19.1.8. email.header
: Internationalized headers¶
Source code: Lib/email/header.py
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 email
package supports these standards
in its email.header
and email.charset
modules.
If you want to include non-ASCII characters in your email headers, say in the
Subject or To fields, you should use the
Header
class and assign the field in the Message
object to an instance of Header
instead of using a string for the header
value. Import the Header
class from the 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
>>> msg.as_string()
'Subject: =?iso-8859-1?q?p=F6stal?=\n\n'
Notice here how we wanted the Subject field to contain a non-ASCII
character? We did this by creating a Header
instance and passing in
the character set that the byte string was encoded in. When the subsequent
Message
instance was flattened, the 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 Header
class description:
-
class
email.header.
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 withappend()
method calls. s may be an instance ofbytes
orstr
, but see theappend()
documentation for semantics.Optional charset serves two purposes: it has the same meaning as the charset argument to the
append()
method. It also sets the default character set for all subsequentappend()
calls that omit the charset argument. If charset is not provided in the constructor (the default), theus-ascii
character set is used both as s’s initial charset and as the default for subsequentappend()
calls.The maximum line length can be specified explicitly 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. 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
append()
method.-
append
(s, charset=None, errors='strict')¶ Append the string s to the MIME header.
Optional charset, if given, should be a
Charset
instance (seeemail.charset
) or the name of a character set, which will be converted to aCharset
instance. A value ofNone
(the default) means that the charset given in the constructor is used.s may be an instance of
bytes
orstr
. If it is an instance ofbytes
, then charset is the encoding of that byte string, and aUnicodeError
will be raised if the string cannot be decoded with that character set.If s is an instance of
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.
-
encode
(splitchars=';, \t', maxlinelen=None, linesep='\n')¶ 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 which should be given extra weight by the splitting algorithm during normal header wrapping. This is in very rough support of RFC 2822’s “higher level syntactic breaks”: split points preceded by a splitchar are preferred during line splitting, with the characters preferred in the order in which they appear in the string. Space and tab may be included in the string to indicate whether preference should be given to one over the other as a split point when other split chars do not appear in the line being split. Splitchars does not affect RFC 2047 encoded lines.
maxlinelen, if given, overrides the instance’s value for the maximum line length.
linesep specifies the characters used to separate the lines of the folded header. It defaults to the most useful value for Python application code (
\n
), but\r\n
can be specified in order to produce headers with RFC-compliant line separators.Distinto en la versión 3.2: Added the linesep argument.
The
Header
class also provides a number of methods to support standard operators and built-in functions.-
__str__
()¶ Returns an approximation of the
Header
as a string, using an unlimited line length. All pieces are converted to unicode using the specified encoding and joined together appropriately. Any pieces with a charset of'unknown-8bit'
are decoded as ASCII using the'replace'
error handler.Distinto en la versión 3.2: Added handling for the
'unknown-8bit'
charset.
-
The email.header
module also provides the following convenient functions.
-
email.header.
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 isNone
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?=') [(b'p\xf6stal', 'iso-8859-1')]
-
email.header.
make_header
(decoded_seq, maxlinelen=None, header_name=None, continuation_ws=' ')¶ Create a
Header
instance from a sequence of pairs as returned bydecode_header()
.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
Header
instance. Optional maxlinelen, header_name, and continuation_ws are as in theHeader
constructor.