/python/Lib/email/header.py

# Copyright (C) 2002-2006 Python Software Foundation

# Author: Ben Gertzfield, Barry Warsaw

# Contact: email-sig@python.org



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



__all__ = [

    'Header',

    'decode_header',

    'make_header',

    ]



import re

import binascii



import email.quoprimime

import email.base64mime



from email.errors import HeaderParseError

from email.charset import Charset



NL = '\n'

SPACE = ' '

USPACE = u' '

SPACE8 = ' ' * 8

UEMPTYSTRING = u''



MAXLINELEN = 76



USASCII = Charset('us-ascii')

UTF8 = Charset('utf-8')



# 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 ?=

  (?=[ \t]|$)           # whitespace or the end of the string

  ''', re.VERBOSE | re.IGNORECASE | re.MULTILINE)



# Field name regexp, including trailing colon, but not separating whitespace,

# according to RFC 2822.  Character range is from tilde to exclamation mark.

# For use with .match()

fcre = re.compile(r'[\041-\176]+:$')







# 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.



    An email.Errors.HeaderParseError may be raised when certain decoding error

    occurs (e.g. a base64 decoding exception).

    """

    # 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] + SPACE + unenc, None)

                else:

                    decoded.append((unenc, None))

            if parts:

                charset, encoding = [s.lower() for s in parts[0:2]]

                encoded = parts[2]

                dec = None

                if encoding == 'q':

                    dec = email.quoprimime.header_decode(encoded)

                elif encoding == 'b':

                    try:

                        dec = email.base64mime.decode(encoded)

                    except binascii.Error:

                        # Turn this into a higher level exception.  BAW: Right

                        # now we throw the lower level exception away but

                        # when/if we get exception chaining, we'll preserve it.

                        raise HeaderParseError

                if dec is None:

                    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







def make_header(decoded_seq, maxlinelen=None, header_name=None,

                continuation_ws=' '):

    """Create a Header from a sequence of pairs as returned by decode_header()



    decode_header() takes a header value string and returns a sequence of

    pairs of the format (decoded_string, charset) where charset is the string

    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

    the Header constructor.

    """

    h = Header(maxlinelen=maxlinelen, header_name=header_name,

               continuation_ws=continuation_ws)

    for s, charset in decoded_seq:

        # None means us-ascii but we can simply pass it on to h.append()

        if charset is not None and not isinstance(charset, Charset):

            charset = Charset(charset)

        h.append(s, charset)

    return h







class Header:

    def __init__(self, s=None, charset=None,

                 maxlinelen=None, header_name=None,

                 continuation_ws=' ', errors='strict'):

        """Create a MIME-compliant header that can contain many character sets.



        Optional s is the initial header value.  If None, the initial header

        value is not set.  You can later append to the header with .append()

        method calls.  s may be a byte string or a Unicode string, 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 us-ascii

        charset is used both as s's initial charset and as the default for

        subsequent .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. `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.



        errors is passed through to the .append() call.

        """

        if charset is None:

            charset = USASCII

        if not isinstance(charset, Charset):

            charset = 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 = []

        if s is not None:

            self.append(s, charset, errors)

        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 __unicode__(self):

        """Helper for the built-in unicode function."""

        uchunks = []

        lastcs = None

        for s, charset in self._chunks:

            # We must preserve spaces between encoded and non-encoded word

            # boundaries, which means for us we need to add a space when we go

            # from a charset to None/us-ascii, or from None/us-ascii to a

            # charset.  Only do this for the second and subsequent chunks.

            nextcs = charset

            if uchunks:

                if lastcs not in (None, 'us-ascii'):

                    if nextcs in (None, 'us-ascii'):

                        uchunks.append(USPACE)

                        nextcs = None

                elif nextcs not in (None, 'us-ascii'):

                    uchunks.append(USPACE)

            lastcs = nextcs

            uchunks.append(unicode(s, str(charset)))

        return UEMPTYSTRING.join(uchunks)



    # Rich comparison operators for equality only.  BAW: does it make sense to

    # have or explicitly disable <, <=, >, >= operators?

    def __eq__(self, other):

        # other may be a Header or a string.  Both are fine so coerce

        # ourselves to a string, swap the args and do another comparison.

        return other == self.encode()



    def __ne__(self, other):

        return not self == other



    def append(self, s, charset=None, errors='strict'):

        """Append a string to the MIME header.



        Optional charset, if given, should be a Charset instance or the name

        of a character set (which will be converted to a Charset instance).  A

        value of None (the default) means that the charset given in the

        constructor is used.



        s may be a byte string or a Unicode string.  If it is a byte string

        (i.e. isinstance(s, str) is true), then charset is the encoding of

        that byte string, and a UnicodeError will be raised if the string

        cannot be decoded with that charset.  If s is a Unicode string, then

        charset is a hint specifying the character set of the characters in

        the string.  In this case, when producing an RFC 2822 compliant header

        using RFC 2047 rules, the Unicode string will be encoded using the

        following charsets in order: us-ascii, the charset hint, utf-8.  The

        first character set not to provoke a UnicodeError is used.



        Optional `errors' is passed as the third argument to any unicode() or

        ustr.encode() call.

        """

        if charset is None:

            charset = self._charset

        elif not isinstance(charset, Charset):

            charset = Charset(charset)

        # If the charset is our faux 8bit charset, leave the string unchanged

        if charset != '8bit':

            # We need to test that the string can be converted to unicode and

            # back to a byte string, given the input and output codecs of the

            # charset.

            if isinstance(s, str):

                # Possibly raise UnicodeError if the byte string can't be

                # converted to a unicode with the input codec of the charset.

                incodec = charset.input_codec or 'us-ascii'

                ustr = unicode(s, incodec, errors)

                # Now make sure that the unicode could be converted back to a

                # byte string with the output codec, which may be different

                # than the iput coded.  Still, use the original byte string.

                outcodec = charset.output_codec or 'us-ascii'

                ustr.encode(outcodec, errors)

            elif isinstance(s, unicode):

                # Now we have to be sure the unicode string can be converted

                # to a byte string with a reasonable output codec.  We want to

                # use the byte string in the chunk.

                for charset in USASCII, charset, UTF8:

                    try:

                        outcodec = charset.output_codec or 'us-ascii'

                        s = s.encode(outcodec, errors)

                        break

                    except UnicodeError:

                        pass

                else:

                    assert False, 'utf-8 conversion failed'

        self._chunks.append((s, charset))



    def _split(self, s, charset, maxlinelen, splitchars):

        # Split up a header safely for use with encode_chunks.

        splittable = charset.to_splittable(s)

        encoded = charset.from_splittable(splittable, True)

        elen = charset.encoded_header_len(encoded)

        # If the line's encoded length first, just return it

        if elen <= maxlinelen:

            return [(encoded, charset)]

        # If we have undetermined raw 8bit characters sitting in a byte

        # string, we really don't know what the right thing to do is.  We

        # can't really split it because it might be multibyte data which we

        # could break if we split it between pairs.  The least harm seems to

        # be to not split the header at all, but that means they could go out

        # longer than maxlinelen.

        if charset == '8bit':

            return [(s, 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._split_ascii(s, charset, maxlinelen, splitchars)

        # 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 = maxlinelen

            first = charset.from_splittable(splittable[:splitpnt], False)

            last = charset.from_splittable(splittable[splitpnt:], False)

        else:

            # Binary search for split point

            first, last = _binsplit(splittable, charset, maxlinelen)

        # first is of the proper length so just wrap it in the appropriate

        # chrome.  last must be recursively split.

        fsplittable = charset.to_splittable(first)

        fencoded = charset.from_splittable(fsplittable, True)

        chunk = [(fencoded, charset)]

        return chunk + self._split(last, charset, self._maxlinelen, splitchars)



    def _split_ascii(self, s, charset, firstlen, splitchars):

        chunks = _split_ascii(s, firstlen, self._maxlinelen,

                              self._continuation_ws, splitchars)

        return zip(chunks, [charset]*len(chunks))



    def _encode_chunks(self, newchunks, maxlinelen):

        # 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 newchunks:

            if not header:

                continue

            if charset is None or charset.header_encoding is None:

                s = header

            else:

                s = charset.header_encode(header)

            # Don't add more folding whitespace than necessary

            if chunks and chunks[-1].endswith(' '):

                extra = ''

            else:

                extra = ' '

            _max_append(chunks, s, maxlinelen, extra)

        joiner = NL + self._continuation_ws

        return joiner.join(chunks)



    def encode(self, splitchars=';, '):

        """Encode a message header into an RFC-compliant format.



        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.



        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.

        """

        newchunks = []

        maxlinelen = self._firstlinelen

        lastlen = 0

        for s, charset in self._chunks:

            # The first bit of the next chunk should be just long enough to

            # fill the next line.  Don't forget the space separating the

            # encoded words.

            targetlen = maxlinelen - lastlen - 1

            if targetlen < charset.encoded_header_len(''):

                # Stick it on the next line

                targetlen = maxlinelen

            newchunks += self._split(s, charset, targetlen, splitchars)

            lastchunk, lastcharset = newchunks[-1]

            lastlen = lastcharset.encoded_header_len(lastchunk)

        return self._encode_chunks(newchunks, maxlinelen)







def _split_ascii(s, firstlen, restlen, continuation_ws, splitchars):

    lines = []

    maxlen = firstlen

    for line in s.splitlines():

        # Ignore any leading whitespace (i.e. continuation whitespace) already

        # on the line, since we'll be adding our own.

        line = line.lstrip()

        if len(line) < maxlen:

            lines.append(line)

            maxlen = restlen

            continue

        # 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 commas, then

        # whitespace.

        for ch in splitchars:

            if ch in line:

                break

        else:

            # There's nothing useful to split the line on, not even spaces, so

            # just append this line unchanged

            lines.append(line)

            maxlen = restlen

            continue

        # Now split the line on the character plus trailing whitespace

        cre = re.compile(r'%s\s*' % ch)

        if ch in ';,':

            eol = ch

        else:

            eol = ''

        joiner = eol + ' '

        joinlen = len(joiner)

        wslen = len(continuation_ws.replace('\t', SPACE8))

        this = []

        linelen = 0

        for part in cre.split(line):

            curlen = linelen + max(0, len(this)-1) * joinlen

            partlen = len(part)

            onfirstline = not lines

            # We don't want to split after the field name, if we're on the

            # first line and the field name is present in the header string.

            if ch == ' ' and onfirstline and \

                   len(this) == 1 and fcre.match(this[0]):

                this.append(part)

                linelen += partlen

            elif curlen + partlen > maxlen:

                if this:

                    lines.append(joiner.join(this) + eol)

                # If this part is longer than maxlen and we aren't already

                # splitting on whitespace, try to recursively split this line

                # on whitespace.

                if partlen > maxlen and ch != ' ':

                    subl = _split_ascii(part, maxlen, restlen,

                                        continuation_ws, ' ')

                    lines.extend(subl[:-1])

                    this = [subl[-1]]

                else:

                    this = [part]

                linelen = wslen + len(this[-1])

                maxlen = restlen

            else:

                this.append(part)

                linelen += partlen

        # Put any left over parts on a line by themselves

        if this:

            lines.append(joiner.join(this))

    return lines







def _binsplit(splittable, charset, maxlinelen):

    i = 0

    j = len(splittable)

    while i < j:

        # Invariants:

        # 1. splittable[:k] fits for all k <= i (note that we *assume*,

        #    at the start, that splittable[:0] fits).

        # 2. splittable[:k] does not fit for any k > j (at the start,

        #    this means we shouldn't look at any k > len(splittable)).

        # 3. We don't know about splittable[:k] for k in i+1..j.

        # 4. We want to set i to the largest k that fits, with i <= k <= j.

        #

        m = (i+j+1) >> 1  # ceiling((i+j)/2); i < m <= j

        chunk = charset.from_splittable(splittable[:m], True)

        chunklen = charset.encoded_header_len(chunk)

        if chunklen <= maxlinelen:

            # m is acceptable, so is a new lower bound.

            i = m

        else:

            # m is not acceptable, so final i must be < m.

            j = m - 1

    # i == j.  Invariant #1 implies that splittable[:i] fits, and

    # invariant #2 implies that splittable[:i+1] does not fit, so i

    # is what we're looking for.

    first = charset.from_splittable(splittable[:i], False)

    last  = charset.from_splittable(splittable[i:], False)

    return first, last