:mod:`email.charset`: Representing character sets
.. module:: email.charset :synopsis: Character Sets
Source code: :source:`Lib/email/charset.py`
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.
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.