"email.charset"：表示字元集合
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**原始碼：**Lib/email/charset.py

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此模組是舊版 ("Compat32") email API的其中一部份，在新版的 API 只有使用
別名表。

此章節的其餘文字是原始模組的說明文件

此模組提供一個 class（類別） "Charset" 來表示電子郵件中的字元集合和字
元集合轉換，以及字元集合登錄檔 (registry) 和其他許多便捷的方法以運用此
登錄檔。在 "email" 套件中有許多其他模組使用 "Charset" 的實例。

從 "email.charset" 模組中 import 此類別

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*.

   header_encode(string)

      Header-encode the string *string*.

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

   header_encode_lines(string, maxlengths)

      Header-encode a *string* by converting it first to bytes.

      This is similar to "header_encode()" except that the string is
      fit into maximum line lengths as given by the argument
      *maxlengths*, which must be an iterator: each element returned
      from this iterator will provide the next maximum line length.

   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 "encode()" method.
