cpython/Doc/library/email.generator.rst

:mod:`email.generator`: Generating MIME documents
-------------------------------------------------

.. module:: email.generator
   :synopsis: Generate flat text email messages from a message structure.


One of the most common tasks is to generate the flat text of the email message
represented by a message object structure.  You will need to do this if you want
to send your message via the :mod:`smtplib` module or the :mod:`nntplib` module,
or print the message on the console.  Taking a message object structure and
producing a flat text document is the job of the :class:`Generator` class.

Again, as with the :mod:`email.parser` module, you aren't limited to the
functionality of the bundled generator; you could write one from scratch
yourself.  However the bundled generator knows how to generate most email in a
standards-compliant way, should handle MIME and non-MIME email messages just
fine, and is designed so that the transformation from flat text, to a message
structure via the :class:`~email.parser.Parser` class, and back to flat text,
is idempotent (the input is identical to the output) [#]_.  On the other hand,
using the Generator on a :class:`~email.message.Message` constructed by program
may result in changes to the :class:`~email.message.Message` object as defaults
are filled in.

Here are the public methods of the :class:`Generator` class, imported from the
:mod:`email.generator` module:


.. class:: Generator(outfp[, mangle_from_[, maxheaderlen]])

   The constructor for the :class:`Generator` class takes a file-like object called
   *outfp* for an argument.  *outfp* must support the :meth:`write` method and be
   usable as the output file in a Python extended print statement.

   Optional *mangle_from_* is a flag that, when ``True``, puts a ``>`` character in
   front of any line in the body that starts exactly as ``From``, i.e. ``From``
   followed by a space at the beginning of the line.  This is the only guaranteed
   portable way to avoid having such lines be mistaken for a Unix mailbox format
   envelope header separator (see `WHY THE CONTENT-LENGTH FORMAT IS BAD
   <https://www.jwz.org/doc/content-length.html>`_ for details).  *mangle_from_*
   defaults to ``True``, but you might want to set this to ``False`` if you are not
   writing Unix mailbox format files.

   Optional *maxheaderlen* specifies the longest length for a non-continued header.
   When a header line is longer than *maxheaderlen* (in characters, with tabs
   expanded to 8 spaces), the header will be split as defined in the
   :class:`~email.header.Header` class.  Set to zero to disable header wrapping.
   The default is 78, as recommended (but not required) by :rfc:`2822`.

   The other public :class:`Generator` methods are:


   .. method:: flatten(msg[, unixfrom])

      Print the textual representation of the message object structure rooted at
      *msg* to the output file specified when the :class:`Generator` instance
      was created.  Subparts are visited depth-first and the resulting text will
      be properly MIME encoded.

      Optional *unixfrom* is a flag that forces the printing of the envelope
      header delimiter before the first :rfc:`2822` header of the root message
      object.  If the root object has no envelope header, a standard one is
      crafted.  By default, this is set to ``False`` to inhibit the printing of
      the envelope delimiter.

      Note that for subparts, no envelope header is ever printed.

      .. versionadded:: 2.2.2


   .. method:: clone(fp)

      Return an independent clone of this :class:`Generator` instance with the
      exact same options.

      .. versionadded:: 2.2.2


   .. method:: write(s)

      Write the string *s* to the underlying file object, i.e. *outfp* passed to
      :class:`Generator`'s constructor.  This provides just enough file-like API
      for :class:`Generator` instances to be used in extended print statements.

As a convenience, see the methods :meth:`Message.as_string` and
``str(aMessage)``, a.k.a. :meth:`Message.__str__`, which simplify the generation
of a formatted string representation of a message object.  For more detail, see
:mod:`email.message`.

The :mod:`email.generator` module also provides a derived class, called
:class:`DecodedGenerator` which is like the :class:`Generator` base class,
except that non-\ :mimetype:`text` parts are substituted with a format string
representing the part.


.. class:: DecodedGenerator(outfp[, mangle_from_[, maxheaderlen[, fmt]]])

   This class, derived from :class:`Generator` walks through all the subparts of a
   message.  If the subpart is of main type :mimetype:`text`, then it prints the
   decoded payload of the subpart. Optional *_mangle_from_* and *maxheaderlen* are
   as with the :class:`Generator` base class.

   If the subpart is not of main type :mimetype:`text`, optional *fmt* is a format
   string that is used instead of the message payload. *fmt* is expanded with the
   following keywords, ``%(keyword)s`` format:

   * ``type`` -- Full MIME type of the non-\ :mimetype:`text` part

   * ``maintype`` -- Main MIME type of the non-\ :mimetype:`text` part

   * ``subtype`` -- Sub-MIME type of the non-\ :mimetype:`text` part

   * ``filename`` -- Filename of the non-\ :mimetype:`text` part

   * ``description`` -- Description associated with the non-\ :mimetype:`text` part

   * ``encoding`` -- Content transfer encoding of the non-\ :mimetype:`text` part

   The default value for *fmt* is ``None``, meaning ::

      [Non-text (%(type)s) part of message omitted, filename %(filename)s]

   .. versionadded:: 2.2.2

.. versionchanged:: 2.5
   The previously deprecated method :meth:`__call__` was removed.


.. rubric:: Footnotes

.. [#] This statement assumes that you use the appropriate setting for the
       ``unixfrom`` argument, and that you set maxheaderlen=0 (which will
       preserve whatever the input line lengths were).  It is also not strictly
       true, since in many cases runs of whitespace in headers are collapsed
       into single blanks.  The latter is a bug that will eventually be fixed.