19.1.2. email.parser: Parsing email messages

Source code: Lib/email/parser.py

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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.

New in version 3.2.

Changed in version 3.3: Added the policy keyword.

Changed in version 3.6: _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.

Changed in version 3.3: 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 sematnics 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.

Changed in version 3.3: Removed the strict argument that was deprecated in 2.4. Added the policy keyword.

Changed in version 3.6: _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.

New in version 3.2.

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

Exactly like BytesParser, except that headersonly defaults to True.

New in version 3.3.

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

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

Changed in version 3.3: Removed the strict argument. Added the policy keyword.

Changed in version 3.6: _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 strict are interpreted as with the BytesParser class constructor.

New in version 3.2.

Changed in version 3.3: Removed the strict argument. Added the policy keyword.

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.

New in version 3.2.

Changed in version 3.3: 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.

Changed in version 3.3: 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.

Changed in version 3.3: Removed the strict argument. Added the policy keyword.

Changed in version 3.6: _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.