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# 18.1.6. `email`: 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.

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

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.

  `encoded_header_len`()
    Return the length of the encoded header string, properly calculating for quoted-printable or base64 encoding.
  `header_encode`(string)

    Header-encode the string string.

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

  `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 `decode()` method

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