"email.message"：表示電子郵件訊息
*********************************

**原始碼：**Lib/email/message.py

======================================================================

在 3.6 版被加入: [1]

The central class in the "email" package is the "EmailMessage" class,
imported from the "email.message" module.  It is the base class for
the "email" object model.  "EmailMessage" provides the core
functionality for setting and querying header fields, for accessing
message bodies, and for creating or modifying structured messages.

An email message consists of *headers* and a *payload* (which is also
referred to as the *content*).  Headers are **RFC 5322** or **RFC
6532** style field 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 an "EmailMessage" object is that of
an ordered dictionary of headers coupled with a *payload* that
represents the **RFC 5322** body of the message, which might be a list
of sub-"EmailMessage" objects.  In addition to the normal dictionary
methods for accessing the header names and values, there are methods
for accessing specialized information from the headers (for example
the MIME content type), for operating on the payload, for generating a
serialized version of the message, and for recursively walking over
the object tree.

The "EmailMessage" dictionary-like interface is indexed by the header
names, which must be ASCII values.  The values of the dictionary are
strings with some extra methods.  Headers are stored and returned in
case-preserving form, but field names are matched case-insensitively.
The keys are ordered, but unlike a real dict, there can be duplicates.
Additional methods are provided for working with headers that have
duplicate keys.

The *payload* is either a string or bytes object, in the case of
simple message objects, or a list of "EmailMessage" objects, for MIME
container documents such as *multipart/** and *message/rfc822* message
objects.

class email.message.EmailMessage(policy=default)

   If *policy* is specified use the rules it specifies to update and
   serialize the representation of the message.  If *policy* is not
   set, use the "default" policy, which follows the rules of the email
   RFCs except for line endings (instead of the RFC mandated "\r\n",
   it uses the Python standard "\n" line endings).  For more
   information see the "policy" documentation.

   as_string(unixfrom=False, maxheaderlen=None, 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 with the base "Message" class *maxheaderlen* is
      accepted, but defaults to "None", which means that by default
      the line length is controlled by the "max_line_length" of the
      policy.  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 "EmailMessage"
      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
      be the most useful way to serialize messages in your
      application, especially if you are dealing with multiple
      messages.  See "email.generator.Generator" for a more flexible
      API for serializing messages.  Note also that this method is
      restricted to producing messages serialized as "7 bit clean"
      when "utf8" is "False", which is the default.

      在 3.6 版的變更: the default behavior when *maxheaderlen* is not
      specified was changed from defaulting to 0 to defaulting to the
      value of *max_line_length* from the policy.

   __str__()

      Equivalent to "as_string(policy=self.policy.clone(utf8=True))".
      Allows "str(msg)" to produce a string containing the serialized
      message in a readable format.

      在 3.4 版的變更: the method was changed to use "utf8=True", thus
      producing an **RFC 6531**-like message representation, instead
      of being a direct alias for "as_string()".

   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 "EmailMessage"
      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
      be the most useful way to serialize messages in your
      application, especially if you are dealing with multiple
      messages.  See "email.generator.BytesGenerator" for a more
      flexible API for serializing messages.

   __bytes__()

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

   is_multipart()

      Return "True" if the message's payload is a list of
      sub-"EmailMessage" 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
      "EmailMessage" is of type "message/rfc822".

   set_unixfrom(unixfrom)

      Set the message's envelope header to *unixfrom*, which should be
      a string.  (See "mboxMessage" for a brief description of this
      header.)

   get_unixfrom()

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

   The following methods implement the mapping-like interface for
   accessing the message's 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 an "EmailMessage" object, headers are
   always returned in the order they appeared in the original message,
   or in which they were added to the message later.  Any header
   deleted and then re-added is always appended to the end of the
   header list.

   These semantic differences are intentional and are biased toward
   convenience in the most common use cases.

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

   __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 without regard to case and *name* does not
      include the trailing colon.  Used for the "in" operator.  For
      example:

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

   __getitem__(name)

      Return the value of the named header field.  *name* does 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 headers named *name*.

      Using the standard (non-"compat32") policies, the returned value
      is an instance of a subclass of
      "email.headerregistry.BaseHeader".

   __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 headers.

      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!'

      If the "policy" defines certain headers to be unique (as the
      standard policies do), this method may raise a "ValueError" when
      an attempt is made to assign a value to such a header when one
      already exists.  This behavior is intentional for consistency's
      sake, but do not depend on it as we may choose to make such
      assignments do an automatic deletion of the existing header in
      the future.

   __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 (*failobj* defaults to "None").

   Here are some additional useful header related 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, the charset and
      language may be explicitly controlled by specifying the value 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".

      以下是個範例：

         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 of the extended interface with non-ASCII characters:

         msg.add_header('Content-Disposition', 'attachment',
                        filename=('iso-8859-1', '', 'Fußballer.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
      of the original header.  If no matching header is found, raise a
      "KeyError".

   get_content_type()

      Return the message's content type, coerced to lower case of the
      form *maintype/subtype*.  If there is no *Content-Type* header
      in the message return the value returned by
      "get_default_type()".  If the *Content-Type* header is invalid,
      return "text/plain".

      (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, so it only affects the return value of the
      "get_content_type" methods when no *Content-Type* header is
      present in the message.

   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, replace its value with *value*.
      When *header* is "Content-Type" (the default) and the header
      does not yet exist in the message, add it, set its value to
      *text/plain*, and append the new parameter value.  Optional
      *header* specifies an alternative header to *Content-Type*.

      If the value contains non-ASCII characters, the charset and
      language may be explicitly specified using the optional
      *charset* and *language* parameters.  Optional *language*
      specifies the **RFC 2231** language, defaulting to the empty
      string.  Both *charset* and *language* should be strings.  The
      default is to use the "utf8" *charset* and "None" for the
      *language*.

      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.

      Use of the *requote* parameter with "EmailMessage" objects is
      deprecated.

      Note that existing parameter values of headers may be accessed
      through the "params" attribute of the header value (for example,
      "msg['Content-Type'].params['charset']").

      在 3.4 版的變更: "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.  Optional *header* specifies an
      alternative to *Content-Type*.

      Use of the *requote* parameter with "EmailMessage" objects is
      deprecated.

   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 from 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.

   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.

   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.  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*.

   is_attachment()

      Return "True" if there is a *Content-Disposition* header and its
      (case insensitive) value is "attachment", "False" otherwise.

      在 3.4.2 版的變更: is_attachment is now a method instead of a
      property, for consistency with "is_multipart()".

   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**.

      在 3.5 版被加入.

   The following methods relate to interrogating and manipulating the
   content (payload) of the message.

   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:

         >>> from email.iterators import _structure
         >>> 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.

   get_body(preferencelist=('related', 'html', 'plain'))

      Return the MIME part that is the best candidate to be the "body"
      of the message.

      *preferencelist* must be a sequence of strings from the set
      "related", "html", and "plain", and indicates the order of
      preference for the content type of the part returned.

      Start looking for candidate matches with the object on which the
      "get_body" method is called.

      If "related" is not included in *preferencelist*, consider the
      root part (or subpart of the root part) of any related
      encountered as a candidate if the (sub-)part matches a
      preference.

      When encountering a "multipart/related", check the "start"
      parameter and if a part with a matching *Content-ID* is found,
      consider only it when looking for candidate matches.  Otherwise
      consider only the first (default root) part of the
      "multipart/related".

      If a part has a *Content-Disposition* header, only consider the
      part a candidate match if the value of the header is "inline".

      If none of the candidates matches any of the preferences in
      *preferencelist*, return "None".

      Notes: (1) For most applications the only *preferencelist*
      combinations that really make sense are "('plain',)", "('html',
      'plain')", and the default "('related', 'html', 'plain')".  (2)
      Because matching starts with the object on which "get_body" is
      called, calling "get_body" on a "multipart/related" will return
      the object itself unless *preferencelist* has a non-default
      value. (3) Messages (or message parts) that do not specify a
      *Content-Type* or whose *Content-Type* header is invalid will be
      treated as if they are of type "text/plain", which may
      occasionally cause "get_body" to return unexpected results.

   iter_attachments()

      Return an iterator over all of the immediate sub-parts of the
      message that are not candidate "body" parts.  That is, skip the
      first occurrence of each of "text/plain", "text/html",
      "multipart/related", or "multipart/alternative" (unless they are
      explicitly marked as attachments via *Content-Disposition:
      attachment*), and return all remaining parts.  When applied
      directly to a "multipart/related", return an iterator over the
      all the related parts except the root part (ie: the part pointed
      to by the "start" parameter, or the first part if there is no
      "start" parameter or the "start" parameter doesn't match the
      *Content-ID* of any of the parts).  When applied directly to a
      "multipart/alternative" or a non-"multipart", return an empty
      iterator.

   iter_parts()

      Return an iterator over all of the immediate sub-parts of the
      message, which will be empty for a non-"multipart".  (See also
      "walk()".)

   get_content(*args, content_manager=None, **kw)

      Call the "get_content()" method of the *content_manager*,
      passing self as the message object, and passing along any other
      arguments or keywords as additional arguments.  If
      *content_manager* is not specified, use the "content_manager"
      specified by the current "policy".

   set_content(*args, content_manager=None, **kw)

      Call the "set_content()" method of the *content_manager*,
      passing self as the message object, and passing along any other
      arguments or keywords as additional arguments.  If
      *content_manager* is not specified, use the "content_manager"
      specified by the current "policy".

   make_related(boundary=None)

      Convert a non-"multipart" message into a "multipart/related"
      message, moving any existing *Content-* headers and payload into
      a (new) first part of the "multipart".  If *boundary* is
      specified, use it as the boundary string in the multipart,
      otherwise leave the boundary to be automatically created when it
      is needed (for example, when the message is serialized).

   make_alternative(boundary=None)

      Convert a non-"multipart" or a "multipart/related" into a
      "multipart/alternative", moving any existing *Content-* headers
      and payload into a (new) first part of the "multipart".  If
      *boundary* is specified, use it as the boundary string in the
      multipart, otherwise leave the boundary to be automatically
      created when it is needed (for example, when the message is
      serialized).

   make_mixed(boundary=None)

      Convert a non-"multipart", a "multipart/related", or a
      "multipart-alternative" into a "multipart/mixed", moving any
      existing *Content-* headers and payload into a (new) first part
      of the "multipart".  If *boundary* is specified, use it as the
      boundary string in the multipart, otherwise leave the boundary
      to be automatically created when it is needed (for example, when
      the message is serialized).

   add_related(*args, content_manager=None, **kw)

      If the message is a "multipart/related", create a new message
      object, pass all of the arguments to its "set_content()" method,
      and "attach()" it to the "multipart".  If the message is a
      non-"multipart", call "make_related()" and then proceed as
      above.  If the message is any other type of "multipart", raise a
      "TypeError". If *content_manager* is not specified, use the
      "content_manager" specified by the current "policy". If the
      added part has no *Content-Disposition* header, add one with the
      value "inline".

   add_alternative(*args, content_manager=None, **kw)

      If the message is a "multipart/alternative", create a new
      message object, pass all of the arguments to its "set_content()"
      method, and "attach()" it to the "multipart".  If the message is
      a non-"multipart" or "multipart/related", call
      "make_alternative()" and then proceed as above.  If the message
      is any other type of "multipart", raise a "TypeError". If
      *content_manager* is not specified, use the "content_manager"
      specified by the current "policy".

   add_attachment(*args, content_manager=None, **kw)

      If the message is a "multipart/mixed", create a new message
      object, pass all of the arguments to its "set_content()" method,
      and "attach()" it to the "multipart".  If the message is a
      non-"multipart", "multipart/related", or
      "multipart/alternative", call "make_mixed()" and then proceed as
      above. If *content_manager* is not specified, use the
      "content_manager" specified by the current "policy".  If the
      added part has no *Content-Disposition* header, add one with the
      value "attachment".  This method can be used both for explicit
      attachments (*Content-Disposition: attachment*) and "inline"
      attachments (*Content-Disposition: inline*), by passing
      appropriate options to the "content_manager".

   clear()

      Remove the payload and all of the headers.

   clear_content()

      Remove the payload and all of the *!Content-* headers, leaving
      all other headers intact and in their original order.

   "EmailMessage" objects have the following instance attributes:

   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.  As with the
      "preamble", if there is no epilog text this attribute will be
      "None".

   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.

class email.message.MIMEPart(policy=default)

   This class represents a subpart of a MIME message.  It is identical
   to "EmailMessage", except that no *MIME-Version* headers are added
   when "set_content()" is called, since sub-parts do not need their
   own *MIME-Version* headers.

-[ 註解 ]-

[1] Originally added in 3.4 as a *provisional module*.  Docs for
    legacy message class moved to email.message.Message: Representing
    an email message using the compat32 API.
