| :mod:`email`: Internationalized headers |
| --------------------------------------- |
| |
| .. module:: email.header |
| :synopsis: Representing non-ASCII headers |
| |
| |
| :rfc:`2822` is the base standard that describes the format of email messages. |
| It derives from the older :rfc:`822` standard which came into widespread use at |
| a time when most email was composed of ASCII characters only. :rfc:`2822` is a |
| specification written assuming email contains only 7-bit ASCII characters. |
| |
| Of course, as email has been deployed worldwide, it has become |
| internationalized, such that language specific character sets can now be used in |
| email messages. The base standard still requires email messages to be |
| transferred using only 7-bit ASCII characters, so a slew of RFCs have been |
| written describing how to encode email containing non-ASCII characters into |
| :rfc:`2822`\ -compliant format. These RFCs include :rfc:`2045`, :rfc:`2046`, |
| :rfc:`2047`, and :rfc:`2231`. The :mod:`email` package supports these standards |
| in its :mod:`email.header` and :mod:`email.charset` modules. |
| |
| If you want to include non-ASCII characters in your email headers, say in the |
| :mailheader:`Subject` or :mailheader:`To` fields, you should use the |
| :class:`Header` class and assign the field in the :class:`~email.message.Message` |
| object to an instance of :class:`Header` instead of using a string for the header |
| value. Import the :class:`Header` class from the :mod:`email.header` module. |
| For example:: |
| |
| >>> from email.message import Message |
| >>> from email.header import Header |
| >>> msg = Message() |
| >>> h = Header('p\xf6stal', 'iso-8859-1') |
| >>> msg['Subject'] = h |
| >>> print(msg.as_string()) |
| Subject: =?iso-8859-1?q?p=F6stal?= |
| |
| |
| |
| Notice here how we wanted the :mailheader:`Subject` field to contain a non-ASCII |
| character? We did this by creating a :class:`Header` instance and passing in |
| the character set that the byte string was encoded in. When the subsequent |
| :class:`~email.message.Message` instance was flattened, the :mailheader:`Subject` |
| field was properly :rfc:`2047` encoded. MIME-aware mail readers would show this |
| header using the embedded ISO-8859-1 character. |
| |
| Here is the :class:`Header` class description: |
| |
| |
| .. class:: Header(s=None, charset=None, maxlinelen=None, header_name=None, continuation_ws=' ', errors='strict') |
| |
| Create a MIME-compliant header that can contain strings in different character |
| sets. |
| |
| Optional *s* is the initial header value. If ``None`` (the default), the |
| initial header value is not set. You can later append to the header with |
| :meth:`append` method calls. *s* may be an instance of :class:`bytes` or |
| :class:`str`, but see the :meth:`append` documentation for semantics. |
| |
| Optional *charset* serves two purposes: it has the same meaning as the *charset* |
| argument to the :meth:`append` method. It also sets the default character set |
| for all subsequent :meth:`append` calls that omit the *charset* argument. If |
| *charset* is not provided in the constructor (the default), the ``us-ascii`` |
| character set is used both as *s*'s initial charset and as the default for |
| subsequent :meth:`append` calls. |
| |
| The maximum line length can be specified explicit via *maxlinelen*. For |
| splitting the first line to a shorter value (to account for the field header |
| which isn't included in *s*, e.g. :mailheader:`Subject`) pass in the name of the |
| field in *header_name*. The default *maxlinelen* is 76, and the default value |
| for *header_name* is ``None``, meaning it is not taken into account for the |
| first line of a long, split header. |
| |
| Optional *continuation_ws* must be :rfc:`2822`\ -compliant folding |
| whitespace, and is usually either a space or a hard tab character. This |
| character will be prepended to continuation lines. *continuation_ws* |
| defaults to a single space character. |
| |
| Optional *errors* is passed straight through to the :meth:`append` method. |
| |
| |
| .. method:: append(s, charset=None, errors='strict') |
| |
| Append the string *s* to the MIME header. |
| |
| Optional *charset*, if given, should be a :class:`~email.charset.Charset` |
| instance (see :mod:`email.charset`) or the name of a character set, which |
| will be converted to a :class:`~email.charset.Charset` instance. A value |
| of ``None`` (the default) means that the *charset* given in the constructor |
| is used. |
| |
| *s* may be an instance of :class:`bytes` or :class:`str`. If it is an |
| instance of :class:`bytes`, then *charset* is the encoding of that byte |
| string, and a :exc:`UnicodeError` will be raised if the string cannot be |
| decoded with that character set. |
| |
| If *s* is an instance of :class:`str`, then *charset* is a hint specifying |
| the character set of the characters in the string. |
| |
| In either case, when producing an :rfc:`2822`\ -compliant header using |
| :rfc:`2047` rules, the string will be encoded using the output codec of |
| the charset. If the string cannot be encoded using the output codec, a |
| UnicodeError will be raised. |
| |
| Optional *errors* is passed as the errors argument to the decode call |
| if *s* is a byte string. |
| |
| .. method:: encode(splitchars=';, \\t', maxlinelen=None) |
| |
| Encode a message header into an RFC-compliant format, possibly wrapping |
| long lines and encapsulating non-ASCII parts in base64 or quoted-printable |
| encodings. Optional *splitchars* is a string containing characters to |
| split long ASCII lines on, in rough support of :rfc:`2822`'s *highest |
| level syntactic breaks*. This doesn't affect :rfc:`2047` encoded lines. |
| |
| *maxlinelen*, if given, overrides the instance's value for the maximum |
| line length. |
| |
| |
| The :class:`Header` class also provides a number of methods to support |
| standard operators and built-in functions. |
| |
| .. method:: __str__() |
| |
| A helper for :class:`str`'s :func:`encode` method. Returns the header as |
| a Unicode string. |
| |
| |
| .. method:: __eq__(other) |
| |
| This method allows you to compare two :class:`Header` instances for |
| equality. |
| |
| |
| .. method:: __ne__(other) |
| |
| This method allows you to compare two :class:`Header` instances for |
| inequality. |
| |
| The :mod:`email.header` module also provides the following convenient functions. |
| |
| |
| .. function:: decode_header(header) |
| |
| Decode a message header value without converting the character set. The header |
| value is in *header*. |
| |
| This function returns a list of ``(decoded_string, charset)`` pairs containing |
| each of the decoded parts of the header. *charset* is ``None`` for non-encoded |
| parts of the header, otherwise a lower case string containing the name of the |
| character set specified in the encoded string. |
| |
| Here's an example:: |
| |
| >>> from email.header import decode_header |
| >>> decode_header('=?iso-8859-1?q?p=F6stal?=') |
| [('p\xf6stal', 'iso-8859-1')] |
| |
| |
| .. function:: make_header(decoded_seq, maxlinelen=None, header_name=None, continuation_ws=' ') |
| |
| Create a :class:`Header` instance from a sequence of pairs as returned by |
| :func:`decode_header`. |
| |
| :func:`decode_header` takes a header value string and returns a sequence of |
| pairs of the format ``(decoded_string, charset)`` where *charset* is the name of |
| the character set. |
| |
| This function takes one of those sequence of pairs and returns a |
| :class:`Header` instance. Optional *maxlinelen*, *header_name*, and |
| *continuation_ws* are as in the :class:`Header` constructor. |
| |