
``email``: Representing character sets
**************************************

This module provides a class ``Charset`` for representing character
sets and character set conversions in email messages, as well as a
character set registry and several convenience methods for
manipulating this registry. Instances of ``Charset`` are used in
several other modules within the ``email`` package.

Import this class from the ``email.charset`` module.

class class email.charset.Charset(input_charset=DEFAULT_CHARSET)

   Map character sets to their email properties.

   This class provides information about the requirements imposed on
   email for a specific character set.  It also provides convenience
   routines for converting between character sets, given the
   availability of the applicable codecs.  Given a character set, it
   will do its best to provide information on how to use that
   character set in an email message in an RFC-compliant way.

   Certain character sets must be encoded with quoted-printable or
   base64 when used in email headers or bodies.  Certain character
   sets must be converted outright, and are not allowed in email.

   Optional *input_charset* is as described below; it is always
   coerced to lower case.  After being alias normalized it is also
   used as a lookup into the registry of character sets to find out
   the header encoding, body encoding, and output conversion codec to
   be used for the character set.  For example, if *input_charset* is
   ``iso-8859-1``, then headers and bodies will be encoded using
   quoted-printable and no output conversion codec is necessary.  If
   *input_charset* is ``euc-jp``, then headers will be encoded with
   base64, bodies will not be encoded, but output text will be
   converted from the ``euc-jp`` character set to the ``iso-2022-jp``
   character set.

   ``Charset`` instances have the following data attributes:

   input_charset

      The initial character set specified.  Common aliases are
      converted to their *official* email names (e.g. ``latin_1`` is
      converted to ``iso-8859-1``).  Defaults to 7-bit ``us-ascii``.

   header_encoding

      If the character set must be encoded before it can be used in an
      email header, this attribute will be set to ``Charset.QP`` (for
      quoted-printable), ``Charset.BASE64`` (for base64 encoding), or
      ``Charset.SHORTEST`` for the shortest of QP or BASE64 encoding.
      Otherwise, it will be ``None``.

   body_encoding

      Same as *header_encoding*, but describes the encoding for the
      mail message's body, which indeed may be different than the
      header encoding. ``Charset.SHORTEST`` is not allowed for
      *body_encoding*.

   output_charset

      Some character sets must be converted before they can be used in
      email headers or bodies.  If the *input_charset* is one of them,
      this attribute will contain the name of the character set output
      will be converted to. Otherwise, it will be ``None``.

   input_codec

      The name of the Python codec used to convert the *input_charset*
      to Unicode.  If no conversion codec is necessary, this attribute
      will be ``None``.

   output_codec

      The name of the Python codec used to convert Unicode to the
      *output_charset*.  If no conversion codec is necessary, this
      attribute will have the same value as the *input_codec*.

   ``Charset`` instances also have the following methods:

   get_body_encoding()

      Return the content transfer encoding used for body encoding.

      This is either the string ``quoted-printable`` or ``base64``
      depending on the encoding used, or it is a function, in which
      case you should call the function with a single argument, the
      Message object being encoded.  The function should then set the
      *Content-Transfer-Encoding* header itself to whatever is
      appropriate.

      Returns the string ``quoted-printable`` if *body_encoding* is
      ``QP``, returns the string ``base64`` if *body_encoding* is
      ``BASE64``, and returns the string ``7bit`` otherwise.

   get_output_charset()

      Return the output character set.

      This is the *output_charset* attribute if that is not ``None``,
      otherwise it is *input_charset*.

   encoded_header_len()

      Return the length of the encoded header string, properly
      calculating for quoted-printable or base64 encoding.

   header_encode(string)

      Header-encode the string *string*.

      The type of encoding (base64 or quoted-printable) will be based
      on the *header_encoding* attribute.

   body_encode(string)

      Body-encode the string *string*.

      The type of encoding (base64 or quoted-printable) will be based
      on the *body_encoding* attribute.

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

   __str__()

      Returns *input_charset* as a string coerced to lower case.
      ``__repr__()`` is an alias for ``__str__()``.

   __eq__(other)

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

   __ne__(other)

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

The ``email.charset`` module also provides the following functions for
adding new entries to the global character set, alias, and codec
registries:

email.charset.add_charset(charset, header_enc=None, body_enc=None, output_charset=None)

   Add character properties to the global registry.

   *charset* is the input character set, and must be the canonical
   name of a character set.

   Optional *header_enc* and *body_enc* is either ``Charset.QP`` for
   quoted-printable, ``Charset.BASE64`` for base64 encoding,
   ``Charset.SHORTEST`` for the shortest of quoted-printable or base64
   encoding, or ``None`` for no encoding.  ``SHORTEST`` is only valid
   for *header_enc*. The default is ``None`` for no encoding.

   Optional *output_charset* is the character set that the output
   should be in. Conversions will proceed from input charset, to
   Unicode, to the output charset when the method
   ``Charset.convert()`` is called.  The default is to output in the
   same character set as the input.

   Both *input_charset* and *output_charset* must have Unicode codec
   entries in the module's character set-to-codec mapping; use
   ``add_codec()`` to add codecs the module does not know about.  See
   the ``codecs`` module's documentation for more information.

   The global character set registry is kept in the module global
   dictionary ``CHARSETS``.

email.charset.add_alias(alias, canonical)

   Add a character set alias.  *alias* is the alias name, e.g.
   ``latin-1``. *canonical* is the character set's canonical name,
   e.g. ``iso-8859-1``.

   The global charset alias registry is kept in the module global
   dictionary ``ALIASES``.

email.charset.add_codec(charset, codecname)

   Add a codec that map characters in the given character set to and
   from Unicode.

   *charset* is the canonical name of a character set. *codecname* is
   the name of a Python codec, as appropriate for the second argument
   to the ``str``'s ``decode()`` method
