1 # Copyright (C) 2002-2006 Python Software Foundation
2 # Author: Ben Gertzfield, Barry Warsaw
3 # Contact: email-sig@python.org
5 """Header encoding and decoding functionality."""
16 import email.quoprimime
17 import email.base64mime
19 from email.errors import HeaderParseError
20 from email.charset import Charset
30 USASCII = Charset('us-ascii')
31 UTF8 = Charset('utf-8')
33 # Match encoded-word strings in the form =?charset?q?Hello_World?=
34 ecre = re.compile(r'''
36 (?P<charset>[^?]*?) # non-greedy up to the next ? is the charset
38 (?P<encoding>[qb]) # either a "q" or a "b", case insensitive
40 (?P<encoded>.*?) # non-greedy up to the next ?= is the encoded string
42 (?=[ \t]|$) # whitespace or the end of the string
43 ''', re.VERBOSE | re.IGNORECASE | re.MULTILINE)
45 # Field name regexp, including trailing colon, but not separating whitespace,
46 # according to RFC 2822. Character range is from tilde to exclamation mark.
47 # For use with .match()
48 fcre = re.compile(r'[\041-\176]+:$')
50 # Find a header embedded in a putative header value. Used to check for
51 # header injection attack.
52 _embeded_header = re.compile(r'\n[^ \t]+:')
57 _max_append = email.quoprimime._max_append
61 def decode_header(header):
62 """Decode a message header value without converting charset.
64 Returns a list of (decoded_string, charset) pairs containing each of the
65 decoded parts of the header. Charset is None for non-encoded parts of the
66 header, otherwise a lower-case string containing the name of the character
67 set specified in the encoded string.
69 An email.errors.HeaderParseError may be raised when certain decoding error
70 occurs (e.g. a base64 decoding exception).
72 # If no encoding, just return the header
74 if not ecre.search(header):
75 return [(header, None)]
78 for line in header.splitlines():
79 # This line might not have an encoding in it
80 if not ecre.search(line):
81 decoded.append((line, None))
83 parts = ecre.split(line)
85 unenc = parts.pop(0).strip()
87 # Should we continue a long line?
88 if decoded and decoded[-1][1] is None:
89 decoded[-1] = (decoded[-1][0] + SPACE + unenc, None)
91 decoded.append((unenc, None))
93 charset, encoding = [s.lower() for s in parts[0:2]]
97 dec = email.quoprimime.header_decode(encoded)
99 paderr = len(encoded) % 4 # Postel's law: add missing padding
101 encoded += '==='[:4 - paderr]
103 dec = email.base64mime.decode(encoded)
104 except binascii.Error:
105 # Turn this into a higher level exception. BAW: Right
106 # now we throw the lower level exception away but
107 # when/if we get exception chaining, we'll preserve it.
108 raise HeaderParseError
112 if decoded and decoded[-1][1] == charset:
113 decoded[-1] = (decoded[-1][0] + dec, decoded[-1][1])
115 decoded.append((dec, charset))
121 def make_header(decoded_seq, maxlinelen=None, header_name=None,
122 continuation_ws=' '):
123 """Create a Header from a sequence of pairs as returned by decode_header()
125 decode_header() takes a header value string and returns a sequence of
126 pairs of the format (decoded_string, charset) where charset is the string
127 name of the character set.
129 This function takes one of those sequence of pairs and returns a Header
130 instance. Optional maxlinelen, header_name, and continuation_ws are as in
131 the Header constructor.
133 h = Header(maxlinelen=maxlinelen, header_name=header_name,
134 continuation_ws=continuation_ws)
135 for s, charset in decoded_seq:
136 # None means us-ascii but we can simply pass it on to h.append()
137 if charset is not None and not isinstance(charset, Charset):
138 charset = Charset(charset)
145 def __init__(self, s=None, charset=None,
146 maxlinelen=None, header_name=None,
147 continuation_ws=' ', errors='strict'):
148 """Create a MIME-compliant header that can contain many character sets.
150 Optional s is the initial header value. If None, the initial header
151 value is not set. You can later append to the header with .append()
152 method calls. s may be a byte string or a Unicode string, but see the
153 .append() documentation for semantics.
155 Optional charset serves two purposes: it has the same meaning as the
156 charset argument to the .append() method. It also sets the default
157 character set for all subsequent .append() calls that omit the charset
158 argument. If charset is not provided in the constructor, the us-ascii
159 charset is used both as s's initial charset and as the default for
160 subsequent .append() calls.
162 The maximum line length can be specified explicit via maxlinelen. For
163 splitting the first line to a shorter value (to account for the field
164 header which isn't included in s, e.g. `Subject') pass in the name of
165 the field in header_name. The default maxlinelen is 76.
167 continuation_ws must be RFC 2822 compliant folding whitespace (usually
168 either a space or a hard tab) which will be prepended to continuation
171 errors is passed through to the .append() call.
175 if not isinstance(charset, Charset):
176 charset = Charset(charset)
177 self._charset = charset
178 self._continuation_ws = continuation_ws
179 cws_expanded_len = len(continuation_ws.replace('\t', SPACE8))
180 # BAW: I believe `chunks' and `maxlinelen' should be non-public.
183 self.append(s, charset, errors)
184 if maxlinelen is None:
185 maxlinelen = MAXLINELEN
186 if header_name is None:
187 # We don't know anything about the field header so the first line
188 # is the same length as subsequent lines.
189 self._firstlinelen = maxlinelen
191 # The first line should be shorter to take into account the field
192 # header. Also subtract off 2 extra for the colon and space.
193 self._firstlinelen = maxlinelen - len(header_name) - 2
194 # Second and subsequent lines should subtract off the length in
195 # columns of the continuation whitespace prefix.
196 self._maxlinelen = maxlinelen - cws_expanded_len
199 """A synonym for self.encode()."""
202 def __unicode__(self):
203 """Helper for the built-in unicode function."""
206 for s, charset in self._chunks:
207 # We must preserve spaces between encoded and non-encoded word
208 # boundaries, which means for us we need to add a space when we go
209 # from a charset to None/us-ascii, or from None/us-ascii to a
210 # charset. Only do this for the second and subsequent chunks.
213 if lastcs not in (None, 'us-ascii'):
214 if nextcs in (None, 'us-ascii'):
215 uchunks.append(USPACE)
217 elif nextcs not in (None, 'us-ascii'):
218 uchunks.append(USPACE)
220 uchunks.append(unicode(s, str(charset)))
221 return UEMPTYSTRING.join(uchunks)
223 # Rich comparison operators for equality only. BAW: does it make sense to
224 # have or explicitly disable <, <=, >, >= operators?
225 def __eq__(self, other):
226 # other may be a Header or a string. Both are fine so coerce
227 # ourselves to a string, swap the args and do another comparison.
228 return other == self.encode()
230 def __ne__(self, other):
231 return not self == other
233 def append(self, s, charset=None, errors='strict'):
234 """Append a string to the MIME header.
236 Optional charset, if given, should be a Charset instance or the name
237 of a character set (which will be converted to a Charset instance). A
238 value of None (the default) means that the charset given in the
241 s may be a byte string or a Unicode string. If it is a byte string
242 (i.e. isinstance(s, str) is true), then charset is the encoding of
243 that byte string, and a UnicodeError will be raised if the string
244 cannot be decoded with that charset. If s is a Unicode string, then
245 charset is a hint specifying the character set of the characters in
246 the string. In this case, when producing an RFC 2822 compliant header
247 using RFC 2047 rules, the Unicode string will be encoded using the
248 following charsets in order: us-ascii, the charset hint, utf-8. The
249 first character set not to provoke a UnicodeError is used.
251 Optional `errors' is passed as the third argument to any unicode() or
255 charset = self._charset
256 elif not isinstance(charset, Charset):
257 charset = Charset(charset)
258 # If the charset is our faux 8bit charset, leave the string unchanged
259 if charset != '8bit':
260 # We need to test that the string can be converted to unicode and
261 # back to a byte string, given the input and output codecs of the
263 if isinstance(s, str):
264 # Possibly raise UnicodeError if the byte string can't be
265 # converted to a unicode with the input codec of the charset.
266 incodec = charset.input_codec or 'us-ascii'
267 ustr = unicode(s, incodec, errors)
268 # Now make sure that the unicode could be converted back to a
269 # byte string with the output codec, which may be different
270 # than the iput coded. Still, use the original byte string.
271 outcodec = charset.output_codec or 'us-ascii'
272 ustr.encode(outcodec, errors)
273 elif isinstance(s, unicode):
274 # Now we have to be sure the unicode string can be converted
275 # to a byte string with a reasonable output codec. We want to
276 # use the byte string in the chunk.
277 for charset in USASCII, charset, UTF8:
279 outcodec = charset.output_codec or 'us-ascii'
280 s = s.encode(outcodec, errors)
285 assert False, 'utf-8 conversion failed'
286 self._chunks.append((s, charset))
288 def _split(self, s, charset, maxlinelen, splitchars):
289 # Split up a header safely for use with encode_chunks.
290 splittable = charset.to_splittable(s)
291 encoded = charset.from_splittable(splittable, True)
292 elen = charset.encoded_header_len(encoded)
293 # If the line's encoded length first, just return it
294 if elen <= maxlinelen:
295 return [(encoded, charset)]
296 # If we have undetermined raw 8bit characters sitting in a byte
297 # string, we really don't know what the right thing to do is. We
298 # can't really split it because it might be multibyte data which we
299 # could break if we split it between pairs. The least harm seems to
300 # be to not split the header at all, but that means they could go out
301 # longer than maxlinelen.
302 if charset == '8bit':
303 return [(s, charset)]
304 # BAW: I'm not sure what the right test here is. What we're trying to
305 # do is be faithful to RFC 2822's recommendation that ($2.2.3):
307 # "Note: Though structured field bodies are defined in such a way that
308 # folding can take place between many of the lexical tokens (and even
309 # within some of the lexical tokens), folding SHOULD be limited to
310 # placing the CRLF at higher-level syntactic breaks."
312 # For now, I can only imagine doing this when the charset is us-ascii,
313 # although it's possible that other charsets may also benefit from the
314 # higher-level syntactic breaks.
315 elif charset == 'us-ascii':
316 return self._split_ascii(s, charset, maxlinelen, splitchars)
317 # BAW: should we use encoded?
319 # We can split on _maxlinelen boundaries because we know that the
320 # encoding won't change the size of the string
321 splitpnt = maxlinelen
322 first = charset.from_splittable(splittable[:splitpnt], False)
323 last = charset.from_splittable(splittable[splitpnt:], False)
325 # Binary search for split point
326 first, last = _binsplit(splittable, charset, maxlinelen)
327 # first is of the proper length so just wrap it in the appropriate
328 # chrome. last must be recursively split.
329 fsplittable = charset.to_splittable(first)
330 fencoded = charset.from_splittable(fsplittable, True)
331 chunk = [(fencoded, charset)]
332 return chunk + self._split(last, charset, self._maxlinelen, splitchars)
334 def _split_ascii(self, s, charset, firstlen, splitchars):
335 chunks = _split_ascii(s, firstlen, self._maxlinelen,
336 self._continuation_ws, splitchars)
337 return zip(chunks, [charset]*len(chunks))
339 def _encode_chunks(self, newchunks, maxlinelen):
340 # MIME-encode a header with many different charsets and/or encodings.
342 # Given a list of pairs (string, charset), return a MIME-encoded
343 # string suitable for use in a header field. Each pair may have
344 # different charsets and/or encodings, and the resulting header will
345 # accurately reflect each setting.
347 # Each encoding can be email.utils.QP (quoted-printable, for
348 # ASCII-like character sets like iso-8859-1), email.utils.BASE64
349 # (Base64, for non-ASCII like character sets like KOI8-R and
350 # iso-2022-jp), or None (no encoding).
352 # Each pair will be represented on a separate line; the resulting
353 # string will be in the format:
355 # =?charset1?q?Mar=EDa_Gonz=E1lez_Alonso?=\n
356 # =?charset2?b?SvxyZ2VuIEL2aW5n?="
358 for header, charset in newchunks:
361 if charset is None or charset.header_encoding is None:
364 s = charset.header_encode(header)
365 # Don't add more folding whitespace than necessary
366 if chunks and chunks[-1].endswith(' '):
370 _max_append(chunks, s, maxlinelen, extra)
371 joiner = NL + self._continuation_ws
372 return joiner.join(chunks)
374 def encode(self, splitchars=';, '):
375 """Encode a message header into an RFC-compliant format.
377 There are many issues involved in converting a given string for use in
378 an email header. Only certain character sets are readable in most
379 email clients, and as header strings can only contain a subset of
380 7-bit ASCII, care must be taken to properly convert and encode (with
381 Base64 or quoted-printable) header strings. In addition, there is a
382 75-character length limit on any given encoded header field, so
383 line-wrapping must be performed, even with double-byte character sets.
385 This method will do its best to convert the string to the correct
386 character set used in email, and encode and line wrap it safely with
387 the appropriate scheme for that character set.
389 If the given charset is not known or an error occurs during
390 conversion, this function will return the header untouched.
392 Optional splitchars is a string containing characters to split long
393 ASCII lines on, in rough support of RFC 2822's `highest level
394 syntactic breaks'. This doesn't affect RFC 2047 encoded lines.
397 maxlinelen = self._firstlinelen
399 for s, charset in self._chunks:
400 # The first bit of the next chunk should be just long enough to
401 # fill the next line. Don't forget the space separating the
403 targetlen = maxlinelen - lastlen - 1
404 if targetlen < charset.encoded_header_len(''):
405 # Stick it on the next line
406 targetlen = maxlinelen
407 newchunks += self._split(s, charset, targetlen, splitchars)
408 lastchunk, lastcharset = newchunks[-1]
409 lastlen = lastcharset.encoded_header_len(lastchunk)
410 value = self._encode_chunks(newchunks, maxlinelen)
411 if _embeded_header.search(value):
412 raise HeaderParseError("header value appears to contain "
413 "an embedded header: {!r}".format(value))
418 def _split_ascii(s, firstlen, restlen, continuation_ws, splitchars):
421 for line in s.splitlines():
422 # Ignore any leading whitespace (i.e. continuation whitespace) already
423 # on the line, since we'll be adding our own.
425 if len(line) < maxlen:
429 # Attempt to split the line at the highest-level syntactic break
430 # possible. Note that we don't have a lot of smarts about field
431 # syntax; we just try to break on semi-colons, then commas, then
433 for ch in splitchars:
437 # There's nothing useful to split the line on, not even spaces, so
438 # just append this line unchanged
442 # Now split the line on the character plus trailing whitespace
443 cre = re.compile(r'%s\s*' % ch)
449 joinlen = len(joiner)
450 wslen = len(continuation_ws.replace('\t', SPACE8))
453 for part in cre.split(line):
454 curlen = linelen + max(0, len(this)-1) * joinlen
456 onfirstline = not lines
457 # We don't want to split after the field name, if we're on the
458 # first line and the field name is present in the header string.
459 if ch == ' ' and onfirstline and \
460 len(this) == 1 and fcre.match(this[0]):
463 elif curlen + partlen > maxlen:
465 lines.append(joiner.join(this) + eol)
466 # If this part is longer than maxlen and we aren't already
467 # splitting on whitespace, try to recursively split this line
469 if partlen > maxlen and ch != ' ':
470 subl = _split_ascii(part, maxlen, restlen,
471 continuation_ws, ' ')
472 lines.extend(subl[:-1])
476 linelen = wslen + len(this[-1])
481 # Put any left over parts on a line by themselves
483 lines.append(joiner.join(this))
488 def _binsplit(splittable, charset, maxlinelen):
493 # 1. splittable[:k] fits for all k <= i (note that we *assume*,
494 # at the start, that splittable[:0] fits).
495 # 2. splittable[:k] does not fit for any k > j (at the start,
496 # this means we shouldn't look at any k > len(splittable)).
497 # 3. We don't know about splittable[:k] for k in i+1..j.
498 # 4. We want to set i to the largest k that fits, with i <= k <= j.
500 m = (i+j+1) >> 1 # ceiling((i+j)/2); i < m <= j
501 chunk = charset.from_splittable(splittable[:m], True)
502 chunklen = charset.encoded_header_len(chunk)
503 if chunklen <= maxlinelen:
504 # m is acceptable, so is a new lower bound.
507 # m is not acceptable, so final i must be < m.
509 # i == j. Invariant #1 implies that splittable[:i] fits, and
510 # invariant #2 implies that splittable[:i+1] does not fit, so i
511 # is what we're looking for.
512 first = charset.from_splittable(splittable[:i], False)
513 last = charset.from_splittable(splittable[i:], False)