
``email.header``: Internationalized 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 ``email`` package supports these standards in its
``email.header`` and ``email.charset`` modules.

If you want to include non-ASCII characters in your email headers, say
in the *Subject* or *To* fields, you should use the ``Header`` class
and assign the field in the ``Message`` object to an instance of
``Header`` instead of using a string for the header value.  Import the
``Header`` class from the ``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 *Subject* field to contain a non-ASCII
character?  We did this by creating a ``Header`` instance and passing
in the character set that the byte string was encoded in.  When the
subsequent ``Message`` instance was flattened, the *Subject* field was
properly **RFC 2047** encoded.  MIME-aware mail readers would show
this header using the embedded ISO-8859-1 character.

Here is the ``Header`` class description:

class class email.header.Header(s=None, charset=None, maxlinelen=None, header_name=None, continuation_ws=' ', errors='strict')

   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 ``append()`` method calls.  *s* may be an
   instance of ``bytes`` or ``str``, but see the ``append()``
   documentation for semantics.

   Optional *charset* serves two purposes: it has the same meaning as
   the *charset* argument to the ``append()`` method.  It also sets
   the default character set for all subsequent ``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 ``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.
   *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 ``append()``
   method.

   append(s, charset=None, errors='strict')

      Append the string *s* to the MIME header.

      Optional *charset*, if given, should be a ``Charset`` instance
      (see ``email.charset``) or the name of a character set, which
      will be converted to a ``Charset`` instance.  A value of
      ``None`` (the default) means that the *charset* given in the
      constructor is used.

      *s* may be an instance of ``bytes`` or ``str``.  If it is an
      instance of ``bytes``, then *charset* is the encoding of that
      byte string, and a ``UnicodeError`` will be raised if the string
      cannot be decoded with that character set.

      If *s* is an instance of ``str``, then *charset* is a hint
      specifying the character set of the characters in the string.

      In either case, when producing an **RFC 2822**-compliant header
      using **RFC 2047** rules, the string will be encoded using the
      output codec of the charset.  If the string cannot be encoded
      using the output codec, a UnicodeError will be raised.

      Optional *errors* is passed as the errors argument to the decode
      call if *s* is a byte string.

   encode(splitchars=';, \t', maxlinelen=None, linesep='\n')

      Encode a message header into an RFC-compliant format, possibly
      wrapping long lines and encapsulating non-ASCII parts in base64
      or quoted-printable encodings.

      Optional *splitchars* is a string containing characters which
      should be given extra weight by the splitting algorithm during
      normal header wrapping.  This is in very rough support of **RFC
      2822**'s 'higher level syntactic breaks':  split points preceded
      by a splitchar are preferred during line splitting, with the
      characters preferred in the order in which they appear in the
      string.  Space and tab may be included in the string to indicate
      whether preference should be given to one over the other as a
      split point when other split chars do not appear in the line
      being split.  Splitchars does not affect **RFC 2047** encoded
      lines.

      *maxlinelen*, if given, overrides the instance's value for the
      maximum line length.

      *linesep* specifies the characters used to separate the lines of
      the folded header.  It defaults to the most useful value for
      Python application code (``\n``), but ``\r\n`` can be specified
      in order to produce headers with RFC-compliant line separators.

      Changed in version 3.2: Added the *linesep* argument.

   The ``Header`` class also provides a number of methods to support
   standard operators and built-in functions.

   __str__()

      Returns an approximation of the ``Header`` as a string, using an
      unlimited line length.  All pieces are converted to unicode
      using the specified encoding and joined together appropriately.
      Any pieces with a charset of ``'unknown-8bit'`` are decoded as
      ASCII using the ``'replace'`` error handler.

      Changed in version 3.2: Added handling for the
      ``'unknown-8bit'`` charset.

   __eq__(other)

      This method allows you to compare two ``Header`` instances for
      equality.

   __ne__(other)

      This method allows you to compare two ``Header`` instances for
      inequality.

The ``email.header`` module also provides the following convenient
functions.

email.header.decode_header(header)

   Decode a message header value without converting the character set.
   The header value is in *header*.

   This function returns a list of ``(decoded_string, charset)`` pairs
   containing each of the decoded parts of the header.  *charset* is
   ``None`` for non-encoded parts of the header, otherwise a lower
   case string containing the name of the character set specified in
   the encoded string.

   Here's an example:

      >>> from email.header import decode_header
      >>> decode_header('=?iso-8859-1?q?p=F6stal?=')
      [('p\xf6stal', 'iso-8859-1')]

email.header.make_header(decoded_seq, maxlinelen=None, header_name=None, continuation_ws=' ')

   Create a ``Header`` instance from a sequence of pairs as returned
   by ``decode_header()``.

   ``decode_header()`` takes a header value string and returns a
   sequence of pairs of the format ``(decoded_string, charset)`` where
   *charset* is the name of the character set.

   This function takes one of those sequence of pairs and returns a
   ``Header`` instance.  Optional *maxlinelen*, *header_name*, and
   *continuation_ws* are as in the ``Header`` constructor.
