1:mod:`email.header`: Internationalized headers
2----------------------------------------------
3
4.. module:: email.header
5   :synopsis: Representing non-ASCII headers
6
7
8:rfc:`2822` is the base standard that describes the format of email messages.
9It derives from the older :rfc:`822` standard which came into widespread use at
10a time when most email was composed of ASCII characters only.  :rfc:`2822` is a
11specification written assuming email contains only 7-bit ASCII characters.
12
13Of course, as email has been deployed worldwide, it has become
14internationalized, such that language specific character sets can now be used in
15email messages.  The base standard still requires email messages to be
16transferred using only 7-bit ASCII characters, so a slew of RFCs have been
17written describing how to encode email containing non-ASCII characters into
18:rfc:`2822`\ -compliant format. These RFCs include :rfc:`2045`, :rfc:`2046`,
19:rfc:`2047`, and :rfc:`2231`. The :mod:`email` package supports these standards
20in its :mod:`email.header` and :mod:`email.charset` modules.
21
22If you want to include non-ASCII characters in your email headers, say in the
23:mailheader:`Subject` or :mailheader:`To` fields, you should use the
24:class:`Header` class and assign the field in the :class:`~email.message.Message`
25object to an instance of :class:`Header` instead of using a string for the header
26value.  Import the :class:`Header` class from the :mod:`email.header` module.
27For example::
28
29   >>> from email.message import Message
30   >>> from email.header import Header
31   >>> msg = Message()
32   >>> h = Header('p\xf6stal', 'iso-8859-1')
33   >>> msg['Subject'] = h
34   >>> print msg.as_string()
35   Subject: =?iso-8859-1?q?p=F6stal?=
36
37
38
39Notice here how we wanted the :mailheader:`Subject` field to contain a non-ASCII
40character?  We did this by creating a :class:`Header` instance and passing in
41the character set that the byte string was encoded in.  When the subsequent
42:class:`~email.message.Message` instance was flattened, the :mailheader:`Subject`
43field was properly :rfc:`2047` encoded.  MIME-aware mail readers would show this
44header using the embedded ISO-8859-1 character.
45
46.. versionadded:: 2.2.2
47
48Here is the :class:`Header` class description:
49
50
51.. class:: Header([s[, charset[, maxlinelen[, header_name[, continuation_ws[, errors]]]]]])
52
53   Create a MIME-compliant header that can contain strings in different character
54   sets.
55
56   Optional *s* is the initial header value.  If ``None`` (the default), the
57   initial header value is not set.  You can later append to the header with
58   :meth:`append` method calls.  *s* may be a byte string or a Unicode string, but
59   see the :meth:`append` documentation for semantics.
60
61   Optional *charset* serves two purposes: it has the same meaning as the *charset*
62   argument to the :meth:`append` method.  It also sets the default character set
63   for all subsequent :meth:`append` calls that omit the *charset* argument.  If
64   *charset* is not provided in the constructor (the default), the ``us-ascii``
65   character set is used both as *s*'s initial charset and as the default for
66   subsequent :meth:`append` calls.
67
68   The maximum line length can be specified explicitly via *maxlinelen*.  For
69   splitting the first line to a shorter value (to account for the field header
70   which isn't included in *s*, e.g. :mailheader:`Subject`) pass in the name of the
71   field in *header_name*.  The default *maxlinelen* is 76, and the default value
72   for *header_name* is ``None``, meaning it is not taken into account for the
73   first line of a long, split header.
74
75   Optional *continuation_ws* must be :rfc:`2822`\ -compliant folding whitespace,
76   and is usually either a space or a hard tab character. This character will be
77   prepended to continuation lines.  *continuation_ws* defaults to a single
78   space character (" ").
79
80   Optional *errors* is passed straight through to the :meth:`append` method.
81
82
83   .. method:: append(s[, charset[, errors]])
84
85      Append the string *s* to the MIME header.
86
87      Optional *charset*, if given, should be a :class:`~email.charset.Charset`
88      instance (see :mod:`email.charset`) or the name of a character set, which
89      will be converted to a :class:`~email.charset.Charset` instance.  A value
90      of ``None`` (the default) means that the *charset* given in the constructor
91      is used.
92
93      *s* may be a byte string or a Unicode string.  If it is a byte string
94      (i.e.  ``isinstance(s, str)`` is true), then *charset* is the encoding of
95      that byte string, and a :exc:`UnicodeError` will be raised if the string
96      cannot be decoded with that character set.
97
98      If *s* is a Unicode string, then *charset* is a hint specifying the
99      character set of the characters in the string.  In this case, when
100      producing an :rfc:`2822`\ -compliant header using :rfc:`2047` rules, the
101      Unicode string will be encoded using the following charsets in order:
102      ``us-ascii``, the *charset* hint, ``utf-8``.  The first character set to
103      not provoke a :exc:`UnicodeError` is used.
104
105      Optional *errors* is passed through to any :func:`unicode` or
106      :meth:`unicode.encode` call, and defaults to "strict".
107
108
109   .. method:: encode([splitchars])
110
111      Encode a message header into an RFC-compliant format, possibly wrapping
112      long lines and encapsulating non-ASCII parts in base64 or quoted-printable
113      encodings.  Optional *splitchars* is a string containing characters to
114      split long ASCII lines on, in rough support of :rfc:`2822`'s *highest
115      level syntactic breaks*.  This doesn't affect :rfc:`2047` encoded lines.
116
117   The :class:`Header` class also provides a number of methods to support
118   standard operators and built-in functions.
119
120
121   .. method:: __str__()
122
123      A synonym for :meth:`Header.encode`.  Useful for ``str(aHeader)``.
124
125
126   .. method:: __unicode__()
127
128      A helper for the built-in :func:`unicode` function.  Returns the header as
129      a Unicode string.
130
131
132   .. method:: __eq__(other)
133
134      This method allows you to compare two :class:`Header` instances for
135      equality.
136
137
138   .. method:: __ne__(other)
139
140      This method allows you to compare two :class:`Header` instances for
141      inequality.
142
143The :mod:`email.header` module also provides the following convenient functions.
144
145
146.. function:: decode_header(header)
147
148   Decode a message header value without converting the character set. The header
149   value is in *header*.
150
151   This function returns a list of ``(decoded_string, charset)`` pairs containing
152   each of the decoded parts of the header.  *charset* is ``None`` for non-encoded
153   parts of the header, otherwise a lower case string containing the name of the
154   character set specified in the encoded string.
155
156   Here's an example::
157
158      >>> from email.header import decode_header
159      >>> decode_header('=?iso-8859-1?q?p=F6stal?=')
160      [('p\xf6stal', 'iso-8859-1')]
161
162
163.. function:: make_header(decoded_seq[, maxlinelen[, header_name[, continuation_ws]]])
164
165   Create a :class:`Header` instance from a sequence of pairs as returned by
166   :func:`decode_header`.
167
168   :func:`decode_header` takes a header value string and returns a sequence of
169   pairs of the format ``(decoded_string, charset)`` where *charset* is the name of
170   the character set.
171
172   This function takes one of those sequence of pairs and returns a :class:`Header`
173   instance.  Optional *maxlinelen*, *header_name*, and *continuation_ws* are as in
174   the :class:`Header` constructor.
175
176