Lib/email/Header.py

# Copyright (C) 2002 Python Software Foundation
# Author: che@debian.org (Ben Gertzfield)

"""Header encoding and decoding functionality."""

import re
import email.quopriMIME
import email.base64MIME
from email.Charset import Charset

try:
    from email._compat22 import _floordiv
except SyntaxError:
    # Python 2.1 spells integer division differently
    from email._compat21 import _floordiv

CRLFSPACE = '\r\n '
CRLF = '\r\n'
NL = '\n'
SPACE8 = ' ' * 8
EMPTYSTRING = ''

MAXLINELEN = 76

ENCODE = 1
DECODE = 2

# Match encoded-word strings in the form =?charset?q?Hello_World?=
ecre = re.compile(r'''
  =\?                   # literal =?
  (?P<charset>[^?]*?)   # non-greedy up to the next ? is the charset
  \?                    # literal ?
  (?P<encoding>[qb])    # either a "q" or a "b", case insensitive
  \?                    # literal ?
  (?P<encoded>.*?)      # non-greedy up to the next ?= is the encoded string
  \?=                   # literal ?=
  ''', re.VERBOSE | re.IGNORECASE)



# Helpers
_max_append = email.quopriMIME._max_append



def decode_header(header):
    """Decode a message header value without converting charset.

    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.
    """
    # If no encoding, just return the header
    header = str(header)
    if not ecre.search(header):
        return [(header, None)]

    decoded = []
    dec = ''
    for line in header.splitlines():
        # This line might not have an encoding in it
        if not ecre.search(line):
            decoded.append((line, None))
            continue

        parts = ecre.split(line)
        while parts:
            unenc = parts.pop(0).strip()
            if unenc:
                # Should we continue a long line?
                if decoded and decoded[-1][1] is None:
                    decoded[-1] = (decoded[-1][0] + dec, None)
                else:
                    decoded.append((unenc, None))
            if parts:
                charset, encoding = [s.lower() for s in parts[0:2]]
                encoded = parts[2]
                dec = ''
                if encoding == 'q':
                    dec = email.quopriMIME.header_decode(encoded)
                elif encoding == 'b':
                    dec = email.base64MIME.decode(encoded)
                else:
                    dec = encoded

                if decoded and decoded[-1][1] == charset:
                    decoded[-1] = (decoded[-1][0] + dec, decoded[-1][1])
                else:
                    decoded.append((dec, charset))
            del parts[0:3]
    return decoded



class Header:
    def __init__(self, s, charset=None, maxlinelen=None, header_name=None,
                 continuation_ws=' '):
        """Create a MIME-compliant header that can contain many languages.

        Specify the initial header value in s.  Specify its character set as a
        Charset object in the charset argument.  If None, a default Charset
        instance will be used.

        You can later append to the header with append(s, charset) below;
        charset does not have to be the same as the one initially specified
        here.  In fact, it's optional, and if not given, defaults to the
        charset specified in the constructor.

        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. `Subject') pass in the name of
        the field in header_name.  The default maxlinelen is 76.

        continuation_ws must be RFC 2822 compliant folding whitespace (usually
        either a space or a hard tab) which will be prepended to continuation
        lines.
        """
        if charset is None:
            charset = Charset()
        self._charset = charset
        self._continuation_ws = continuation_ws
        cws_expanded_len = len(continuation_ws.replace('\t', SPACE8))
        # BAW: I believe `chunks' and `maxlinelen' should be non-public.
        self._chunks = []
        self.append(s, charset)
        if maxlinelen is None:
            maxlinelen = MAXLINELEN
        if header_name is None:
            # We don't know anything about the field header so the first line
            # is the same length as subsequent lines.
            self._firstlinelen = maxlinelen
        else:
            # The first line should be shorter to take into account the field
            # header.  Also subtract off 2 extra for the colon and space.
            self._firstlinelen = maxlinelen - len(header_name) - 2
        # Second and subsequent lines should subtract off the length in
        # columns of the continuation whitespace prefix.
        self._maxlinelen = maxlinelen - cws_expanded_len

    def __str__(self):
        """A synonym for self.encode()."""
        return self.encode()

    def append(self, s, charset=None):
        """Append string s with Charset charset to the MIME header.

        charset defaults to the one given in the class constructor.
        """
        if charset is None:
            charset = self._charset
        self._chunks.append((s, charset))

    def _split(self, s, charset, firstline=0):
        # Split up a header safely for use with encode_chunks.  BAW: this
        # appears to be a private convenience method.
        splittable = charset.to_splittable(s)
        encoded = charset.from_splittable(splittable)
        elen = charset.encoded_header_len(encoded)

        if elen <= self._maxlinelen:
            return [(encoded, charset)]
        # BAW: I'm not sure what the right test here is.  What we're trying to
        # do is be faithful to RFC 2822's recommendation that ($2.2.3):
        #
        # "Note: Though structured field bodies are defined in such a way that
        #  folding can take place between many of the lexical tokens (and even
        #  within some of the lexical tokens), folding SHOULD be limited to
        #  placing the CRLF at higher-level syntactic breaks."
        #
        # For now, I can only imagine doing this when the charset is us-ascii,
        # although it's possible that other charsets may also benefit from the
        # higher-level syntactic breaks.
        #
        elif charset == 'us-ascii':
            return self._ascii_split(s, charset, firstline)
        # BAW: should we use encoded?
        elif elen == len(s):
            # We can split on _maxlinelen boundaries because we know that the
            # encoding won't change the size of the string
            splitpnt = self._maxlinelen
            first = charset.from_splittable(splittable[:splitpnt], 0)
            last = charset.from_splittable(splittable[splitpnt:], 0)
        else:
            # Divide and conquer.
            halfway = _floordiv(len(splittable), 2)
            first = charset.from_splittable(splittable[:halfway], 0)
            last = charset.from_splittable(splittable[halfway:], 0)
        # Do the split
        return self._split(first, charset, firstline) + \
               self._split(last, charset)

    def _ascii_split(self, s, charset, firstline):
        # Attempt to split the line at the highest-level syntactic break
        # possible.  Note that we don't have a lot of smarts about field
        # syntax; we just try to break on semi-colons, then whitespace.
        rtn = []
        lines = s.splitlines()
        while lines:
            line = lines.pop(0)
            if firstline:
                maxlinelen = self._firstlinelen
                firstline = 0
            else:
                line = line.lstrip()
                maxlinelen = self._maxlinelen
            # Short lines can remain unchanged
            if len(line.replace('\t', SPACE8)) <= maxlinelen:
                rtn.append(line)
            else:
                oldlen = len(line)
                # Try to break the line on semicolons, but if that doesn't
                # work, try to split on folding whitespace.
                while len(line) > maxlinelen:
                    i = line.rfind(';', 0, maxlinelen)
                    if i < 0:
                        break
                    rtn.append(line[:i] + ';')
                    line = line[i+1:]
                # Is the remaining stuff still longer than maxlinelen?
                if len(line) <= maxlinelen:
                    # Splitting on semis worked
                    rtn.append(line)
                    continue
                # Splitting on semis didn't finish the job.  If it did any
                # work at all, stick the remaining junk on the front of the
                # `lines' sequence and let the next pass do its thing.
                if len(line) <> oldlen:
                    lines.insert(0, line)
                    continue
                # Otherwise, splitting on semis didn't help at all.
                parts = re.split(r'(\s+)', line)
                if len(parts) == 1 or (len(parts) == 3 and
                                       parts[0].endswith(':')):
                    # This line can't be split on whitespace.  There's now
                    # little we can do to get this into maxlinelen.  BAW:
                    # We're still potentially breaking the RFC by possibly
                    # allowing lines longer than the absolute maximum of 998
                    # characters.  For now, let it slide.
                    #
                    # len(parts) will be 1 if this line has no `Field: '
                    # prefix, otherwise it will be len(3).
                    rtn.append(line)
                    continue
                # There is whitespace we can split on.
                first = parts.pop(0)
                sublines = [first]
                acc = len(first)
                while parts:
                    len0 = len(parts[0])
                    len1 = len(parts[1])
                    if acc + len0 + len1 <= maxlinelen:
                        sublines.append(parts.pop(0))
                        sublines.append(parts.pop(0))
                        acc += len0 + len1
                    else:
                        # Split it here, but don't forget to ignore the
                        # next whitespace-only part
                        if first <> '':
                            rtn.append(EMPTYSTRING.join(sublines))
                        del parts[0]
                        first = parts.pop(0)
                        sublines = [first]
                        acc = len(first)
                rtn.append(EMPTYSTRING.join(sublines))
        return [(chunk, charset) for chunk in rtn]

    def _encode_chunks(self):
        """MIME-encode a header with many different charsets and/or encodings.

        Given a list of pairs (string, charset), return a MIME-encoded string
        suitable for use in a header field.  Each pair may have different
        charsets and/or encodings, and the resulting header will accurately
        reflect each setting.

        Each encoding can be email.Utils.QP (quoted-printable, for ASCII-like
        character sets like iso-8859-1), email.Utils.BASE64 (Base64, for
        non-ASCII like character sets like KOI8-R and iso-2022-jp), or None
        (no encoding).

        Each pair will be represented on a separate line; the resulting string
        will be in the format:

        "=?charset1?q?Mar=EDa_Gonz=E1lez_Alonso?=\n
          =?charset2?b?SvxyZ2VuIEL2aW5n?="
        """
        chunks = []
        for header, charset in self._chunks:
            if charset is None or charset.header_encoding is None:
                # There's no encoding for this chunk's charsets
                _max_append(chunks, header, self._maxlinelen)
            else:
                _max_append(chunks, charset.header_encode(header, 0),
                            self._maxlinelen, ' ')
        joiner = NL + self._continuation_ws
        return joiner.join(chunks)

    def encode(self):
        """Encode a message header, possibly converting charset and encoding.

        There are many issues involved in converting a given string for use in
        an email header.  Only certain character sets are readable in most
        email clients, and as header strings can only contain a subset of
        7-bit ASCII, care must be taken to properly convert and encode (with
        Base64 or quoted-printable) header strings.  In addition, there is a
        75-character length limit on any given encoded header field, so
        line-wrapping must be performed, even with double-byte character sets.

        This method will do its best to convert the string to the correct
        character set used in email, and encode and line wrap it safely with
        the appropriate scheme for that character set.

        If the given charset is not known or an error occurs during
        conversion, this function will return the header untouched.
        """
        newchunks = []
        for s, charset in self._chunks:
            newchunks += self._split(s, charset, 1)
        self._chunks = newchunks
        return self._encode_chunks()