Doc/lib/emailparser.tex

\declaremodule{standard}{email.parser}
\modulesynopsis{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{Message} objects and
stringing them together via \method{attach()} and
\method{set_payload()} calls, or they can be created by parsing a flat text
representation of the email message.

The \module{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{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 \code{True} from its
\method{is_multipart()} method, and the subparts can be accessed via
the \method{get_payload()} and \method{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\footnote{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.}.

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 \module{email} package's bundled
parser and the \class{Message} class, so your custom parser can create
message object trees any way it finds necessary.

\subsubsection{FeedParser API}

\versionadded{2.4}

The \class{FeedParser}, imported from the \module{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 \var{defects}
attribute with a list of any problems it found in a message.  See the
\refmodule{email.errors} module for the list of defects that it can find.

Here is the API for the \class{FeedParser}:

\begin{classdesc}{FeedParser}{\optional{_factory}}
Create a \class{FeedParser} instance.  Optional \var{_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.
\end{classdesc}

\begin{methoddesc}[FeedParser]{feed}{data}
Feed the \class{FeedParser} some more data.  \var{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).
\end{methoddesc}

\begin{methoddesc}[FeedParser]{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}.
\end{methoddesc}

\subsubsection{Parser class API}

The \class{Parser} class, imported from the \module{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
\module{email.parser} module also provides a second class, called
\class{HeaderParser} which can be used if you're only interested in
the headers of the message. \class{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.
\class{HeaderParser} has the same API as the \class{Parser} class.

\begin{classdesc}{Parser}{\optional{_class}}
The constructor for the \class{Parser} class takes an optional
argument \var{_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{Message} (see
\refmodule{email.message}).  The factory will be called without
arguments.

The optional \var{strict} flag is ignored.  \deprecated{2.4}{Because the
\class{Parser} class is a backward compatible API wrapper around the
new-in-Python 2.4 \class{FeedParser}, \emph{all} parsing is effectively
non-strict.  You should simply stop passing a \var{strict} flag to the
\class{Parser} constructor.}

\versionchanged[The \var{strict} flag was added]{2.2.2}
\versionchanged[The \var{strict} flag was deprecated]{2.4}
\end{classdesc}

The other public \class{Parser} methods are:

\begin{methoddesc}[Parser]{parse}{fp\optional{, headersonly}}
Read all the data from the file-like object \var{fp}, parse the
resulting text, and return the root message object.  \var{fp} must
support both the \method{readline()} and the \method{read()} methods
on file-like objects.

The text contained in \var{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 \var{headersonly} is as with the \method{parse()} method.

\versionchanged[The \var{headersonly} flag was added]{2.2.2}
\end{methoddesc}

\begin{methoddesc}[Parser]{parsestr}{text\optional{, headersonly}}
Similar to the \method{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 \var{text} in a \class{StringIO}
instance first and calling \method{parse()}.

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

\versionchanged[The \var{headersonly} flag was added]{2.2.2}
\end{methoddesc}

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 \module{email}
package namespace.

\begin{funcdesc}{message_from_string}{s\optional{, _class\optional{, strict}}}
Return a message object structure from a string.  This is exactly
equivalent to \code{Parser().parsestr(s)}.  Optional \var{_class} and
\var{strict} are interpreted as with the \class{Parser} class constructor.

\versionchanged[The \var{strict} flag was added]{2.2.2}
\end{funcdesc}

\begin{funcdesc}{message_from_file}{fp\optional{, _class\optional{, strict}}}
Return a message object structure tree from an open file object.  This
is exactly equivalent to \code{Parser().parse(fp)}.  Optional
\var{_class} and \var{strict} are interpreted as with the
\class{Parser} class constructor.

\versionchanged[The \var{strict} flag was added]{2.2.2}
\end{funcdesc}

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

\begin{verbatim}
>>> import email
>>> msg = email.message_from_string(myString)
\end{verbatim}

\subsubsection{Additional notes}

Here are some notes on the parsing semantics:

\begin{itemize}
\item Most non-\mimetype{multipart} type messages are parsed as a single
      message object with a string payload.  These objects will return
      \code{False} for \method{is_multipart()}.  Their
      \method{get_payload()} method will return a string object.

\item 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
      \code{True} for \method{is_multipart()} and their
      \method{get_payload()} method will return the list of
      \class{Message} subparts.

\item 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
      \method{is_multipart()} method will return \code{True}.  The
      single element in the list payload will be a sub-message object.

\item 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
      \method{is_multipart()} method may return \code{False}.  If such
      messages were parsed with the \class{FeedParser}, they will have an
      instance of the \class{MultipartInvariantViolationDefect} class in their
      \var{defects} attribute list.  See \refmodule{email.errors} for
      details.
\end{itemize}