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# 18.1.2. `email.parser`: Parsing email messages

Message object structures can be created in one of two ways: they can be created from whole cloth by instantiating `Message` objects and stringing them together via `attach()` and `set_payload()` calls, or they can be created by parsing a flat text 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 string or a file object, and the parser will return to you the root `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 `is_multipart()` method, and the subparts can be accessed via the `get_payload()` and `walk()` methods.

There are actually two parser interfaces available for use, the classic `Parser` API and the incremental `FeedParser` API. The classic `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. `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 `FeedParser` can consume and parse the message incrementally, and only returns the root object when you close the parser 1.

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 `email` package’s bundled parser and the `Message` class, so your custom parser can create message object trees any way it finds necessary.

## 18.1.2.1. FeedParser API

2.4 新版功能.

The `FeedParser`, 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 (e.g. a socket). The `FeedParser` can of course be used to parse an email message fully contained in a string or a file, but the classic `Parser` API may be more convenient for such use cases. The semantics and results of the two parser APIs are identical.

The `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 `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 `email.errors` module for the list of defects that it can find.

Here is the API for the `FeedParser`:

class `email.parser.``FeedParser`([_factory])

  Create a `FeedParser` instance. Optional _factory is a no-argument callable that will be called whenever a new message object is needed. It defaults to the `email.message.Message` class.

  `feed`(data)

    Feed the `FeedParser` some more data. data should be a string containing one or more lines. The lines can be partial and the `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).

  `close`()

    Closing a `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 `FeedParser`.

## 18.1.2.2. Parser class API

The `Parser` 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 string or file. The `email.parser` module also provides a second class, called `HeaderParser` which can be used if you’re only interested in the headers of the message. `HeaderParser` can be much faster in these situations, since it does not attempt to parse the message body, instead setting the payload to the raw body as a string. `HeaderParser` has the same API as the `Parser` class.

class `email.parser.``Parser`([_class])

  The constructor for the `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 `Message` (see `email.message`). The factory will be called without arguments.

  The optional strict flag is ignored.

  2.4 版后已移除: Because the `Parser` class is a backward compatible API wrapper around the new-in-Python 2.4 `FeedParser`, all parsing is effectively non-strict. You should simply stop passing a strict flag to the `Parser` constructor.

  在 2.2.2 版更改: The strict flag was added.

  在 2.4 版更改: The strict flag was deprecated.

  The other public `Parser` methods are:

  `parse`(fp[, headersonly])

    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 `readline()` and the `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 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).

    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.

    在 2.2.2 版更改: The headersonly flag was added.

  `parsestr`(text[, headersonly])

    Similar to the `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 `StringIO` instance first and calling `parse()`.

    Optional headersonly is as with the `parse()` method.

    在 2.2.2 版更改: The headersonly flag was added.

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

`email.``message_from_string`(s[, _class[, strict]])

  Return a message object structure from a string. This is exactly equivalent to `Parser().parsestr(s)`. Optional _class and strict are interpreted as with the `Parser` class constructor.

  在 2.2.2 版更改: The strict flag was added.

`email.``message_from_file`(fp[, _class[, strict]])

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

  在 2.2.2 版更改: The strict flag was added.

Here’s an example of how you might use this at an interactive Python prompt:

    >>> import email
    >>> msg = email.message_from_string(myString)

## 18.1.2.3. 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()`. Their `get_payload()` method will return a string object.

- 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 their `get_payload()` method will return the list of `Message` subparts.

- Most messages with a content type of message/* (e.g. 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 in the list payload 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.

Footnotes

1

  As of email package version 3.0, introduced in Python 2.4, the classic `Parser` was re-implemented in terms of the `FeedParser`, so the semantics and results are identical between the two parsers.