19.1.11. "email.header": Internationalized headers
**************************************************

**Source code:** Lib/email/header.py

======================================================================

This module is part of the legacy ("Compat32") email API.  In the
current API encoding and decoding of headers is handled transparently
by the dictionary-like API of the "EmailMessage" class.  In addition
to uses in legacy code, this module can be useful in applications that
need to completely control the character sets used when encoding
headers.

The remaining text in this section is the original documentation of
the module.

**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
   >>> msg.as_string()
   'Subject: =?iso-8859-1?q?p=F6stal?=\n\n'

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 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.

      3.2 版更變: 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.

      3.2 版更變: 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?=')
      [(b'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.
