:mod:`email`: Representing character sets
-----------------------------------------

.. module:: email.charset
   :synopsis: Character Sets


This module provides a class :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 :class:`Charset` are used in several other modules within the
:mod:`email` package.

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

.. versionadded:: 2.2.2


.. class:: 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.

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


.. data:: 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``.


.. data:: 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``.


.. data:: 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*.


.. data:: 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``.


.. data:: 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``.


.. data:: 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*.

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


.. method:: Charset.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 :mailheader:`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.


.. method:: Charset.convert(s)

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


.. method:: Charset.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'``.


.. method:: Charset.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*.


.. method:: Charset.get_output_charset()

   Return the output character set.

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


.. method:: Charset.encoded_header_len()

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


.. method:: Charset.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 :class:`Header` class to
   deal with these issues (see :mod:`email.header`).  *convert* defaults to
   ``False``.

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


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


.. method:: Charset.__str__()

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


.. method:: Charset.__eq__(other)

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


.. method:: Header.__ne__(other)

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

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


.. function:: 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 :meth:`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 :func:`add_codec` to add codecs the
   module does not know about.  See the :mod:`codecs` module's documentation for
   more information.

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


.. function:: 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``.


.. function:: 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 :func:`unicode`
   built-in, or to the :meth:`encode` method of a Unicode string.

