18.1.5. "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.

Novo na versão 2.2.2.

Here is the "Header" class description:

class email.header.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 "append()" method calls.  *s* may be a byte string or a
   Unicode string, 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[, errors]])

      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 a byte string or a Unicode string.  If it is a byte
      string (i.e.  "isinstance(s, str)" is true), 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 a Unicode string, then *charset* is a hint specifying
      the character set of the characters in the string.  In this
      case, when producing an **RFC 2822**-compliant header using
      **RFC 2047** rules, the Unicode string will be encoded using the
      following charsets in order: "us-ascii", the *charset* hint,
      "utf-8".  The first character set to not provoke a
      "UnicodeError" is used.

      Optional *errors* is passed through to any "unicode()" or
      "unicode.encode()" call, and defaults to “strict”.

   encode([splitchars])

      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 to split long ASCII lines on, in
      rough support of **RFC 2822**’s *highest level syntactic
      breaks*.  This doesn’t affect **RFC 2047** encoded lines.

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

   __str__()

      A synonym for "Header.encode()".  Useful for "str(aHeader)".

   __unicode__()

      A helper for the built-in "unicode()" function.  Returns the
      header as a Unicode string.

   __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[, header_name[, 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.
