"email.message.Message": Representing an email message using the "compat32" API
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The "Message" class is very similar to the "EmailMessage" class,
without the methods added by that class, and with the default behavior
of certain other methods being slightly different.  We also document
here some methods that, while supported by the "EmailMessage" class,
are not recommended unless you are dealing with legacy code.

The philosophy and structure of the two classes is otherwise the same.

This document describes the behavior under the default (for "Message")
policy "Compat32".  If you are going to use another policy, you should
be using the "EmailMessage" class instead.

An email message consists of *headers* and a *payload*.  Headers must
be **RFC 5233** style names and values, where the field name and value
are separated by a colon.  The colon is not part of either the field
name or the field value.  The payload may be a simple text message, or
a binary object, or a structured sequence of sub-messages each with
their own set of headers and their own payload.  The latter type of
payload is indicated by the message having a MIME type such as
*multipart/** or *message/rfc822*.

The conceptual model provided by a "Message" object is that of an
ordered dictionary of headers with additional methods for accessing
both specialized information from the headers, for accessing the
payload, for generating a serialized version of the message, and for
recursively walking over the object tree.  Note that duplicate headers
are supported but special methods must be used to access them.

The "Message" pseudo-dictionary is indexed by the header names, which
must be ASCII values.  The values of the dictionary are strings that
are supposed to contain only ASCII characters; there is some special
handling for non-ASCII input, but it doesn't always produce the
correct results.  Headers are stored and returned in case-preserving
form, but field names are matched case-insensitively.  There may also
be a single envelope header, also known as the *Unix-From* header or
the "From_" header.  The *payload* is either a string or bytes, in the
case of simple message objects, or a list of "Message" objects, for
MIME container documents (e.g. *multipart/** and *message/rfc822*).

Here are the methods of the "Message" class:

class email.message.Message(policy=compat32)

   If *policy* is specified (it must be an instance of a "policy"
   class) use the rules it specifies to update and serialize the
   representation of the message.  If *policy* is not set, use the
   "compat32" policy, which maintains backward compatibility with the
   Python 3.2 version of the email package.  For more information see
   the "policy" documentation.

   在 3.3 版更改: The *policy* keyword argument was added.

   as_string(unixfrom=False, maxheaderlen=0, policy=None)

      Return the entire message flattened as a string.  When optional
      *unixfrom* is true, the envelope header is included in the
      returned string. *unixfrom* defaults to "False".  For backward
      compatibility reasons, *maxheaderlen* defaults to "0", so if you
      want a different value you must override it explicitly (the
      value specified for *max_line_length* in the policy will be
      ignored by this method).  The *policy* argument may be used to
      override the default policy obtained from the message instance.
      This can be used to control some of the formatting produced by
      the method, since the specified *policy* will be passed to the
      "Generator".

      Flattening the message may trigger changes to the "Message" if
      defaults need to be filled in to complete the transformation to
      a string (for example, MIME boundaries may be generated or
      modified).

      Note that this method is provided as a convenience and may not
      always format the message the way you want.  For example, by
      default it does not do the mangling of lines that begin with
      "From" that is required by the unix mbox format.  For more
      flexibility, instantiate a "Generator" instance and use its
      "flatten()" method directly.  For example:

         from io import StringIO
         from email.generator import Generator
         fp = StringIO()
         g = Generator(fp, mangle_from_=True, maxheaderlen=60)
         g.flatten(msg)
         text = fp.getvalue()

      If the message object contains binary data that is not encoded
      according to RFC standards, the non-compliant data will be
      replaced by unicode "unknown character" code points.  (See also
      "as_bytes()" and "BytesGenerator".)

      在 3.4 版更改: the *policy* keyword argument was added.

   __str__()

      Equivalent to "as_string()".  Allows "str(msg)" to produce a
      string containing the formatted message.

   as_bytes(unixfrom=False, policy=None)

      Return the entire message flattened as a bytes object.  When
      optional *unixfrom* is true, the envelope header is included in
      the returned string.  *unixfrom* defaults to "False".  The
      *policy* argument may be used to override the default policy
      obtained from the message instance. This can be used to control
      some of the formatting produced by the method, since the
      specified *policy* will be passed to the "BytesGenerator".

      Flattening the message may trigger changes to the "Message" if
      defaults need to be filled in to complete the transformation to
      a string (for example, MIME boundaries may be generated or
      modified).

      Note that this method is provided as a convenience and may not
      always format the message the way you want.  For example, by
      default it does not do the mangling of lines that begin with
      "From" that is required by the unix mbox format.  For more
      flexibility, instantiate a "BytesGenerator" instance and use its
      "flatten()" method directly. For example:

         from io import BytesIO
         from email.generator import BytesGenerator
         fp = BytesIO()
         g = BytesGenerator(fp, mangle_from_=True, maxheaderlen=60)
         g.flatten(msg)
         text = fp.getvalue()

      3.4 新版功能.

   __bytes__()

      Equivalent to "as_bytes()".  Allows "bytes(msg)" to produce a
      bytes object containing the formatted message.

      3.4 新版功能.

   is_multipart()

      Return "True" if the message's payload is a list of
      sub-"Message" objects, otherwise return "False".  When
      "is_multipart()" returns "False", the payload should be a string
      object (which might be a CTE encoded binary payload).  (Note
      that "is_multipart()" returning "True" does not necessarily mean
      that "msg.get_content_maintype() == 'multipart'" will return the
      "True". For example, "is_multipart" will return "True" when the
      "Message" is of type "message/rfc822".)

   set_unixfrom(unixfrom)

      Set the message's envelope header to *unixfrom*, which should be
      a string.

   get_unixfrom()

      Return the message's envelope header.  Defaults to "None" if the
      envelope header was never set.

   attach(payload)

      Add the given *payload* to the current payload, which must be
      "None" or a list of "Message" objects before the call. After the
      call, the payload will always be a list of "Message" objects.
      If you want to set the payload to a scalar object (e.g. a
      string), use "set_payload()" instead.

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by "set_content()" and the related
      "make" and "add" methods.

   get_payload(i=None, decode=False)

      Return the current payload, which will be a list of "Message"
      objects when "is_multipart()" is "True", or a string when
      "is_multipart()" is "False".  If the payload is a list and you
      mutate the list object, you modify the message's payload in
      place.

      With optional argument *i*, "get_payload()" will return the
      *i*-th element of the payload, counting from zero, if
      "is_multipart()" is "True".  An "IndexError" will be raised if
      *i* is less than 0 or greater than or equal to the number of
      items in the payload.  If the payload is a string (i.e.
      "is_multipart()" is "False") and *i* is given, a "TypeError" is
      raised.

      Optional *decode* is a flag indicating whether the payload
      should be decoded or not, according to the *Content-Transfer-
      Encoding* header. 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 *Content-Transfer-Encoding* header is missing, the payload is
      returned as-is (undecoded).  In all cases the returned value is
      binary data.  If the message is a multipart and the *decode*
      flag is "True", then "None" is returned.  If the payload is
      base64 and it was not perfectly formed (missing padding,
      characters outside the base64 alphabet), then an appropriate
      defect will be added to the message's defect property
      ("InvalidBase64PaddingDefect" or
      "InvalidBase64CharactersDefect", respectively).

      When *decode* is "False" (the default) the body is returned as a
      string without decoding the *Content-Transfer-Encoding*.
      However, for a *Content-Transfer-Encoding* of 8bit, an attempt
      is made to decode the original bytes using the "charset"
      specified by the *Content-Type* header, using the "replace"
      error handler. If no "charset" is specified, or if the "charset"
      given is not recognized by the email package, the body is
      decoded using the default ASCII charset.

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by "get_content()" and "iter_parts()".

   set_payload(payload, charset=None)

      Set the entire message object's payload to *payload*.  It is the
      client's responsibility to ensure the payload invariants.
      Optional *charset* sets the message's default character set; see
      "set_charset()" for details.

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by "set_content()".

   set_charset(charset)

      Set the character set of the payload to *charset*, which can
      either be a "Charset" instance (see "email.charset"), 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*
      header (the message will not be otherwise modified).  Anything
      else will generate a "TypeError".

      If there is no existing *MIME-Version* header one will be added.
      If there is no existing *Content-Type* header, one will be added
      with a value of *text/plain*.  Whether the *Content-Type* header
      already exists or not, its "charset" parameter will be set to
      *charset.output_charset*.   If *charset.input_charset* and
      *charset.output_charset* differ, the payload will be re-encoded
      to the *output_charset*.  If there is no existing *Content-
      Transfer-Encoding* header, then the payload will be transfer-
      encoded, if needed, using the specified "Charset", and a header
      with the appropriate value will be added.  If a *Content-
      Transfer-Encoding* header already exists, the payload is assumed
      to already be correctly encoded using that *Content-Transfer-
      Encoding* and is not modified.

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by the *charset* parameter of the
      "email.emailmessage.EmailMessage.set_content()" method.

   get_charset()

      Return the "Charset" instance associated with the message's
      payload.

      This is a legacy method.  On the "EmailMessage" class it always
      returns "None".

   The following methods implement a mapping-like interface for
   accessing the message's **RFC 2822** headers.  Note that there are
   some semantic differences between these methods and a normal
   mapping (i.e. dictionary) interface.  For example, in a dictionary
   there are no duplicate keys, but here there may be duplicate
   message headers.  Also, in dictionaries there is no guaranteed
   order to the keys returned by "keys()", but in a "Message" object,
   headers are always returned in the order they appeared in the
   original message, or were added to the message later.  Any header
   deleted and then re-added are always appended to the end of the
   header list.

   These semantic differences are intentional and are biased toward
   maximal convenience.

   Note that in all cases, any envelope header present in the message
   is not included in the mapping interface.

   In a model generated from bytes, any header values that (in
   contravention of the RFCs) contain non-ASCII bytes will, when
   retrieved through this interface, be represented as "Header"
   objects with a charset of *unknown-8bit*.

   __len__()

      Return the total number of headers, including duplicates.

   __contains__(name)

      Return "True" if the message object has a field named *name*.
      Matching is done case-insensitively and *name* should not
      include the trailing colon. Used for the "in" operator, e.g.:

         if 'message-id' in myMessage:
            print('Message-ID:', myMessage['message-id'])

   __getitem__(name)

      Return the value of the named header field.  *name* should not
      include the colon field separator.  If the header is missing,
      "None" is returned; a "KeyError" is never raised.

      Note that if the named field appears more than once in the
      message's headers, exactly which of those field values will be
      returned is undefined.  Use the "get_all()" method to get the
      values of all the extant named headers.

   __setitem__(name, val)

      Add a header to the message with field name *name* and value
      *val*.  The field is appended to the end of the message's
      existing fields.

      Note that this does *not* overwrite or delete any existing
      header with the same name.  If you want to ensure that the new
      header is the only one present in the message with field name
      *name*, delete the field first, e.g.:

         del msg['subject']
         msg['subject'] = 'Python roolz!'

   __delitem__(name)

      Delete all occurrences of the field with name *name* from the
      message's headers.  No exception is raised if the named field
      isn't present in the headers.

   keys()

      Return a list of all the message's header field names.

   values()

      Return a list of all the message's field values.

   items()

      Return a list of 2-tuples containing all the message's field
      headers and values.

   get(name, failobj=None)

      Return the value of the named header field.  This is identical
      to "__getitem__()" except that optional *failobj* is returned if
      the named header is missing (defaults to "None").

   Here are some additional useful methods:

   get_all(name, failobj=None)

      Return a list of all the values for the field named *name*. If
      there are no such named headers in the message, *failobj* is
      returned (defaults to "None").

   add_header(_name, _value, **_params)

      Extended header setting.  This method is similar to
      "__setitem__()" except that additional header parameters can be
      provided as keyword arguments.  *_name* is the header field to
      add and *_value* is the *primary* value for the header.

      For each item in the keyword argument dictionary *_params*, the
      key is taken as the parameter name, with underscores converted
      to dashes (since dashes are illegal in Python identifiers).
      Normally, the parameter will be added as "key="value"" unless
      the value is "None", in which case only the key will be added.
      If the value contains non-ASCII characters, it can be specified
      as a three tuple in the format "(CHARSET, LANGUAGE, VALUE)",
      where "CHARSET" is a string naming the charset to be used to
      encode the value, "LANGUAGE" can usually be set to "None" or the
      empty string (see **RFC 2231** for other possibilities), and
      "VALUE" is the string value containing non-ASCII code points.
      If a three tuple is not passed and the value contains non-ASCII
      characters, it is automatically encoded in **RFC 2231** format
      using a "CHARSET" of "utf-8" and a "LANGUAGE" of "None".

      Here's an example:

         msg.add_header('Content-Disposition', 'attachment', filename='bud.gif')

      This will add a header that looks like

         Content-Disposition: attachment; filename="bud.gif"

      An example with non-ASCII characters:

         msg.add_header('Content-Disposition', 'attachment',
                        filename=('iso-8859-1', '', 'Fußballer.ppt'))

      Which produces

         Content-Disposition: attachment; filename*="iso-8859-1''Fu%DFballer.ppt"

   replace_header(_name, _value)

      Replace a header.  Replace the first header found in the message
      that matches *_name*, retaining header order and field name
      case.  If no matching header was found, a "KeyError" is raised.

   get_content_type()

      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,
      "get_content_type()" 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*.  If the *Content-Type*
      header has an invalid type specification, **RFC 2045** mandates
      that the default type be *text/plain*.

   get_content_maintype()

      Return the message's main content type.  This is the *maintype*
      part of the string returned by "get_content_type()".

   get_content_subtype()

      Return the message's sub-content type.  This is the *subtype*
      part of the string returned by "get_content_type()".

   get_default_type()

      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*.

   set_default_type(ctype)

      Set the default content type.  *ctype* should either be
      *text/plain* or *message/rfc822*, although this is not enforced.
      The default content type is not stored in the *Content-Type*
      header.

   get_params(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,
      otherwise the value is as described in "get_param()" and is
      unquoted if optional *unquote* is "True" (the default).

      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*.

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by the *params* property of the
      individual header objects returned by the header access methods.

   get_param(param, failobj=None, header='content-type', unquote=True)

      Return the value of the *Content-Type* header's parameter
      *param* as a string.  If the message has no *Content-Type*
      header or if there is no such parameter, then *failobj* is
      returned (defaults to "None").

      Optional *header* if given, specifies the message header to use
      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".

      If your application doesn't care whether the parameter was
      encoded as in **RFC 2231**, you can collapse the parameter value
      by calling "email.utils.collapse_rfc2231_value()", passing in
      the return value from "get_param()".  This will return a
      suitably decoded Unicode string when the value is a tuple, or
      the original string unquoted if it isn't.  For example:

         rawparam = msg.get_param('foo')
         param = email.utils.collapse_rfc2231_value(rawparam)

      In any case, the parameter value (either the returned string, or
      the "VALUE" item in the 3-tuple) is always unquoted, unless
      *unquote* is set to "False".

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by the *params* property of the
      individual header objects returned by the header access methods.

   set_param(param, value, header='Content-Type', requote=True, charset=None, language='', replace=False)

      Set a parameter in the *Content-Type* header.  If the parameter
      already exists in the header, its value will be replaced with
      *value*.  If the *Content-Type* header as not yet been defined
      for this message, it will be set to *text/plain* and the new
      parameter value will be appended as per **RFC 2045**.

      Optional *header* specifies an alternative header to *Content-
      Type*, and all parameters will be quoted as necessary unless
      optional *requote* is "False" (the default is "True").

      If optional *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 *replace* is "False" (the default) the header is moved to the
      end of the list of headers.  If *replace* is "True", the header
      will be updated in place.

      在 3.4 版更改: "replace" keyword was added.

   del_param(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" (the default is "True").  Optional
      *header* specifies an alternative to *Content-Type*.

   set_type(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 a *MIME-Version* header is
      also added.

      This is a legacy method.  On the "EmailMessage" class its
      functionality is replaced by the "make_" and "add_" methods.

   get_filename(failobj=None)

      Return the value of the "filename" parameter of the *Content-
      Disposition* header of the message.  If the header does not have
      a "filename" parameter, this method falls back to looking for
      the "name" parameter on the *Content-Type* header.  If neither
      is found, or the header is missing, then *failobj* is returned.
      The returned string will always be unquoted as per
      "email.utils.unquote()".

   get_boundary(failobj=None)

      Return the value of the "boundary" parameter of the *Content-
      Type* header of the message, or *failobj* if either the header
      is missing, or has no "boundary" parameter.  The returned string
      will always be unquoted as per "email.utils.unquote()".

   set_boundary(boundary)

      Set the "boundary" parameter of the *Content-Type* header to
      *boundary*.  "set_boundary()" will always quote *boundary* if
      necessary.  A "HeaderParseError" is raised if the message object
      has no *Content-Type* header.

      Note that using this method is subtly different than deleting
      the old *Content-Type* header and adding a new one with the new
      boundary via "add_header()", because "set_boundary()" preserves
      the order of the *Content-Type* header in the list of headers.
      However, it does *not* preserve any continuation lines which may
      have been present in the original *Content-Type* header.

   get_content_charset(failobj=None)

      Return the "charset" parameter of the *Content-Type* header,
      coerced to lower case.  If there is no *Content-Type* header, or
      if that header has no "charset" parameter, *failobj* is
      returned.

      Note that this method differs from "get_charset()" which returns
      the "Charset" instance for the default encoding of the message
      body.

   get_charsets(failobj=None)

      Return a list containing the character set names in the message.
      If the message is a *multipart*, then the list will contain one
      element for each subpart in the payload, otherwise, it will be a
      list of length 1.

      Each item in the list will be a string which is the value of the
      "charset" parameter in the *Content-Type* header for the
      represented subpart.  However, if the subpart has no *Content-
      Type* header, no "charset" parameter, or is not of the *text*
      main MIME type, then that item in the returned list will be
      *failobj*.

   get_content_disposition()

      Return the lowercased value (without parameters) of the
      message's *Content-Disposition* header if it has one, or "None".
      The possible values for this method are *inline*, *attachment*
      or "None" if the message follows **RFC 2183**.

      3.5 新版功能.

   walk()

      The "walk()" method is an all-purpose generator which can be
      used to iterate over all the parts and subparts of a message
      object tree, in depth-first traversal order.  You will typically
      use "walk()" as the iterator in a "for" loop; each iteration
      returns the next subpart.

      Here's an example that prints the MIME type of every part of a
      multipart message structure:

         >>> for part in msg.walk():
         ...     print(part.get_content_type())
         multipart/report
         text/plain
         message/delivery-status
         text/plain
         text/plain
         message/rfc822
         text/plain

      "walk" iterates over the subparts of any part where
      "is_multipart()" returns "True", even though
      "msg.get_content_maintype() == 'multipart'" may return "False".
      We can see this in our example by making use of the "_structure"
      debug helper function:

         >>> for part in msg.walk():
         ...     print(part.get_content_maintype() == 'multipart',
         ...           part.is_multipart())
         True True
         False False
         False True
         False False
         False False
         False True
         False False
         >>> _structure(msg)
         multipart/report
             text/plain
             message/delivery-status
                 text/plain
                 text/plain
             message/rfc822
                 text/plain

      Here the "message" parts are not "multiparts", but they do
      contain subparts. "is_multipart()" returns "True" and "walk"
      descends into the subparts.

   "Message" objects can also optionally contain two instance
   attributes, which can be used when generating the plain text of a
   MIME message.

   preamble

      The format of a MIME document allows for some text between the
      blank line following the headers, and the first multipart
      boundary string. Normally, this text is never visible in a MIME-
      aware mail reader because it falls outside the standard MIME
      armor.  However, when viewing the raw text of the message, or
      when viewing the message in a non-MIME aware reader, this text
      can become visible.

      The *preamble* attribute contains this leading extra-armor text
      for MIME documents.  When the "Parser" discovers some text after
      the headers but before the first boundary string, it assigns
      this text to the message's *preamble* attribute.  When the
      "Generator" is writing out the plain text representation of a
      MIME message, and it finds the message has a *preamble*
      attribute, it will write this text in the area between the
      headers and the first boundary.  See "email.parser" and
      "email.generator" for details.

      Note that if the message object has no preamble, the *preamble*
      attribute will be "None".

   epilogue

      The *epilogue* attribute acts the same way as the *preamble*
      attribute, except that it contains text that appears between the
      last boundary and the end of the message.

      You do not need to set the epilogue to the empty string in order
      for the "Generator" to print a newline at the end of the file.

   defects

      The *defects* attribute contains a list of all the problems
      found when parsing this message.  See "email.errors" for a
      detailed description of the possible parsing defects.
