## 12.2.6 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.

New in version 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.

`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.

`__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:

`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`.

`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`.

`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.