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/Lib/email/message.py

http://unladen-swallow.googlecode.com/
Python | 790 lines | 724 code | 14 blank | 52 comment | 21 complexity | d7c39457414798d54a8f90bbca57b8b1 MD5 | raw file
  1# Copyright (C) 2001-2006 Python Software Foundation
  2# Author: Barry Warsaw
  3# Contact: email-sig@python.org
  4
  5"""Basic message object for the email package object model."""
  6
  7__all__ = ['Message']
  8
  9import re
 10import uu
 11import binascii
 12import warnings
 13from cStringIO import StringIO
 14
 15# Intrapackage imports
 16import email.charset
 17from email import utils
 18from email import errors
 19
 20SEMISPACE = '; '
 21
 22# Regular expression that matches `special' characters in parameters, the
 23# existence of which force quoting of the parameter value.
 24tspecials = re.compile(r'[ \(\)<>@,;:\\"/\[\]\?=]')
 25
 26
 27# Helper functions
 28def _splitparam(param):
 29    # Split header parameters.  BAW: this may be too simple.  It isn't
 30    # strictly RFC 2045 (section 5.1) compliant, but it catches most headers
 31    # found in the wild.  We may eventually need a full fledged parser
 32    # eventually.
 33    a, sep, b = param.partition(';')
 34    if not sep:
 35        return a.strip(), None
 36    return a.strip(), b.strip()
 37
 38def _formatparam(param, value=None, quote=True):
 39    """Convenience function to format and return a key=value pair.
 40
 41    This will quote the value if needed or if quote is true.
 42    """
 43    if value is not None and len(value) > 0:
 44        # A tuple is used for RFC 2231 encoded parameter values where items
 45        # are (charset, language, value).  charset is a string, not a Charset
 46        # instance.
 47        if isinstance(value, tuple):
 48            # Encode as per RFC 2231
 49            param += '*'
 50            value = utils.encode_rfc2231(value[2], value[0], value[1])
 51        # BAW: Please check this.  I think that if quote is set it should
 52        # force quoting even if not necessary.
 53        if quote or tspecials.search(value):
 54            return '%s="%s"' % (param, utils.quote(value))
 55        else:
 56            return '%s=%s' % (param, value)
 57    else:
 58        return param
 59
 60def _parseparam(s):
 61    plist = []
 62    while s[:1] == ';':
 63        s = s[1:]
 64        end = s.find(';')
 65        while end > 0 and s.count('"', 0, end) % 2:
 66            end = s.find(';', end + 1)
 67        if end < 0:
 68            end = len(s)
 69        f = s[:end]
 70        if '=' in f:
 71            i = f.index('=')
 72            f = f[:i].strip().lower() + '=' + f[i+1:].strip()
 73        plist.append(f.strip())
 74        s = s[end:]
 75    return plist
 76
 77
 78def _unquotevalue(value):
 79    # This is different than utils.collapse_rfc2231_value() because it doesn't
 80    # try to convert the value to a unicode.  Message.get_param() and
 81    # Message.get_params() are both currently defined to return the tuple in
 82    # the face of RFC 2231 parameters.
 83    if isinstance(value, tuple):
 84        return value[0], value[1], utils.unquote(value[2])
 85    else:
 86        return utils.unquote(value)
 87
 88
 89
 90class Message:
 91    """Basic message object.
 92
 93    A message object is defined as something that has a bunch of RFC 2822
 94    headers and a payload.  It may optionally have an envelope header
 95    (a.k.a. Unix-From or From_ header).  If the message is a container (i.e. a
 96    multipart or a message/rfc822), then the payload is a list of Message
 97    objects, otherwise it is a string.
 98
 99    Message objects implement part of the `mapping' interface, which assumes
100    there is exactly one occurrance of the header per message.  Some headers
101    do in fact appear multiple times (e.g. Received) and for those headers,
102    you must use the explicit API to set or get all the headers.  Not all of
103    the mapping methods are implemented.
104    """
105    def __init__(self):
106        self._headers = []
107        self._unixfrom = None
108        self._payload = None
109        self._charset = None
110        # Defaults for multipart messages
111        self.preamble = self.epilogue = None
112        self.defects = []
113        # Default content type
114        self._default_type = 'text/plain'
115
116    def __str__(self):
117        """Return the entire formatted message as a string.
118        This includes the headers, body, and envelope header.
119        """
120        return self.as_string(unixfrom=True)
121
122    def as_string(self, unixfrom=False):
123        """Return the entire formatted message as a string.
124        Optional `unixfrom' when True, means include the Unix From_ envelope
125        header.
126
127        This is a convenience method and may not generate the message exactly
128        as you intend because by default it mangles lines that begin with
129        "From ".  For more flexibility, use the flatten() method of a
130        Generator instance.
131        """
132        from email.generator import Generator
133        fp = StringIO()
134        g = Generator(fp)
135        g.flatten(self, unixfrom=unixfrom)
136        return fp.getvalue()
137
138    def is_multipart(self):
139        """Return True if the message consists of multiple parts."""
140        return isinstance(self._payload, list)
141
142    #
143    # Unix From_ line
144    #
145    def set_unixfrom(self, unixfrom):
146        self._unixfrom = unixfrom
147
148    def get_unixfrom(self):
149        return self._unixfrom
150
151    #
152    # Payload manipulation.
153    #
154    def attach(self, payload):
155        """Add the given payload to the current payload.
156
157        The current payload will always be a list of objects after this method
158        is called.  If you want to set the payload to a scalar object, use
159        set_payload() instead.
160        """
161        if self._payload is None:
162            self._payload = [payload]
163        else:
164            self._payload.append(payload)
165
166    def get_payload(self, i=None, decode=False):
167        """Return a reference to the payload.
168
169        The payload will either be a list object or a string.  If you mutate
170        the list object, you modify the message's payload in place.  Optional
171        i returns that index into the payload.
172
173        Optional decode is a flag indicating whether the payload should be
174        decoded or not, according to the Content-Transfer-Encoding header
175        (default is False).
176
177        When True and the message is not a multipart, the payload will be
178        decoded if this header's value is `quoted-printable' or `base64'.  If
179        some other encoding is used, or the header is missing, or if the
180        payload has bogus data (i.e. bogus base64 or uuencoded data), the
181        payload is returned as-is.
182
183        If the message is a multipart and the decode flag is True, then None
184        is returned.
185        """
186        if i is None:
187            payload = self._payload
188        elif not isinstance(self._payload, list):
189            raise TypeError('Expected list, got %s' % type(self._payload))
190        else:
191            payload = self._payload[i]
192        if decode:
193            if self.is_multipart():
194                return None
195            cte = self.get('content-transfer-encoding', '').lower()
196            if cte == 'quoted-printable':
197                return utils._qdecode(payload)
198            elif cte == 'base64':
199                try:
200                    return utils._bdecode(payload)
201                except binascii.Error:
202                    # Incorrect padding
203                    return payload
204            elif cte in ('x-uuencode', 'uuencode', 'uue', 'x-uue'):
205                sfp = StringIO()
206                try:
207                    uu.decode(StringIO(payload+'\n'), sfp, quiet=True)
208                    payload = sfp.getvalue()
209                except uu.Error:
210                    # Some decoding problem
211                    return payload
212        # Everything else, including encodings with 8bit or 7bit are returned
213        # unchanged.
214        return payload
215
216    def set_payload(self, payload, charset=None):
217        """Set the payload to the given value.
218
219        Optional charset sets the message's default character set.  See
220        set_charset() for details.
221        """
222        self._payload = payload
223        if charset is not None:
224            self.set_charset(charset)
225
226    def set_charset(self, charset):
227        """Set the charset of the payload to a given character set.
228
229        charset can be a Charset instance, a string naming a character set, or
230        None.  If it is a string it will be converted to a Charset instance.
231        If charset is None, the charset parameter will be removed from the
232        Content-Type field.  Anything else will generate a TypeError.
233
234        The message will be assumed to be of type text/* encoded with
235        charset.input_charset.  It will be converted to charset.output_charset
236        and encoded properly, if needed, when generating the plain text
237        representation of the message.  MIME headers (MIME-Version,
238        Content-Type, Content-Transfer-Encoding) will be added as needed.
239
240        """
241        if charset is None:
242            self.del_param('charset')
243            self._charset = None
244            return
245        if isinstance(charset, basestring):
246            charset = email.charset.Charset(charset)
247        if not isinstance(charset, email.charset.Charset):
248            raise TypeError(charset)
249        # BAW: should we accept strings that can serve as arguments to the
250        # Charset constructor?
251        self._charset = charset
252        if not self.has_key('MIME-Version'):
253            self.add_header('MIME-Version', '1.0')
254        if not self.has_key('Content-Type'):
255            self.add_header('Content-Type', 'text/plain',
256                            charset=charset.get_output_charset())
257        else:
258            self.set_param('charset', charset.get_output_charset())
259        if str(charset) != charset.get_output_charset():
260            self._payload = charset.body_encode(self._payload)
261        if not self.has_key('Content-Transfer-Encoding'):
262            cte = charset.get_body_encoding()
263            try:
264                cte(self)
265            except TypeError:
266                self._payload = charset.body_encode(self._payload)
267                self.add_header('Content-Transfer-Encoding', cte)
268
269    def get_charset(self):
270        """Return the Charset instance associated with the message's payload.
271        """
272        return self._charset
273
274    #
275    # MAPPING INTERFACE (partial)
276    #
277    def __len__(self):
278        """Return the total number of headers, including duplicates."""
279        return len(self._headers)
280
281    def __getitem__(self, name):
282        """Get a header value.
283
284        Return None if the header is missing instead of raising an exception.
285
286        Note that if the header appeared multiple times, exactly which
287        occurrance gets returned is undefined.  Use get_all() to get all
288        the values matching a header field name.
289        """
290        return self.get(name)
291
292    def __setitem__(self, name, val):
293        """Set the value of a header.
294
295        Note: this does not overwrite an existing header with the same field
296        name.  Use __delitem__() first to delete any existing headers.
297        """
298        self._headers.append((name, val))
299
300    def __delitem__(self, name):
301        """Delete all occurrences of a header, if present.
302
303        Does not raise an exception if the header is missing.
304        """
305        name = name.lower()
306        newheaders = []
307        for k, v in self._headers:
308            if k.lower() != name:
309                newheaders.append((k, v))
310        self._headers = newheaders
311
312    def __contains__(self, name):
313        return name.lower() in [k.lower() for k, v in self._headers]
314
315    def has_key(self, name):
316        """Return true if the message contains the header."""
317        missing = object()
318        return self.get(name, missing) is not missing
319
320    def keys(self):
321        """Return a list of all the message's header field names.
322
323        These will be sorted in the order they appeared in the original
324        message, or were added to the message, and may contain duplicates.
325        Any fields deleted and re-inserted are always appended to the header
326        list.
327        """
328        return [k for k, v in self._headers]
329
330    def values(self):
331        """Return a list of all the message's header values.
332
333        These will be sorted in the order they appeared in the original
334        message, or were added to the message, and may contain duplicates.
335        Any fields deleted and re-inserted are always appended to the header
336        list.
337        """
338        return [v for k, v in self._headers]
339
340    def items(self):
341        """Get all the message's header fields and values.
342
343        These will be sorted in the order they appeared in the original
344        message, or were added to the message, and may contain duplicates.
345        Any fields deleted and re-inserted are always appended to the header
346        list.
347        """
348        return self._headers[:]
349
350    def get(self, name, failobj=None):
351        """Get a header value.
352
353        Like __getitem__() but return failobj instead of None when the field
354        is missing.
355        """
356        name = name.lower()
357        for k, v in self._headers:
358            if k.lower() == name:
359                return v
360        return failobj
361
362    #
363    # Additional useful stuff
364    #
365
366    def get_all(self, name, failobj=None):
367        """Return a list of all the values for the named field.
368
369        These will be sorted in the order they appeared in the original
370        message, and may contain duplicates.  Any fields deleted and
371        re-inserted are always appended to the header list.
372
373        If no such fields exist, failobj is returned (defaults to None).
374        """
375        values = []
376        name = name.lower()
377        for k, v in self._headers:
378            if k.lower() == name:
379                values.append(v)
380        if not values:
381            return failobj
382        return values
383
384    def add_header(self, _name, _value, **_params):
385        """Extended header setting.
386
387        name is the header field to add.  keyword arguments can be used to set
388        additional parameters for the header field, with underscores converted
389        to dashes.  Normally the parameter will be added as key="value" unless
390        value is None, in which case only the key will be added.
391
392        Example:
393
394        msg.add_header('content-disposition', 'attachment', filename='bud.gif')
395        """
396        parts = []
397        for k, v in _params.items():
398            if v is None:
399                parts.append(k.replace('_', '-'))
400            else:
401                parts.append(_formatparam(k.replace('_', '-'), v))
402        if _value is not None:
403            parts.insert(0, _value)
404        self._headers.append((_name, SEMISPACE.join(parts)))
405
406    def replace_header(self, _name, _value):
407        """Replace a header.
408
409        Replace the first matching header found in the message, retaining
410        header order and case.  If no matching header was found, a KeyError is
411        raised.
412        """
413        _name = _name.lower()
414        for i, (k, v) in zip(range(len(self._headers)), self._headers):
415            if k.lower() == _name:
416                self._headers[i] = (k, _value)
417                break
418        else:
419            raise KeyError(_name)
420
421    #
422    # Use these three methods instead of the three above.
423    #
424
425    def get_content_type(self):
426        """Return the message's content type.
427
428        The returned string is coerced to lower case of the form
429        `maintype/subtype'.  If there was no Content-Type header in the
430        message, the default type as given by get_default_type() will be
431        returned.  Since according to RFC 2045, messages always have a default
432        type this will always return a value.
433
434        RFC 2045 defines a message's default type to be text/plain unless it
435        appears inside a multipart/digest container, in which case it would be
436        message/rfc822.
437        """
438        missing = object()
439        value = self.get('content-type', missing)
440        if value is missing:
441            # This should have no parameters
442            return self.get_default_type()
443        ctype = _splitparam(value)[0].lower()
444        # RFC 2045, section 5.2 says if its invalid, use text/plain
445        if ctype.count('/') != 1:
446            return 'text/plain'
447        return ctype
448
449    def get_content_maintype(self):
450        """Return the message's main content type.
451
452        This is the `maintype' part of the string returned by
453        get_content_type().
454        """
455        ctype = self.get_content_type()
456        return ctype.split('/')[0]
457
458    def get_content_subtype(self):
459        """Returns the message's sub-content type.
460
461        This is the `subtype' part of the string returned by
462        get_content_type().
463        """
464        ctype = self.get_content_type()
465        return ctype.split('/')[1]
466
467    def get_default_type(self):
468        """Return the `default' content type.
469
470        Most messages have a default content type of text/plain, except for
471        messages that are subparts of multipart/digest containers.  Such
472        subparts have a default content type of message/rfc822.
473        """
474        return self._default_type
475
476    def set_default_type(self, ctype):
477        """Set the `default' content type.
478
479        ctype should be either "text/plain" or "message/rfc822", although this
480        is not enforced.  The default content type is not stored in the
481        Content-Type header.
482        """
483        self._default_type = ctype
484
485    def _get_params_preserve(self, failobj, header):
486        # Like get_params() but preserves the quoting of values.  BAW:
487        # should this be part of the public interface?
488        missing = object()
489        value = self.get(header, missing)
490        if value is missing:
491            return failobj
492        params = []
493        for p in _parseparam(';' + value):
494            try:
495                name, val = p.split('=', 1)
496                name = name.strip()
497                val = val.strip()
498            except ValueError:
499                # Must have been a bare attribute
500                name = p.strip()
501                val = ''
502            params.append((name, val))
503        params = utils.decode_params(params)
504        return params
505
506    def get_params(self, failobj=None, header='content-type', unquote=True):
507        """Return the message's Content-Type parameters, as a list.
508
509        The elements of the returned list are 2-tuples of key/value pairs, as
510        split on the `=' sign.  The left hand side of the `=' is the key,
511        while the right hand side is the value.  If there is no `=' sign in
512        the parameter the value is the empty string.  The value is as
513        described in the get_param() method.
514
515        Optional failobj is the object to return if there is no Content-Type
516        header.  Optional header is the header to search instead of
517        Content-Type.  If unquote is True, the value is unquoted.
518        """
519        missing = object()
520        params = self._get_params_preserve(missing, header)
521        if params is missing:
522            return failobj
523        if unquote:
524            return [(k, _unquotevalue(v)) for k, v in params]
525        else:
526            return params
527
528    def get_param(self, param, failobj=None, header='content-type',
529                  unquote=True):
530        """Return the parameter value if found in the Content-Type header.
531
532        Optional failobj is the object to return if there is no Content-Type
533        header, or the Content-Type header has no such parameter.  Optional
534        header is the header to search instead of Content-Type.
535
536        Parameter keys are always compared case insensitively.  The return
537        value can either be a string, or a 3-tuple if the parameter was RFC
538        2231 encoded.  When it's a 3-tuple, the elements of the value are of
539        the form (CHARSET, LANGUAGE, VALUE).  Note that both CHARSET and
540        LANGUAGE can be None, in which case you should consider VALUE to be
541        encoded in the us-ascii charset.  You can usually ignore LANGUAGE.
542
543        Your application should be prepared to deal with 3-tuple return
544        values, and can convert the parameter to a Unicode string like so:
545
546            param = msg.get_param('foo')
547            if isinstance(param, tuple):
548                param = unicode(param[2], param[0] or 'us-ascii')
549
550        In any case, the parameter value (either the returned string, or the
551        VALUE item in the 3-tuple) is always unquoted, unless unquote is set
552        to False.
553        """
554        if not self.has_key(header):
555            return failobj
556        for k, v in self._get_params_preserve(failobj, header):
557            if k.lower() == param.lower():
558                if unquote:
559                    return _unquotevalue(v)
560                else:
561                    return v
562        return failobj
563
564    def set_param(self, param, value, header='Content-Type', requote=True,
565                  charset=None, language=''):
566        """Set a parameter in the Content-Type header.
567
568        If the parameter already exists in the header, its value will be
569        replaced with the new value.
570
571        If header is Content-Type and has not yet been defined for this
572        message, it will be set to "text/plain" and the new parameter and
573        value will be appended as per RFC 2045.
574
575        An alternate header can specified in the header argument, and all
576        parameters will be quoted as necessary unless requote is False.
577
578        If charset is specified, the parameter will be encoded according to RFC
579        2231.  Optional language specifies the RFC 2231 language, defaulting
580        to the empty string.  Both charset and language should be strings.
581        """
582        if not isinstance(value, tuple) and charset:
583            value = (charset, language, value)
584
585        if not self.has_key(header) and header.lower() == 'content-type':
586            ctype = 'text/plain'
587        else:
588            ctype = self.get(header)
589        if not self.get_param(param, header=header):
590            if not ctype:
591                ctype = _formatparam(param, value, requote)
592            else:
593                ctype = SEMISPACE.join(
594                    [ctype, _formatparam(param, value, requote)])
595        else:
596            ctype = ''
597            for old_param, old_value in self.get_params(header=header,
598                                                        unquote=requote):
599                append_param = ''
600                if old_param.lower() == param.lower():
601                    append_param = _formatparam(param, value, requote)
602                else:
603                    append_param = _formatparam(old_param, old_value, requote)
604                if not ctype:
605                    ctype = append_param
606                else:
607                    ctype = SEMISPACE.join([ctype, append_param])
608        if ctype != self.get(header):
609            del self[header]
610            self[header] = ctype
611
612    def del_param(self, param, header='content-type', requote=True):
613        """Remove the given parameter completely from the Content-Type header.
614
615        The header will be re-written in place without the parameter or its
616        value. All values will be quoted as necessary unless requote is
617        False.  Optional header specifies an alternative to the Content-Type
618        header.
619        """
620        if not self.has_key(header):
621            return
622        new_ctype = ''
623        for p, v in self.get_params(header=header, unquote=requote):
624            if p.lower() != param.lower():
625                if not new_ctype:
626                    new_ctype = _formatparam(p, v, requote)
627                else:
628                    new_ctype = SEMISPACE.join([new_ctype,
629                                                _formatparam(p, v, requote)])
630        if new_ctype != self.get(header):
631            del self[header]
632            self[header] = new_ctype
633
634    def set_type(self, type, header='Content-Type', requote=True):
635        """Set the main type and subtype for the Content-Type header.
636
637        type must be a string in the form "maintype/subtype", otherwise a
638        ValueError is raised.
639
640        This method replaces the Content-Type header, keeping all the
641        parameters in place.  If requote is False, this leaves the existing
642        header's quoting as is.  Otherwise, the parameters will be quoted (the
643        default).
644
645        An alternative header can be specified in the header argument.  When
646        the Content-Type header is set, we'll always also add a MIME-Version
647        header.
648        """
649        # BAW: should we be strict?
650        if not type.count('/') == 1:
651            raise ValueError
652        # Set the Content-Type, you get a MIME-Version
653        if header.lower() == 'content-type':
654            del self['mime-version']
655            self['MIME-Version'] = '1.0'
656        if not self.has_key(header):
657            self[header] = type
658            return
659        params = self.get_params(header=header, unquote=requote)
660        del self[header]
661        self[header] = type
662        # Skip the first param; it's the old type.
663        for p, v in params[1:]:
664            self.set_param(p, v, header, requote)
665
666    def get_filename(self, failobj=None):
667        """Return the filename associated with the payload if present.
668
669        The filename is extracted from the Content-Disposition header's
670        `filename' parameter, and it is unquoted.  If that header is missing
671        the `filename' parameter, this method falls back to looking for the
672        `name' parameter.
673        """
674        missing = object()
675        filename = self.get_param('filename', missing, 'content-disposition')
676        if filename is missing:
677            filename = self.get_param('name', missing, 'content-disposition')
678        if filename is missing:
679            return failobj
680        return utils.collapse_rfc2231_value(filename).strip()
681
682    def get_boundary(self, failobj=None):
683        """Return the boundary associated with the payload if present.
684
685        The boundary is extracted from the Content-Type header's `boundary'
686        parameter, and it is unquoted.
687        """
688        missing = object()
689        boundary = self.get_param('boundary', missing)
690        if boundary is missing:
691            return failobj
692        # RFC 2046 says that boundaries may begin but not end in w/s
693        return utils.collapse_rfc2231_value(boundary).rstrip()
694
695    def set_boundary(self, boundary):
696        """Set the boundary parameter in Content-Type to 'boundary'.
697
698        This is subtly different than deleting the Content-Type header and
699        adding a new one with a new boundary parameter via add_header().  The
700        main difference is that using the set_boundary() method preserves the
701        order of the Content-Type header in the original message.
702
703        HeaderParseError is raised if the message has no Content-Type header.
704        """
705        missing = object()
706        params = self._get_params_preserve(missing, 'content-type')
707        if params is missing:
708            # There was no Content-Type header, and we don't know what type
709            # to set it to, so raise an exception.
710            raise errors.HeaderParseError('No Content-Type header found')
711        newparams = []
712        foundp = False
713        for pk, pv in params:
714            if pk.lower() == 'boundary':
715                newparams.append(('boundary', '"%s"' % boundary))
716                foundp = True
717            else:
718                newparams.append((pk, pv))
719        if not foundp:
720            # The original Content-Type header had no boundary attribute.
721            # Tack one on the end.  BAW: should we raise an exception
722            # instead???
723            newparams.append(('boundary', '"%s"' % boundary))
724        # Replace the existing Content-Type header with the new value
725        newheaders = []
726        for h, v in self._headers:
727            if h.lower() == 'content-type':
728                parts = []
729                for k, v in newparams:
730                    if v == '':
731                        parts.append(k)
732                    else:
733                        parts.append('%s=%s' % (k, v))
734                newheaders.append((h, SEMISPACE.join(parts)))
735
736            else:
737                newheaders.append((h, v))
738        self._headers = newheaders
739
740    def get_content_charset(self, failobj=None):
741        """Return the charset parameter of the Content-Type header.
742
743        The returned string is always coerced to lower case.  If there is no
744        Content-Type header, or if that header has no charset parameter,
745        failobj is returned.
746        """
747        missing = object()
748        charset = self.get_param('charset', missing)
749        if charset is missing:
750            return failobj
751        if isinstance(charset, tuple):
752            # RFC 2231 encoded, so decode it, and it better end up as ascii.
753            pcharset = charset[0] or 'us-ascii'
754            try:
755                # LookupError will be raised if the charset isn't known to
756                # Python.  UnicodeError will be raised if the encoded text
757                # contains a character not in the charset.
758                charset = unicode(charset[2], pcharset).encode('us-ascii')
759            except (LookupError, UnicodeError):
760                charset = charset[2]
761        # charset character must be in us-ascii range
762        try:
763            if isinstance(charset, str):
764                charset = unicode(charset, 'us-ascii')
765            charset = charset.encode('us-ascii')
766        except UnicodeError:
767            return failobj
768        # RFC 2046, $4.1.2 says charsets are not case sensitive
769        return charset.lower()
770
771    def get_charsets(self, failobj=None):
772        """Return a list containing the charset(s) used in this message.
773
774        The returned list of items describes the Content-Type headers'
775        charset parameter for this message and all the subparts in its
776        payload.
777
778        Each item will either be a string (the value of the charset parameter
779        in the Content-Type header of that part) or the value of the
780        'failobj' parameter (defaults to None), if the part does not have a
781        main MIME type of "text", or the charset is not defined.
782
783        The list will contain one string for each part of the message, plus
784        one for the container message (i.e. self), so that a non-multipart
785        message will still return a list of length 1.
786        """
787        return [part.get_content_charset(failobj) for part in self.walk()]
788
789    # I.e. def walk(self): ...
790    from email.iterators import walk