source: python/trunk/Lib/email/header.py@ 391

Last change on this file since 391 was 391, checked in by dmik, 11 years ago

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1# Copyright (C) 2002-2006 Python Software Foundation
2# Author: Ben Gertzfield, Barry Warsaw
3# Contact: email-sig@python.org
4
5"""Header encoding and decoding functionality."""
6
7__all__ = [
8 'Header',
9 'decode_header',
10 'make_header',
11 ]
12
13import re
14import binascii
15
16import email.quoprimime
17import email.base64mime
18
19from email.errors import HeaderParseError
20from email.charset import Charset
21
22NL = '\n'
23SPACE = ' '
24USPACE = u' '
25SPACE8 = ' ' * 8
26UEMPTYSTRING = u''
27
28MAXLINELEN = 76
29
30USASCII = Charset('us-ascii')
31UTF8 = Charset('utf-8')
32
33# Match encoded-word strings in the form =?charset?q?Hello_World?=
34ecre = re.compile(r'''
35 =\? # literal =?
36 (?P<charset>[^?]*?) # non-greedy up to the next ? is the charset
37 \? # literal ?
38 (?P<encoding>[qb]) # either a "q" or a "b", case insensitive
39 \? # literal ?
40 (?P<encoded>.*?) # non-greedy up to the next ?= is the encoded string
41 \?= # literal ?=
42 (?=[ \t]|$) # whitespace or the end of the string
43 ''', re.VERBOSE | re.IGNORECASE | re.MULTILINE)
44
45# Field name regexp, including trailing colon, but not separating whitespace,
46# according to RFC 2822. Character range is from tilde to exclamation mark.
47# For use with .match()
48fcre = re.compile(r'[\041-\176]+:$')
49
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]+:')
53
54
55
56
57# Helpers
58_max_append = email.quoprimime._max_append
59
60
61
62
63def decode_header(header):
64 """Decode a message header value without converting charset.
65
66 Returns a list of (decoded_string, charset) pairs containing each of the
67 decoded parts of the header. Charset is None for non-encoded parts of the
68 header, otherwise a lower-case string containing the name of the character
69 set specified in the encoded string.
70
71 An email.errors.HeaderParseError may be raised when certain decoding error
72 occurs (e.g. a base64 decoding exception).
73 """
74 # If no encoding, just return the header
75 header = str(header)
76 if not ecre.search(header):
77 return [(header, None)]
78 decoded = []
79 dec = ''
80 for line in header.splitlines():
81 # This line might not have an encoding in it
82 if not ecre.search(line):
83 decoded.append((line, None))
84 continue
85 parts = ecre.split(line)
86 while parts:
87 unenc = parts.pop(0).strip()
88 if unenc:
89 # Should we continue a long line?
90 if decoded and decoded[-1][1] is None:
91 decoded[-1] = (decoded[-1][0] + SPACE + unenc, None)
92 else:
93 decoded.append((unenc, None))
94 if parts:
95 charset, encoding = [s.lower() for s in parts[0:2]]
96 encoded = parts[2]
97 dec = None
98 if encoding == 'q':
99 dec = email.quoprimime.header_decode(encoded)
100 elif encoding == 'b':
101 paderr = len(encoded) % 4 # Postel's law: add missing padding
102 if paderr:
103 encoded += '==='[:4 - paderr]
104 try:
105 dec = email.base64mime.decode(encoded)
106 except binascii.Error:
107 # Turn this into a higher level exception. BAW: Right
108 # now we throw the lower level exception away but
109 # when/if we get exception chaining, we'll preserve it.
110 raise HeaderParseError
111 if dec is None:
112 dec = encoded
113
114 if decoded and decoded[-1][1] == charset:
115 decoded[-1] = (decoded[-1][0] + dec, decoded[-1][1])
116 else:
117 decoded.append((dec, charset))
118 del parts[0:3]
119 return decoded
120
121
122
123
124def make_header(decoded_seq, maxlinelen=None, header_name=None,
125 continuation_ws=' '):
126 """Create a Header from a sequence of pairs as returned by decode_header()
127
128 decode_header() takes a header value string and returns a sequence of
129 pairs of the format (decoded_string, charset) where charset is the string
130 name of the character set.
131
132 This function takes one of those sequence of pairs and returns a Header
133 instance. Optional maxlinelen, header_name, and continuation_ws are as in
134 the Header constructor.
135 """
136 h = Header(maxlinelen=maxlinelen, header_name=header_name,
137 continuation_ws=continuation_ws)
138 for s, charset in decoded_seq:
139 # None means us-ascii but we can simply pass it on to h.append()
140 if charset is not None and not isinstance(charset, Charset):
141 charset = Charset(charset)
142 h.append(s, charset)
143 return h
144
145
146
147
148class Header:
149 def __init__(self, s=None, charset=None,
150 maxlinelen=None, header_name=None,
151 continuation_ws=' ', errors='strict'):
152 """Create a MIME-compliant header that can contain many character sets.
153
154 Optional s is the initial header value. If None, the initial header
155 value is not set. You can later append to the header with .append()
156 method calls. s may be a byte string or a Unicode string, but see the
157 .append() documentation for semantics.
158
159 Optional charset serves two purposes: it has the same meaning as the
160 charset argument to the .append() method. It also sets the default
161 character set for all subsequent .append() calls that omit the charset
162 argument. If charset is not provided in the constructor, the us-ascii
163 charset is used both as s's initial charset and as the default for
164 subsequent .append() calls.
165
166 The maximum line length can be specified explicit via maxlinelen. For
167 splitting the first line to a shorter value (to account for the field
168 header which isn't included in s, e.g. `Subject') pass in the name of
169 the field in header_name. The default maxlinelen is 76.
170
171 continuation_ws must be RFC 2822 compliant folding whitespace (usually
172 either a space or a hard tab) which will be prepended to continuation
173 lines.
174
175 errors is passed through to the .append() call.
176 """
177 if charset is None:
178 charset = USASCII
179 if not isinstance(charset, Charset):
180 charset = Charset(charset)
181 self._charset = charset
182 self._continuation_ws = continuation_ws
183 cws_expanded_len = len(continuation_ws.replace('\t', SPACE8))
184 # BAW: I believe `chunks' and `maxlinelen' should be non-public.
185 self._chunks = []
186 if s is not None:
187 self.append(s, charset, errors)
188 if maxlinelen is None:
189 maxlinelen = MAXLINELEN
190 if header_name is None:
191 # We don't know anything about the field header so the first line
192 # is the same length as subsequent lines.
193 self._firstlinelen = maxlinelen
194 else:
195 # The first line should be shorter to take into account the field
196 # header. Also subtract off 2 extra for the colon and space.
197 self._firstlinelen = maxlinelen - len(header_name) - 2
198 # Second and subsequent lines should subtract off the length in
199 # columns of the continuation whitespace prefix.
200 self._maxlinelen = maxlinelen - cws_expanded_len
201
202 def __str__(self):
203 """A synonym for self.encode()."""
204 return self.encode()
205
206 def __unicode__(self):
207 """Helper for the built-in unicode function."""
208 uchunks = []
209 lastcs = None
210 for s, charset in self._chunks:
211 # We must preserve spaces between encoded and non-encoded word
212 # boundaries, which means for us we need to add a space when we go
213 # from a charset to None/us-ascii, or from None/us-ascii to a
214 # charset. Only do this for the second and subsequent chunks.
215 nextcs = charset
216 if uchunks:
217 if lastcs not in (None, 'us-ascii'):
218 if nextcs in (None, 'us-ascii'):
219 uchunks.append(USPACE)
220 nextcs = None
221 elif nextcs not in (None, 'us-ascii'):
222 uchunks.append(USPACE)
223 lastcs = nextcs
224 uchunks.append(unicode(s, str(charset)))
225 return UEMPTYSTRING.join(uchunks)
226
227 # Rich comparison operators for equality only. BAW: does it make sense to
228 # have or explicitly disable <, <=, >, >= operators?
229 def __eq__(self, other):
230 # other may be a Header or a string. Both are fine so coerce
231 # ourselves to a string, swap the args and do another comparison.
232 return other == self.encode()
233
234 def __ne__(self, other):
235 return not self == other
236
237 def append(self, s, charset=None, errors='strict'):
238 """Append a string to the MIME header.
239
240 Optional charset, if given, should be a Charset instance or the name
241 of a character set (which will be converted to a Charset instance). A
242 value of None (the default) means that the charset given in the
243 constructor is used.
244
245 s may be a byte string or a Unicode string. If it is a byte string
246 (i.e. isinstance(s, str) is true), then charset is the encoding of
247 that byte string, and a UnicodeError will be raised if the string
248 cannot be decoded with that charset. If s is a Unicode string, then
249 charset is a hint specifying the character set of the characters in
250 the string. In this case, when producing an RFC 2822 compliant header
251 using RFC 2047 rules, the Unicode string will be encoded using the
252 following charsets in order: us-ascii, the charset hint, utf-8. The
253 first character set not to provoke a UnicodeError is used.
254
255 Optional `errors' is passed as the third argument to any unicode() or
256 ustr.encode() call.
257 """
258 if charset is None:
259 charset = self._charset
260 elif not isinstance(charset, Charset):
261 charset = Charset(charset)
262 # If the charset is our faux 8bit charset, leave the string unchanged
263 if charset != '8bit':
264 # We need to test that the string can be converted to unicode and
265 # back to a byte string, given the input and output codecs of the
266 # charset.
267 if isinstance(s, str):
268 # Possibly raise UnicodeError if the byte string can't be
269 # converted to a unicode with the input codec of the charset.
270 incodec = charset.input_codec or 'us-ascii'
271 ustr = unicode(s, incodec, errors)
272 # Now make sure that the unicode could be converted back to a
273 # byte string with the output codec, which may be different
274 # than the iput coded. Still, use the original byte string.
275 outcodec = charset.output_codec or 'us-ascii'
276 ustr.encode(outcodec, errors)
277 elif isinstance(s, unicode):
278 # Now we have to be sure the unicode string can be converted
279 # to a byte string with a reasonable output codec. We want to
280 # use the byte string in the chunk.
281 for charset in USASCII, charset, UTF8:
282 try:
283 outcodec = charset.output_codec or 'us-ascii'
284 s = s.encode(outcodec, errors)
285 break
286 except UnicodeError:
287 pass
288 else:
289 assert False, 'utf-8 conversion failed'
290 self._chunks.append((s, charset))
291
292 def _split(self, s, charset, maxlinelen, splitchars):
293 # Split up a header safely for use with encode_chunks.
294 splittable = charset.to_splittable(s)
295 encoded = charset.from_splittable(splittable, True)
296 elen = charset.encoded_header_len(encoded)
297 # If the line's encoded length first, just return it
298 if elen <= maxlinelen:
299 return [(encoded, charset)]
300 # If we have undetermined raw 8bit characters sitting in a byte
301 # string, we really don't know what the right thing to do is. We
302 # can't really split it because it might be multibyte data which we
303 # could break if we split it between pairs. The least harm seems to
304 # be to not split the header at all, but that means they could go out
305 # longer than maxlinelen.
306 if charset == '8bit':
307 return [(s, charset)]
308 # BAW: I'm not sure what the right test here is. What we're trying to
309 # do is be faithful to RFC 2822's recommendation that ($2.2.3):
310 #
311 # "Note: Though structured field bodies are defined in such a way that
312 # folding can take place between many of the lexical tokens (and even
313 # within some of the lexical tokens), folding SHOULD be limited to
314 # placing the CRLF at higher-level syntactic breaks."
315 #
316 # For now, I can only imagine doing this when the charset is us-ascii,
317 # although it's possible that other charsets may also benefit from the
318 # higher-level syntactic breaks.
319 elif charset == 'us-ascii':
320 return self._split_ascii(s, charset, maxlinelen, splitchars)
321 # BAW: should we use encoded?
322 elif elen == len(s):
323 # We can split on _maxlinelen boundaries because we know that the
324 # encoding won't change the size of the string
325 splitpnt = maxlinelen
326 first = charset.from_splittable(splittable[:splitpnt], False)
327 last = charset.from_splittable(splittable[splitpnt:], False)
328 else:
329 # Binary search for split point
330 first, last = _binsplit(splittable, charset, maxlinelen)
331 # first is of the proper length so just wrap it in the appropriate
332 # chrome. last must be recursively split.
333 fsplittable = charset.to_splittable(first)
334 fencoded = charset.from_splittable(fsplittable, True)
335 chunk = [(fencoded, charset)]
336 return chunk + self._split(last, charset, self._maxlinelen, splitchars)
337
338 def _split_ascii(self, s, charset, firstlen, splitchars):
339 chunks = _split_ascii(s, firstlen, self._maxlinelen,
340 self._continuation_ws, splitchars)
341 return zip(chunks, [charset]*len(chunks))
342
343 def _encode_chunks(self, newchunks, maxlinelen):
344 # MIME-encode a header with many different charsets and/or encodings.
345 #
346 # Given a list of pairs (string, charset), return a MIME-encoded
347 # string suitable for use in a header field. Each pair may have
348 # different charsets and/or encodings, and the resulting header will
349 # accurately reflect each setting.
350 #
351 # Each encoding can be email.utils.QP (quoted-printable, for
352 # ASCII-like character sets like iso-8859-1), email.utils.BASE64
353 # (Base64, for non-ASCII like character sets like KOI8-R and
354 # iso-2022-jp), or None (no encoding).
355 #
356 # Each pair will be represented on a separate line; the resulting
357 # string will be in the format:
358 #
359 # =?charset1?q?Mar=EDa_Gonz=E1lez_Alonso?=\n
360 # =?charset2?b?SvxyZ2VuIEL2aW5n?="
361 chunks = []
362 for header, charset in newchunks:
363 if not header:
364 continue
365 if charset is None or charset.header_encoding is None:
366 s = header
367 else:
368 s = charset.header_encode(header)
369 # Don't add more folding whitespace than necessary
370 if chunks and chunks[-1].endswith(' '):
371 extra = ''
372 else:
373 extra = ' '
374 _max_append(chunks, s, maxlinelen, extra)
375 joiner = NL + self._continuation_ws
376 return joiner.join(chunks)
377
378 def encode(self, splitchars=';, '):
379 """Encode a message header into an RFC-compliant format.
380
381 There are many issues involved in converting a given string for use in
382 an email header. Only certain character sets are readable in most
383 email clients, and as header strings can only contain a subset of
384 7-bit ASCII, care must be taken to properly convert and encode (with
385 Base64 or quoted-printable) header strings. In addition, there is a
386 75-character length limit on any given encoded header field, so
387 line-wrapping must be performed, even with double-byte character sets.
388
389 This method will do its best to convert the string to the correct
390 character set used in email, and encode and line wrap it safely with
391 the appropriate scheme for that character set.
392
393 If the given charset is not known or an error occurs during
394 conversion, this function will return the header untouched.
395
396 Optional splitchars is a string containing characters to split long
397 ASCII lines on, in rough support of RFC 2822's `highest level
398 syntactic breaks'. This doesn't affect RFC 2047 encoded lines.
399 """
400 newchunks = []
401 maxlinelen = self._firstlinelen
402 lastlen = 0
403 for s, charset in self._chunks:
404 # The first bit of the next chunk should be just long enough to
405 # fill the next line. Don't forget the space separating the
406 # encoded words.
407 targetlen = maxlinelen - lastlen - 1
408 if targetlen < charset.encoded_header_len(''):
409 # Stick it on the next line
410 targetlen = maxlinelen
411 newchunks += self._split(s, charset, targetlen, splitchars)
412 lastchunk, lastcharset = newchunks[-1]
413 lastlen = lastcharset.encoded_header_len(lastchunk)
414 value = self._encode_chunks(newchunks, maxlinelen)
415 if _embeded_header.search(value):
416 raise HeaderParseError("header value appears to contain "
417 "an embedded header: {!r}".format(value))
418 return value
419
420
421
422
423def _split_ascii(s, firstlen, restlen, continuation_ws, splitchars):
424 lines = []
425 maxlen = firstlen
426 for line in s.splitlines():
427 # Ignore any leading whitespace (i.e. continuation whitespace) already
428 # on the line, since we'll be adding our own.
429 line = line.lstrip()
430 if len(line) < maxlen:
431 lines.append(line)
432 maxlen = restlen
433 continue
434 # Attempt to split the line at the highest-level syntactic break
435 # possible. Note that we don't have a lot of smarts about field
436 # syntax; we just try to break on semi-colons, then commas, then
437 # whitespace.
438 for ch in splitchars:
439 if ch in line:
440 break
441 else:
442 # There's nothing useful to split the line on, not even spaces, so
443 # just append this line unchanged
444 lines.append(line)
445 maxlen = restlen
446 continue
447 # Now split the line on the character plus trailing whitespace
448 cre = re.compile(r'%s\s*' % ch)
449 if ch in ';,':
450 eol = ch
451 else:
452 eol = ''
453 joiner = eol + ' '
454 joinlen = len(joiner)
455 wslen = len(continuation_ws.replace('\t', SPACE8))
456 this = []
457 linelen = 0
458 for part in cre.split(line):
459 curlen = linelen + max(0, len(this)-1) * joinlen
460 partlen = len(part)
461 onfirstline = not lines
462 # We don't want to split after the field name, if we're on the
463 # first line and the field name is present in the header string.
464 if ch == ' ' and onfirstline and \
465 len(this) == 1 and fcre.match(this[0]):
466 this.append(part)
467 linelen += partlen
468 elif curlen + partlen > maxlen:
469 if this:
470 lines.append(joiner.join(this) + eol)
471 # If this part is longer than maxlen and we aren't already
472 # splitting on whitespace, try to recursively split this line
473 # on whitespace.
474 if partlen > maxlen and ch != ' ':
475 subl = _split_ascii(part, maxlen, restlen,
476 continuation_ws, ' ')
477 lines.extend(subl[:-1])
478 this = [subl[-1]]
479 else:
480 this = [part]
481 linelen = wslen + len(this[-1])
482 maxlen = restlen
483 else:
484 this.append(part)
485 linelen += partlen
486 # Put any left over parts on a line by themselves
487 if this:
488 lines.append(joiner.join(this))
489 return lines
490
491
492
493
494def _binsplit(splittable, charset, maxlinelen):
495 i = 0
496 j = len(splittable)
497 while i < j:
498 # Invariants:
499 # 1. splittable[:k] fits for all k <= i (note that we *assume*,
500 # at the start, that splittable[:0] fits).
501 # 2. splittable[:k] does not fit for any k > j (at the start,
502 # this means we shouldn't look at any k > len(splittable)).
503 # 3. We don't know about splittable[:k] for k in i+1..j.
504 # 4. We want to set i to the largest k that fits, with i <= k <= j.
505 #
506 m = (i+j+1) >> 1 # ceiling((i+j)/2); i < m <= j
507 chunk = charset.from_splittable(splittable[:m], True)
508 chunklen = charset.encoded_header_len(chunk)
509 if chunklen <= maxlinelen:
510 # m is acceptable, so is a new lower bound.
511 i = m
512 else:
513 # m is not acceptable, so final i must be < m.
514 j = m - 1
515 # i == j. Invariant #1 implies that splittable[:i] fits, and
516 # invariant #2 implies that splittable[:i+1] does not fit, so i
517 # is what we're looking for.
518 first = charset.from_splittable(splittable[:i], False)
519 last = charset.from_splittable(splittable[i:], False)
520 return first, last
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