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
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
object to an instance of
Header instead of using a string for the header
value. Import the
Header class from the
>>> 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:
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
append()method calls. s may be an instance of
str, but see the
append()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 subsequent
append()calls that omit the charset argument. If charset is not provided in the constructor (the default), the
us-asciicharacter set is used both as s‘s initial charset and as the default for subsequent
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(s, charset=None, errors='strict')¶
Append the string s to the MIME header.
Optional charset, if given, should be a
email.charset) or the name of a character set, which will be converted to a
Charsetinstance. A value of
None(the default) means that the charset given in the constructor is used.
s may be an instance of
str. If it is an instance of
bytes, then charset is the encoding of that byte string, and a
UnicodeErrorwill 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 (
\r\ncan be specified in order to produce headers with RFC-compliant line separators.
Changed in version 3.2: Added the linesep argument.
Headerclass also provides a number of methods to support standard operators and built-in functions.
Returns an approximation of the
Headeras 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
Changed in version 3.2: Added handling for the
email.header module also provides the following convenient functions.
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
Nonefor 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')]
make_header(decoded_seq, maxlinelen=None, header_name=None, continuation_ws=' ')¶
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.