18.1.6. "email.charset": 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.

New in version 2.2.2.

class email.charset.Charset([input_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.

   convert(s)

      Convert the string *s* from the *input_codec* to the
      *output_codec*.

   to_splittable(s)

      Convert a possibly multibyte string to a safely splittable
      format. *s* is the string to split.

      Uses the *input_codec* to try and convert the string to Unicode,
      so it can be safely split on character boundaries (even for
      multibyte characters).

      Returns the string as-is if it isn’t known how to convert *s* to
      Unicode with the *input_charset*.

      Characters that could not be converted to Unicode will be
      replaced with the Unicode replacement character "'U+FFFD'".

   from_splittable(ustr[, to_output])

      Convert a splittable string back into an encoded string.  *ustr*
      is a Unicode string to “unsplit”.

      This method uses the proper codec to try and convert the string
      from Unicode back into an encoded format.  Return the string as-
      is if it is not Unicode, or if it could not be converted from
      Unicode.

      Characters that could not be converted from Unicode will be
      replaced with an appropriate character (usually "'?'").

      If *to_output* is "True" (the default), uses *output_codec* to
      convert to an encoded format.  If *to_output* is "False", it
      uses *input_codec*.

   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(s[, convert])

      Header-encode the string *s*.

      If *convert* is "True", the string will be converted from the
      input charset to the output charset automatically.  This is not
      useful for multibyte character sets, which have line length
      issues (multibyte characters must be split on a character, not a
      byte boundary); use the higher-level "Header" class to deal with
      these issues (see "email.header").  *convert* defaults to
      "False".

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

   body_encode(s[, convert])

      Body-encode the string *s*.

      If *convert* is "True" (the default), the string will be
      converted from the input charset to output charset
      automatically. Unlike "header_encode()", there are no issues
      with byte boundaries and multibyte charsets in email bodies, so
      this is usually pretty safe.

      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[, body_enc[, output_charset]]])

   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 "unicode()" built-in, or to the "encode()" method of a
   Unicode string.
