text: # -*- coding: utf-8 -*-
  # Copyright (C) 2001-2007 Python Software Foundation
  # Author: Barry Warsaw
  # Contact: email-sig@python.org
  
  """Basic message object for the email package object model."""
  from __future__ import absolute_import, division, unicode_literals
  from future.builtins import list, range, str, zip
  
  __all__ = ['Message']
  
  import re
  import uu
  import base64
  import binascii
  from io import BytesIO, StringIO
  
  # Intrapackage imports
  from future.utils import as_native_str
  from future.backports.email import utils
  from future.backports.email import errors
  from future.backports.email._policybase import compat32
  from future.backports.email import charset as _charset
  from future.backports.email._encoded_words import decode_b
  Charset = _charset.Charset
  
  SEMISPACE = '; '
  
  # Regular expression that matches `special' characters in parameters, the
  # existence of which force quoting of the parameter value.
  tspecials = re.compile(r'[ \(\)<>@,;:\\"/\[\]\?=]')
  
  
  def _splitparam(param):
      # Split header parameters.  BAW: this may be too simple.  It isn't
      # strictly RFC 2045 (section 5.1) compliant, but it catches most headers
      # found in the wild.  We may eventually need a full fledged parser.
      # RDM: we might have a Header here; for now just stringify it.
      a, sep, b = str(param).partition(';')
      if not sep:
          return a.strip(), None
      return a.strip(), b.strip()
  
  def _formatparam(param, value=None, quote=True):
      """Convenience function to format and return a key=value pair.
  
      This will quote the value if needed or if quote is true.  If value is a
      three tuple (charset, language, value), it will be encoded according
      to RFC2231 rules.  If it contains non-ascii characters it will likewise
      be encoded according to RFC2231 rules, using the utf-8 charset and
      a null language.
      """
      if value is not None and len(value) > 0:
          # A tuple is used for RFC 2231 encoded parameter values where items
          # are (charset, language, value).  charset is a string, not a Charset
          # instance.  RFC 2231 encoded values are never quoted, per RFC.
          if isinstance(value, tuple):
              # Encode as per RFC 2231
              param += '*'
              value = utils.encode_rfc2231(value[2], value[0], value[1])
              return '%s=%s' % (param, value)
          else:
              try:
                  value.encode('ascii')
              except UnicodeEncodeError:
                  param += '*'
                  value = utils.encode_rfc2231(value, 'utf-8', '')
                  return '%s=%s' % (param, value)
          # BAW: Please check this.  I think that if quote is set it should
          # force quoting even if not necessary.
          if quote or tspecials.search(value):
              return '%s="%s"' % (param, utils.quote(value))
          else:
              return '%s=%s' % (param, value)
      else:
          return param
  
  def _parseparam(s):
      # RDM This might be a Header, so for now stringify it.
      s = ';' + str(s)
      plist = []
      while s[:1] == ';':
          s = s[1:]
          end = s.find(';')
          while end > 0 and (s.count('"', 0, end) - s.count('\\"', 0, end)) % 2:
              end = s.find(';', end + 1)
          if end < 0:
              end = len(s)
          f = s[:end]
          if '=' in f:
              i = f.index('=')
              f = f[:i].strip().lower() + '=' + f[i+1:].strip()
          plist.append(f.strip())
          s = s[end:]
      return plist
  
  
  def _unquotevalue(value):
      # This is different than utils.collapse_rfc2231_value() because it doesn't
      # try to convert the value to a unicode.  Message.get_param() and
      # Message.get_params() are both currently defined to return the tuple in
      # the face of RFC 2231 parameters.
      if isinstance(value, tuple):
          return value[0], value[1], utils.unquote(value[2])
      else:
          return utils.unquote(value)
  
  
  class Message(object):
      """Basic message object.
  
      A message object is defined as something that has a bunch of RFC 2822
      headers and a payload.  It may optionally have an envelope header
      (a.k.a. Unix-From or From_ header).  If the message is a container (i.e. a
      multipart or a message/rfc822), then the payload is a list of Message
      objects, otherwise it is a string.
  
      Message objects implement part of the `mapping' interface, which assumes
      there is exactly one occurrence of the header per message.  Some headers
      do in fact appear multiple times (e.g. Received) and for those headers,
      you must use the explicit API to set or get all the headers.  Not all of
      the mapping methods are implemented.
      """
      def __init__(self, policy=compat32):
          self.policy = policy
          self._headers = list()
          self._unixfrom = None
          self._payload = None
          self._charset = None
          # Defaults for multipart messages
          self.preamble = self.epilogue = None
          self.defects = []
          # Default content type
          self._default_type = 'text/plain'
  
      @as_native_str(encoding='utf-8')
      def __str__(self):
          """Return the entire formatted message as a string.
          This includes the headers, body, and envelope header.
          """
          return self.as_string()
  
      def as_string(self, unixfrom=False, maxheaderlen=0):
          """Return the entire formatted message as a (unicode) string.
          Optional `unixfrom' when True, means include the Unix From_ envelope
          header.
  
          This is a convenience method and may not generate the message exactly
          as you intend.  For more flexibility, use the flatten() method of a
          Generator instance.
          """
          from future.backports.email.generator import Generator
          fp = StringIO()
          g = Generator(fp, mangle_from_=False, maxheaderlen=maxheaderlen)
          g.flatten(self, unixfrom=unixfrom)
          return fp.getvalue()
  
      def is_multipart(self):
          """Return True if the message consists of multiple parts."""
          return isinstance(self._payload, list)
  
      #
      # Unix From_ line
      #
      def set_unixfrom(self, unixfrom):
          self._unixfrom = unixfrom
  
      def get_unixfrom(self):
          return self._unixfrom
  
      #
      # Payload manipulation.
      #
      def attach(self, payload):
          """Add the given payload to the current payload.
  
          The current payload will always be a list of objects after this method
          is called.  If you want to set the payload to a scalar object, use
          set_payload() instead.
          """
          if self._payload is None:
              self._payload = [payload]
          else:
              self._payload.append(payload)
  
      def get_payload(self, i=None, decode=False):
          """Return a reference to the payload.
  
          The payload will either be a list object or a string.  If you mutate
          the list object, you modify the message's payload in place.  Optional
          i returns that index into the payload.
  
          Optional decode is a flag indicating whether the payload should be
          decoded or not, according to the Content-Transfer-Encoding header
          (default is False).
  
          When True and the message is not a multipart, the payload will be
          decoded if this header's value is `quoted-printable' or `base64'.  If
          some other encoding is used, or the header is missing, or if the
          payload has bogus data (i.e. bogus base64 or uuencoded data), the
          payload is returned as-is.
  
          If the message is a multipart and the decode flag is True, then None
          is returned.
          """
          # Here is the logic table for this code, based on the email5.0.0 code:
          #   i     decode  is_multipart  result
          # ------  ------  ------------  ------------------------------
          #  None   True    True          None
          #   i     True    True          None
          #  None   False   True          _payload (a list)
          #   i     False   True          _payload element i (a Message)
          #   i     False   False         error (not a list)
          #   i     True    False         error (not a list)
          #  None   False   False         _payload
          #  None   True    False         _payload decoded (bytes)
          # Note that Barry planned to factor out the 'decode' case, but that
          # isn't so easy now that we handle the 8 bit data, which needs to be
          # converted in both the decode and non-decode path.
          if self.is_multipart():
              if decode:
                  return None
              if i is None:
                  return self._payload
              else:
                  return self._payload[i]
          # For backward compatibility, Use isinstance and this error message
          # instead of the more logical is_multipart test.
          if i is not None and not isinstance(self._payload, list):
              raise TypeError('Expected list, got %s' % type(self._payload))
          payload = self._payload
          # cte might be a Header, so for now stringify it.
          cte = str(self.get('content-transfer-encoding', '')).lower()
          # payload may be bytes here.
          if isinstance(payload, str):
              payload = str(payload)    # for Python-Future, so surrogateescape works
              if utils._has_surrogates(payload):
                  bpayload = payload.encode('ascii', 'surrogateescape')
                  if not decode:
                      try:
                          payload = bpayload.decode(self.get_param('charset', 'ascii'), 'replace')
                      except LookupError:
                          payload = bpayload.decode('ascii', 'replace')
              elif decode:
                  try:
                      bpayload = payload.encode('ascii')
                  except UnicodeError:
                      # This won't happen for RFC compliant messages (messages
                      # containing only ASCII codepoints in the unicode input).
                      # If it does happen, turn the string into bytes in a way
                      # guaranteed not to fail.
                      bpayload = payload.encode('raw-unicode-escape')
          if not decode:
              return payload
          if cte == 'quoted-printable':
              return utils._qdecode(bpayload)
          elif cte == 'base64':
              # XXX: this is a bit of a hack; decode_b should probably be factored
              # out somewhere, but I haven't figured out where yet.
              value, defects = decode_b(b''.join(bpayload.splitlines()))
              for defect in defects:
                  self.policy.handle_defect(self, defect)
              return value
          elif cte in ('x-uuencode', 'uuencode', 'uue', 'x-uue'):
              in_file = BytesIO(bpayload)
              out_file = BytesIO()
              try:
                  uu.decode(in_file, out_file, quiet=True)
                  return out_file.getvalue()
              except uu.Error:
                  # Some decoding problem
                  return bpayload
          if isinstance(payload, str):
              return bpayload
          return payload
  
      def set_payload(self, payload, charset=None):
          """Set the payload to the given value.
  
          Optional charset sets the message's default character set.  See
          set_charset() for details.
          """
          self._payload = payload
          if charset is not None:
              self.set_charset(charset)
  
      def set_charset(self, charset):
          """Set the charset of the payload to a given character set.
  
          charset can be a Charset instance, a string naming a character set, or
          None.  If it is a string it will be converted to a Charset instance.
          If charset is None, the charset parameter will be removed from the
          Content-Type field.  Anything else will generate a TypeError.
  
          The message will be assumed to be of type text/* encoded with
          charset.input_charset.  It will be converted to charset.output_charset
          and encoded properly, if needed, when generating the plain text
          representation of the message.  MIME headers (MIME-Version,
          Content-Type, Content-Transfer-Encoding) will be added as needed.
          """
          if charset is None:
              self.del_param('charset')
              self._charset = None
              return
          if not isinstance(charset, Charset):
              charset = Charset(charset)
          self._charset = charset
          if 'MIME-Version' not in self:
              self.add_header('MIME-Version', '1.0')
          if 'Content-Type' not in self:
              self.add_header('Content-Type', 'text/plain',
                              charset=charset.get_output_charset())
          else:
              self.set_param('charset', charset.get_output_charset())
          if charset != charset.get_output_charset():
              self._payload = charset.body_encode(self._payload)
          if 'Content-Transfer-Encoding' not in self:
              cte = charset.get_body_encoding()
              try:
                  cte(self)
              except TypeError:
                  self._payload = charset.body_encode(self._payload)
                  self.add_header('Content-Transfer-Encoding', cte)
  
      def get_charset(self):
          """Return the Charset instance associated with the message's payload.
          """
          return self._charset
  
      #
      # MAPPING INTERFACE (partial)
      #
      def __len__(self):
          """Return the total number of headers, including duplicates."""
          return len(self._headers)
  
      def __getitem__(self, name):
          """Get a header value.
  
          Return None if the header is missing instead of raising an exception.
  
          Note that if the header appeared multiple times, exactly which
          occurrence gets returned is undefined.  Use get_all() to get all
          the values matching a header field name.
          """
          return self.get(name)
  
      def __setitem__(self, name, val):
          """Set the value of a header.
  
          Note: this does not overwrite an existing header with the same field
          name.  Use __delitem__() first to delete any existing headers.
          """
          max_count = self.policy.header_max_count(name)
          if max_count:
              lname = name.lower()
              found = 0
              for k, v in self._headers:
                  if k.lower() == lname:
                      found += 1
                      if found >= max_count:
                          raise ValueError("There may be at most {} {} headers "
                                           "in a message".format(max_count, name))
          self._headers.append(self.policy.header_store_parse(name, val))
  
      def __delitem__(self, name):
          """Delete all occurrences of a header, if present.
  
          Does not raise an exception if the header is missing.
          """
          name = name.lower()
          newheaders = list()
          for k, v in self._headers:
              if k.lower() != name:
                  newheaders.append((k, v))
          self._headers = newheaders
  
      def __contains__(self, name):
          return name.lower() in [k.lower() for k, v in self._headers]
  
      def __iter__(self):
          for field, value in self._headers:
              yield field
  
      def keys(self):
          """Return a list of all the message's header field names.
  
          These will be sorted in the order they appeared in the original
          message, or were added to the message, and may contain duplicates.
          Any fields deleted and re-inserted are always appended to the header
          list.
          """
          return [k for k, v in self._headers]
  
      def values(self):
          """Return a list of all the message's header values.
  
          These will be sorted in the order they appeared in the original
          message, or were added to the message, and may contain duplicates.
          Any fields deleted and re-inserted are always appended to the header
          list.
          """
          return [self.policy.header_fetch_parse(k, v)
                  for k, v in self._headers]
  
      def items(self):
          """Get all the message's header fields and values.
  
          These will be sorted in the order they appeared in the original
          message, or were added to the message, and may contain duplicates.
          Any fields deleted and re-inserted are always appended to the header
          list.
          """
          return [(k, self.policy.header_fetch_parse(k, v))
                  for k, v in self._headers]
  
      def get(self, name, failobj=None):
          """Get a header value.
  
          Like __getitem__() but return failobj instead of None when the field
          is missing.
          """
          name = name.lower()
          for k, v in self._headers:
              if k.lower() == name:
                  return self.policy.header_fetch_parse(k, v)
          return failobj
  
      #
      # "Internal" methods (public API, but only intended for use by a parser
      # or generator, not normal application code.
      #
  
      def set_raw(self, name, value):
          """Store name and value in the model without modification.
  
          This is an "internal" API, intended only for use by a parser.
          """
          self._headers.append((name, value))
  
      def raw_items(self):
          """Return the (name, value) header pairs without modification.
  
          This is an "internal" API, intended only for use by a generator.
          """
          return iter(self._headers.copy())
  
      #
      # Additional useful stuff
      #
  
      def get_all(self, name, failobj=None):
          """Return a list of all the values for the named field.
  
          These will be sorted in the order they appeared in the original
          message, and may contain duplicates.  Any fields deleted and
          re-inserted are always appended to the header list.
  
          If no such fields exist, failobj is returned (defaults to None).
          """
          values = []
          name = name.lower()
          for k, v in self._headers:
              if k.lower() == name:
                  values.append(self.policy.header_fetch_parse(k, v))
          if not values:
              return failobj
          return values
  
      def add_header(self, _name, _value, **_params):
          """Extended header setting.
  
          name is the header field to add.  keyword arguments can be used to set
          additional parameters for the header field, with underscores converted
          to dashes.  Normally the parameter will be added as key="value" unless
          value is None, in which case only the key will be added.  If a
          parameter value contains non-ASCII characters it can be specified as a
          three-tuple of (charset, language, value), in which case it will be
          encoded according to RFC2231 rules.  Otherwise it will be encoded using
          the utf-8 charset and a language of ''.
  
          Examples:
  
          msg.add_header('content-disposition', 'attachment', filename='bud.gif')
          msg.add_header('content-disposition', 'attachment',
                         filename=('utf-8', '', 'Fußballer.ppt'))
          msg.add_header('content-disposition', 'attachment',
                         filename='Fußballer.ppt'))
          """
          parts = []
          for k, v in _params.items():
              if v is None:
                  parts.append(k.replace('_', '-'))
              else:
                  parts.append(_formatparam(k.replace('_', '-'), v))
          if _value is not None:
              parts.insert(0, _value)
          self[_name] = SEMISPACE.join(parts)
  
      def replace_header(self, _name, _value):
          """Replace a header.
  
          Replace the first matching header found in the message, retaining
          header order and case.  If no matching header was found, a KeyError is
          raised.
          """
          _name = _name.lower()
          for i, (k, v) in zip(range(len(self._headers)), self._headers):
              if k.lower() == _name:
                  self._headers[i] = self.policy.header_store_parse(k, _value)
                  break
          else:
              raise KeyError(_name)
  
      #
      # Use these three methods instead of the three above.
      #
  
      def get_content_type(self):
          """Return the message's content type.
  
          The returned string is coerced to lower case of the form
          `maintype/subtype'.  If there was no Content-Type header in the
          message, the default type as given by get_default_type() will be
          returned.  Since according to RFC 2045, messages always have a default
          type this will always return a value.
  
          RFC 2045 defines a message's default type to be text/plain unless it
          appears inside a multipart/digest container, in which case it would be
          message/rfc822.
          """
          missing = object()
          value = self.get('content-type', missing)
          if value is missing:
              # This should have no parameters
              return self.get_default_type()
          ctype = _splitparam(value)[0].lower()
          # RFC 2045, section 5.2 says if its invalid, use text/plain
          if ctype.count('/') != 1:
              return 'text/plain'
          return ctype
  
      def get_content_maintype(self):
          """Return the message's main content type.
  
          This is the `maintype' part of the string returned by
          get_content_type().
          """
          ctype = self.get_content_type()
          return ctype.split('/')[0]
  
      def get_content_subtype(self):
          """Returns the message's sub-content type.
  
          This is the `subtype' part of the string returned by
          get_content_type().
          """
          ctype = self.get_content_type()
          return ctype.split('/')[1]
  
      def get_default_type(self):
          """Return the `default' content type.
  
          Most messages have a default content type of text/plain, except for
          messages that are subparts of multipart/digest containers.  Such
          subparts have a default content type of message/rfc822.
          """
          return self._default_type
  
      def set_default_type(self, ctype):
          """Set the `default' content type.
  
          ctype should be either "text/plain" or "message/rfc822", although this
          is not enforced.  The default content type is not stored in the
          Content-Type header.
          """
          self._default_type = ctype
  
      def _get_params_preserve(self, failobj, header):
          # Like get_params() but preserves the quoting of values.  BAW:
          # should this be part of the public interface?
          missing = object()
          value = self.get(header, missing)
          if value is missing:
              return failobj
          params = []
          for p in _parseparam(value):
              try:
                  name, val = p.split('=', 1)
                  name = name.strip()
                  val = val.strip()
              except ValueError:
                  # Must have been a bare attribute
                  name = p.strip()
                  val = ''
              params.append((name, val))
          params = utils.decode_params(params)
          return params
  
      def get_params(self, failobj=None, header='content-type', unquote=True):
          """Return the message's Content-Type parameters, as a list.
  
          The elements of the returned list are 2-tuples of key/value pairs, as
          split on the `=' sign.  The left hand side of the `=' is the key,
          while the right hand side is the value.  If there is no `=' sign in
          the parameter the value is the empty string.  The value is as
          described in the get_param() method.
  
          Optional failobj is the object to return if there is no Content-Type
          header.  Optional header is the header to search instead of
          Content-Type.  If unquote is True, the value is unquoted.
          """
          missing = object()
          params = self._get_params_preserve(missing, header)
          if params is missing:
              return failobj
          if unquote:
              return [(k, _unquotevalue(v)) for k, v in params]
          else:
              return params
  
      def get_param(self, param, failobj=None, header='content-type',
                    unquote=True):
          """Return the parameter value if found in the Content-Type header.
  
          Optional failobj is the object to return if there is no Content-Type
          header, or the Content-Type header has no such parameter.  Optional
          header is the header to search instead of Content-Type.
  
          Parameter keys are always compared case insensitively.  The return
          value can either be a string, or a 3-tuple if the parameter was RFC
          2231 encoded.  When it's a 3-tuple, the elements of the value are of
          the form (CHARSET, LANGUAGE, VALUE).  Note that both CHARSET and
          LANGUAGE can be None, in which case you should consider VALUE to be
          encoded in the us-ascii charset.  You can usually ignore LANGUAGE.
          The parameter value (either the returned string, or the VALUE item in
          the 3-tuple) is always unquoted, unless unquote is set to False.
  
          If your application doesn't care whether the parameter was RFC 2231
          encoded, it can turn the return value into a string as follows:
  
              param = msg.get_param('foo')
              param = email.utils.collapse_rfc2231_value(rawparam)
  
          """
          if header not in self:
              return failobj
          for k, v in self._get_params_preserve(failobj, header):
              if k.lower() == param.lower():
                  if unquote:
                      return _unquotevalue(v)
                  else:
                      return v
          return failobj
  
      def set_param(self, param, value, header='Content-Type', requote=True,
                    charset=None, language=''):
          """Set a parameter in the Content-Type header.
  
          If the parameter already exists in the header, its value will be
          replaced with the new value.
  
          If header is Content-Type and has not yet been defined for this
          message, it will be set to "text/plain" and the new parameter and
          value will be appended as per RFC 2045.
  
          An alternate header can specified in the header argument, and all
          parameters will be quoted as necessary unless requote is False.
  
          If charset is specified, the parameter will be encoded according to RFC
          2231.  Optional language specifies the RFC 2231 language, defaulting
          to the empty string.  Both charset and language should be strings.
          """
          if not isinstance(value, tuple) and charset:
              value = (charset, language, value)
  
          if header not in self and header.lower() == 'content-type':
              ctype = 'text/plain'
          else:
              ctype = self.get(header)
          if not self.get_param(param, header=header):
              if not ctype:
                  ctype = _formatparam(param, value, requote)
              else:
                  ctype = SEMISPACE.join(
                      [ctype, _formatparam(param, value, requote)])
          else:
              ctype = ''
              for old_param, old_value in self.get_params(header=header,
                                                          unquote=requote):
                  append_param = ''
                  if old_param.lower() == param.lower():
                      append_param = _formatparam(param, value, requote)
                  else:
                      append_param = _formatparam(old_param, old_value, requote)
                  if not ctype:
                      ctype = append_param
                  else:
                      ctype = SEMISPACE.join([ctype, append_param])
          if ctype != self.get(header):
              del self[header]
              self[header] = ctype
  
      def del_param(self, param, header='content-type', requote=True):
          """Remove the given parameter completely from the Content-Type header.
  
          The header will be re-written in place without the parameter or its
          value. All values will be quoted as necessary unless requote is
          False.  Optional header specifies an alternative to the Content-Type
          header.
          """
          if header not in self:
              return
          new_ctype = ''
          for p, v in self.get_params(header=header, unquote=requote):
              if p.lower() != param.lower():
                  if not new_ctype:
                      new_ctype = _formatparam(p, v, requote)
                  else:
                      new_ctype = SEMISPACE.join([new_ctype,
                                                  _formatparam(p, v, requote)])
          if new_ctype != self.get(header):
              del self[header]
              self[header] = new_ctype
  
      def set_type(self, type, header='Content-Type', requote=True):
          """Set the main type and subtype for the Content-Type header.
  
          type must be a string in the form "maintype/subtype", otherwise a
          ValueError is raised.
  
          This method replaces the Content-Type header, keeping all the
          parameters in place.  If requote is False, this leaves the existing
          header's quoting as is.  Otherwise, the parameters will be quoted (the
          default).
  
          An alternative header can be specified in the header argument.  When
          the Content-Type header is set, we'll always also add a MIME-Version
          header.
          """
          # BAW: should we be strict?
          if not type.count('/') == 1:
              raise ValueError
          # Set the Content-Type, you get a MIME-Version
          if header.lower() == 'content-type':
              del self['mime-version']
              self['MIME-Version'] = '1.0'
          if header not in self:
              self[header] = type
              return
          params = self.get_params(header=header, unquote=requote)
          del self[header]
          self[header] = type
          # Skip the first param; it's the old type.
          for p, v in params[1:]:
              self.set_param(p, v, header, requote)
  
      def get_filename(self, failobj=None):
          """Return the filename associated with the payload if present.
  
          The filename is extracted from the Content-Disposition header's
          `filename' parameter, and it is unquoted.  If that header is missing
          the `filename' parameter, this method falls back to looking for the
          `name' parameter.
          """
          missing = object()
          filename = self.get_param('filename', missing, 'content-disposition')
          if filename is missing:
              filename = self.get_param('name', missing, 'content-type')
          if filename is missing:
              return failobj
          return utils.collapse_rfc2231_value(filename).strip()
  
      def get_boundary(self, failobj=None):
          """Return the boundary associated with the payload if present.
  
          The boundary is extracted from the Content-Type header's `boundary'
          parameter, and it is unquoted.
          """
          missing = object()
          boundary = self.get_param('boundary', missing)
          if boundary is missing:
              return failobj
          # RFC 2046 says that boundaries may begin but not end in w/s
          return utils.collapse_rfc2231_value(boundary).rstrip()
  
      def set_boundary(self, boundary):
          """Set the boundary parameter in Content-Type to 'boundary'.
  
          This is subtly different than deleting the Content-Type header and
          adding a new one with a new boundary parameter via add_header().  The
          main difference is that using the set_boundary() method preserves the
          order of the Content-Type header in the original message.
  
          HeaderParseError is raised if the message has no Content-Type header.
          """
          missing = object()
          params = self._get_params_preserve(missing, 'content-type')
          if params is missing:
              # There was no Content-Type header, and we don't know what type
              # to set it to, so raise an exception.
              raise errors.HeaderParseError('No Content-Type header found')
          newparams = []
          foundp = False
          for pk, pv in params:
              if pk.lower() == 'boundary':
                  newparams.append(('boundary', '"%s"' % boundary))
                  foundp = True
              else:
                  newparams.append((pk, pv))
          if not foundp:
              # The original Content-Type header had no boundary attribute.
              # Tack one on the end.  BAW: should we raise an exception
              # instead???
              newparams.append(('boundary', '"%s"' % boundary))
          # Replace the existing Content-Type header with the new value
          newheaders = []
          for h, v in self._headers:
              if h.lower() == 'content-type':
                  parts = []
                  for k, v in newparams:
                      if v == '':
                          parts.append(k)
                      else:
                          parts.append('%s=%s' % (k, v))
                  val = SEMISPACE.join(parts)
                  newheaders.append(self.policy.header_store_parse(h, val))
  
              else:
                  newheaders.append((h, v))
          self._headers = newheaders
  
      def get_content_charset(self, failobj=None):
          """Return the charset parameter of the Content-Type header.
  
          The returned string is always coerced to lower case.  If there is no
          Content-Type header, or if that header has no charset parameter,
          failobj is returned.
          """
          missing = object()
          charset = self.get_param('charset', missing)
          if charset is missing:
              return failobj
          if isinstance(charset, tuple):
              # RFC 2231 encoded, so decode it, and it better end up as ascii.
              pcharset = charset[0] or 'us-ascii'
              try:
                  # LookupError will be raised if the charset isn't known to
                  # Python.  UnicodeError will be raised if the encoded text
                  # contains a character not in the charset.
                  as_bytes = charset[2].encode('raw-unicode-escape')
                  charset = str(as_bytes, pcharset)
              except (LookupError, UnicodeError):
                  charset = charset[2]
          # charset characters must be in us-ascii range
          try:
              charset.encode('us-ascii')
          except UnicodeError:
              return failobj
          # RFC 2046, $4.1.2 says charsets are not case sensitive
          return charset.lower()
  
      def get_charsets(self, failobj=None):
          """Return a list containing the charset(s) used in this message.
  
          The returned list of items describes the Content-Type headers'
          charset parameter for this message and all the subparts in its
          payload.
  
          Each item will either be a string (the value of the charset parameter
          in the Content-Type header of that part) or the value of the
          'failobj' parameter (defaults to None), if the part does not have a
          main MIME type of "text", or the charset is not defined.
  
          The list will contain one string for each part of the message, plus
          one for the container message (i.e. self), so that a non-multipart
          message will still return a list of length 1.
          """
          return [part.get_content_charset(failobj) for part in self.walk()]
  
      # I.e. def walk(self): ...
      from future.backports.email.iterators import walk
  