:mod:`email.charset`: Representing character sets
-------------------------------------------------
.. module:: email.charset
:synopsis: Character Sets
**Source code:** :source:`Lib/email/charset.py`
--------------
This module is part of the legacy (``Compat32``) email API. In the new
API only the aliases table is used.
The remaining text in this section is the original documentation of the module.
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.
.. class:: 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.
:class:`Charset` instances have the following data attributes:
.. attribute:: 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``.
.. attribute:: 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``.
.. attribute:: 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*.
.. attribute:: 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``.
.. attribute:: 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``.
.. attribute:: 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:: 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:: 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:: header_encode(string)
Header-encode the string *string*.
The type of encoding (base64 or quoted-printable) will be based on the
*header_encoding* attribute.
.. method:: header_encode_lines(string, maxlengths)
Header-encode a *string* by converting it first to bytes.
This is similar to :meth:`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.
.. method:: 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 :class:`Charset` class also provides a number of methods to support
standard operations and built-in functions.
.. method:: __str__()
Returns *input_charset* as a string coerced to lower
case. :meth:`__repr__` is an alias for :meth:`__str__`.
.. method:: __eq__(other)
This method allows you to compare two :class:`Charset` instances for
equality.
.. method:: __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=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 :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 :class:`str`'s
:meth:`~str.encode` method.
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