input.
The *_class* and *strict* arguments are interpreted in the same way as for
the :class:`Parser` constructor.
The *policy* keyword specifies a :mod:`~email.policy` object that
controls a number of aspects of the parser's operation. The default
policy maintains backward compatibility.
.. versionchanged:: 3.3
Removed the *strict* argument. Added the *policy* keyword.
.. method:: parse(fp, headeronly=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 :meth:`readline` and the :meth:`read` methods on file-like
objects.
The bytes contained in *fp* must be formatted as a block of :rfc:`2822`
style headers and header continuation lines, optionally preceded by a
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 :mailheader:`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.
.. method:: parsebytes(bytes, headersonly=False)
Similar to the :meth:`parse` method, except it takes a byte string object
instead of a file-like object. Calling this method on a byte string is
exactly equivalent to wrapping *text* in a :class:`~io.BytesIO` instance
first and calling :meth:`parse`.
Optional *headersonly* is as with the :meth:`parse` method.
.. versionadded:: 3.2
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 :mod:`email` package namespace.
.. currentmodule:: email
.. function:: message_from_string(s, _class=email.message.Message, *, \
policy=policy.default)
Return a message object structure from a string. This is exactly equivalent to
``Parser().parsestr(s)``. *_class* and *policy* are interpreted as
with the :class:`Parser` class constructor.
.. versionchanged:: 3.3
Removed the *strict* argument. Added the *policy* keyword.
.. function:: message_from_bytes(s, _class=email.message.Message, *, \
policy=policy.default)
Return a message object structure from a byte string. This is exactly
equivalent to ``BytesParser().parsebytes(s)``. Optional *_class* and
*strict* are interpreted as with the :class:`Parser` class constructor.
.. versionadded:: 3.2
.. versionchanged:: 3.3
Removed the *strict* argument. Added the *policy* keyword.
.. function:: message_from_file(fp, _class=email.message.Message, *, \
policy=policy.default)
Return a message object structure tree from an open :term:`file object`.
This is exactly equivalent to ``Parser().parse(fp)``. *_class*
and *policy* are interpreted as with the :class:`Parser` class constructor.
.. versionchanged::
Removed the *strict* argument. Added the *policy* keyword.
.. function:: message_from_binary_file(fp, _class=email.message.Message, *, \
policy=policy.default)
Return a message object structure tree from an open binary :term:`file
object`. This is exactly equivalent to ``BytesParser().parse(fp)``.
*_class* and *policy* are interpreted as with the :class:`Parser`
class constructor.
.. versionadded:: 3.2
.. versionchanged:: 3.3
Removed the *strict* argument. Added the *policy* keyword.
Here's an example of how you might use this at an interactive Python prompt::
>>> import email
>>> msg = email.message_from_string(myString)
Additional notes
^^^^^^^^^^^^^^^^
Here are some notes on the parsing semantics:
* Most non-\ :mimetype:`multipart` type messages are parsed as a single message
object with a string payload. These objects will return ``False`` for
:meth:`is_multipart`. Their :meth:`get_payload` method will return a string
object.
* All :mimetype:`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 :meth:`is_multipart` and their
:meth:`get_payload` method will return the list of :class:`~email.message.Message`
subparts.
* Most messages with a content type of :mimetype:`message/\*` (e.g.
:mimetype:`message/delivery-status` and :mimetype:`message/rfc822`) will also be
parsed as container object containing a list payload of length 1. Their
:meth:`is_multipart` method will return ``True``. The single element in the
list payload will be a sub-message object.
* Some non-standards compliant messages may not be internally consistent about
their :mimetype:`multipart`\ -edness. Such messages may have a
:mailheader:`Content-Type` header of type :mimetype:`multipart`, but their
:meth:`is_multipart` method may return ``False``. If such messages were parsed
with the :class:`FeedParser`, they will have an instance of the
:class:`MultipartInvariantViolationDefect` class in their *defects* attribute
list. See :mod:`email.errors` for details.
.. rubric:: Footnotes
.. [#] As of email package version 3.0, introduced in Python 2.4, the classic
:class:`Parser` was re-implemented in terms of the :class:`FeedParser`, so the
semantics and results are identical between the two parsers.