01
     ˆ¿Åƒu±=ÄÃÂ h    :mod:`email.header`: Internationalized headers
----------------------------------------------

.. module:: email.header
   :synopsis: Representing non-ASCII headers


:rfc:`2822` is the base standard that describes the format of email messages.
It derives from the older :rfc:`822` standard which came into widespread use at
a time when most email was composed of ASCII characters only.  :rfc:`2822` is a
specification written assuming email contains only 7-bit ASCII characters.

Of course, as email has been deployed worldwide, it has become
internationalized, such that language specific character sets can now be used in
email messages.  The base standard still requires email messages to be
transferred using only 7-bit ASCII characters, so a slew of RFCs have been
written describing how to encode email containing non-ASCII characters into
:rfc:`2822`\ -compliant format. These RFCs include :rfc:`2045`, :rfc:`2046`,
:rfc:`2047`, and :rfc:`2231`. The :mod:`email` package supports these standards
in its :mod:`email.header` and :mod:`email.charset` modules.

If you want to include non-ASCII characters in your email headers, say in the
:mailheader:`Subject` or :mailheader:`To` fields, you should use the
:class:`Header` class and assign the field in the :class:`~email.message.Message`
object to an instance of :class:`Header` instead of using a string for the header
value.  Import the :class:`Header` class from the :mod:`email.header` module.
For example::

   >>> from email.message import Message
   >>> from email.header import Header
   >>> msg = Message()
   >>> h = Header('p\xf6stal', 'iso-8859-1')
   >>> msg['Subject'] = h
   >>> print msg.as_string()
   Subject: =?iso-8859-1?q?p=F6stal?=


Notice here how we wanted the :mailheader:`Subject` field to contain a non-ASCII
character?  We did this by creating a :class:`Header` instance and passing in
the character set that the byte string was encoded in.  When the subsequent
:class:`~email.message.Message` instance was flattened, the :mailheader:`Subject`
field was properly :rfc:`2047` encoded.  MIME-aware mail readers would show this
header using the embedded ISO-8859-1 character.

.. versionadded:: 2.2.2

Here is the :class:`Header` class description:


.. class:: Header([s[, charset[, maxlinelen[, header_name[, continuation_ws[, errors]]]]]])

   Create a MIME-compliant header that can contain strings in different character
   sets.

   Optional *s* is the initial header value.  If ``None`` (the default), the
   initial header value is not set.  You can later append to the header with
   :meth:`append` method calls.  *s* may be a byte string or a Unicode string, but
   see the :meth:`append` documentation for semantics.

   Optional *charset* serves two purposes: it has the same meaning as the *charset*
   argument to the :meth:`append` method.  It also sets the default character set
   for all subsequent :meth:`append` calls that omit the *charset* argument.  If
   *charset* is not provided in the constructor (the default), the ``us-ascii``
   character set is used both as *s*'s initial charset and as the default for
   subsequent :meth:`append` calls.

   The maximum line length can be specified explicitly via *maxlinelen*.  For
   splitting the first line to a shorter value (to account for the field header
   which isn't included in *s*, e.g. :mailheader:`Subject`) pass in the name of the
   field in *header_name*.  The default *maxlinelen* is 76, and the default value
   for *header_name* is ``None``, meaning it is not taken into account for the
   first line of a long, split header.

   Optional *continuation_ws* must be :rfc:`2822`\ -compliant folding whitespace,
   and is usually either a space or a hard tab character. This character will be
   prepended to continuation lines.  *continuation_ws* defaults to a single
   space character (" ").

   Optional *errors* is passed straight through to the :meth:`append` method.


   .. method:: append(s[, charset[, errors]])

      Append the string *s* to the MIME header.

      Optional *charset