Issue #5170: Fixed regression caused when fixing #5768.

[python.git] / Doc / library / email.message.rst

:mod:`email`: Representing an email message
-------------------------------------------

.. module:: email.message
   :synopsis: The base class representing email messages.


The central class in the :mod:`email` package is the :class:`Message` class,
imported from the :mod:`email.message` module.  It is the base class for the
:mod:`email` object model.  :class:`Message` provides the core functionality for
setting and querying header fields, and for accessing message bodies.

Conceptually, a :class:`Message` object consists of *headers* and *payloads*.
Headers are :rfc:`2822` 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.

Headers are stored and returned in case-preserving form but 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
in the case of simple message objects or a list of :class:`Message` objects for
MIME container documents (e.g. :mimetype:`multipart/\*` and
:mimetype:`message/rfc822`).

:class:`Message` objects provide a mapping style interface for accessing the
message headers, and an explicit interface for accessing both the headers and
the payload.  It provides convenience methods for generating a flat text
representation of the message object tree, for accessing commonly used header
parameters, and for recursively walking over the object tree.

Here are the methods of the :class:`Message` class:


.. class:: Message()

   The constructor takes no arguments.


   .. method:: as_string([unixfrom])

      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``.

      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 mangles
      lines that begin with ``From``.  For more flexibility, instantiate a
      :class:`~email.generator.Generator` instance and use its :meth:`flatten`
      method directly.  For example::

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


   .. method:: __str__()

      Equivalent to ``as_string(unixfrom=True)``.


   .. method:: is_multipart()

      Return ``True`` if the message's payload is a list of sub-\
      :class:`Message` objects, otherwise return ``False``.  When
      :meth:`is_multipart` returns False, the payload should be a string object.


   .. method:: set_unixfrom(unixfrom)

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


   .. method:: get_unixfrom()

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


   .. method:: attach(payload)

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


   .. method:: get_payload([i[, decode]])

      Return the current payload, which will be a list of
      :class:`Message` objects when :meth:`is_multipart` is ``True``, or a
      string when :meth:`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*, :meth:`get_payload` will return the *i*-th
      element of the payload, counting from zero, if :meth:`is_multipart` is
      ``True``.  An :exc:`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.  :meth:`is_multipart` is ``False``) and *i* is
      given, a :exc:`TypeError` is raised.

      Optional *decode* is a flag indicating whether the payload should be
      decoded or not, according to the :mailheader:`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 :mailheader:`Content-Transfer-Encoding`
      header is missing, or if the payload has bogus base64 data, the payload is
      returned as-is (undecoded).  If the message is a multipart and the
      *decode* flag is ``True``, then ``None`` is returned.  The default for
      *decode* is ``False``.


   .. method:: set_payload(payload[, charset])

      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 :meth:`set_charset` for details.

      .. versionchanged:: 2.2.2
         *charset* argument added.


   .. method:: set_charset(charset)

      Set the character set of the payload to *charset*, which can either be a
      :class:`~email.charset.Charset` instance (see :mod:`email.charset`), a
      string naming a character set, or ``None``.  If it is a string, it will
      be converted to a :class:`~email.charset.Charset` instance.  If *charset*
      is ``None``, the ``charset`` parameter will be removed from the
      :mailheader:`Content-Type` header. Anything else will generate a
      :exc:`TypeError`.

      The message will be assumed to be of type :mimetype:`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 (:mailheader:`MIME-Version`,
      :mailheader:`Content-Type`, :mailheader:`Content-Transfer-Encoding`) will
      be added as needed.

      .. versionadded:: 2.2.2


   .. method:: get_charset()

      Return the :class:`~email.charset.Charset` instance associated with the
      message's payload.

      .. versionadded:: 2.2.2

   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 :meth:`keys`, but in a :class:`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.


   .. method:: __len__()

      Return the total number of headers, including duplicates.


   .. method:: __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']


   .. method:: __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
      :exc:`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 :meth:`get_all` method to get the values of all the
      extant named headers.


   .. method:: __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!'


   .. method:: __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.


   .. method:: has_key(name)

      Return true if the message contains a header field named *name*, otherwise
      return false.


   .. method:: keys()

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


   .. method:: values()

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


   .. method:: items()

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


   .. method:: get(name[, failobj])

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

   Here are some additional useful methods:


   .. method:: get_all(name[, failobj])

      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``).


   .. method:: add_header(_name, _value, **_params)

      Extended header setting.  This method is similar to :meth:`__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.

      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"


   .. method:: 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 :exc:`KeyError` is raised.

      .. versionadded:: 2.2.2


   .. method:: get_content_type()

      Return the message's content type.  The returned string is coerced to
      lower case of the form :mimetype:`maintype/subtype`.  If there was no
      :mailheader:`Content-Type` header in the message the default type as given
      by :meth:`get_default_type` will be returned.  Since according to
      :rfc:`2045`, messages always have a default type, :meth:`get_content_type`
      will always return a value.

      :rfc:`2045` defines a message's default type to be :mimetype:`text/plain`
      unless it appears inside a :mimetype:`multipart/digest` container, in
      which case it would be :mimetype:`message/rfc822`.  If the
      :mailheader:`Content-Type` header has an invalid type specification,
      :rfc:`2045` mandates that the default type be :mimetype:`text/plain`.

      .. versionadded:: 2.2.2


   .. method:: get_content_maintype()

      Return the message's main content type.  This is the :mimetype:`maintype`
      part of the string returned by :meth:`get_content_type`.

      .. versionadded:: 2.2.2


   .. method:: get_content_subtype()

      Return the message's sub-content type.  This is the :mimetype:`subtype`
      part of the string returned by :meth:`get_content_type`.

      .. versionadded:: 2.2.2


   .. method:: get_default_type()

      Return the default content type.  Most messages have a default content
      type of :mimetype:`text/plain`, except for messages that are subparts of
      :mimetype:`multipart/digest` containers.  Such subparts have a default
      content type of :mimetype:`message/rfc822`.

      .. versionadded:: 2.2.2


   .. method:: set_default_type(ctype)

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

      .. versionadded:: 2.2.2


   .. method:: get_params([failobj[, header[, unquote]]])

      Return the message's :mailheader:`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 :meth:`get_param` and is unquoted if optional *unquote* is
      ``True`` (the default).

      Optional *failobj* is the object to return if there is no
      :mailheader:`Content-Type` header.  Optional *header* is the header to
      search instead of :mailheader:`Content-Type`.

      .. versionchanged:: 2.2.2
         *unquote* argument added.


   .. method:: get_param(param[, failobj[, header[, unquote]]])

      Return the value of the :mailheader:`Content-Type` header's parameter
      *param* as a string.  If the message has no :mailheader:`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
      :mailheader:`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
      :func:`email.utils.collapse_rfc2231_value`, passing in the return value
      from :meth:`get_param`.  This will return a suitably decoded Unicode
      string whn 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``.

      .. versionchanged:: 2.2.2
         *unquote* argument added, and 3-tuple return value possible.


   .. method:: set_param(param, value[, header[, requote[, charset[, language]]]])

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

      Optional *header* specifies an alternative header to
      :mailheader:`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.

      .. versionadded:: 2.2.2


   .. method:: del_param(param[, header[, requote]])

      Remove the given parameter completely from the :mailheader:`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 :mailheader:`Content-Type`.

      .. versionadded:: 2.2.2


   .. method:: set_type(type[, header][, requote])

      Set the main type and subtype for the :mailheader:`Content-Type`
      header. *type* must be a string in the form :mimetype:`maintype/subtype`,
      otherwise a :exc:`ValueError` is raised.

      This method replaces the :mailheader:`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
      :mailheader:`Content-Type` header is set a :mailheader:`MIME-Version`
      header is also added.

      .. versionadded:: 2.2.2


   .. method:: get_filename([failobj])

      Return the value of the ``filename`` parameter of the
      :mailheader:`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.  If neither is found, or the header is
      missing, then *failobj* is returned.  The returned string will always be
      unquoted as per :func:`email.utils.unquote`.


   .. method:: get_boundary([failobj])

      Return the value of the ``boundary`` parameter of the
      :mailheader:`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 :func:`email.utils.unquote`.


   .. method:: set_boundary(boundary)

      Set the ``boundary`` parameter of the :mailheader:`Content-Type` header to
      *boundary*.  :meth:`set_boundary` will always quote *boundary* if
      necessary.  A :exc:`HeaderParseError` is raised if the message object has
      no :mailheader:`Content-Type` header.

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


   .. method:: get_content_charset([failobj])

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

      Note that this method differs from :meth:`get_charset` which returns the
      :class:`~email.charset.Charset` instance for the default encoding of the message body.

      .. versionadded:: 2.2.2


   .. method:: get_charsets([failobj])

      Return a list containing the character set names in the message.  If the
      message is a :mimetype:`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 :mailheader:`Content-Type` header for the
      represented subpart.  However, if the subpart has no
      :mailheader:`Content-Type` header, no ``charset`` parameter, or is not of
      the :mimetype:`text` main MIME type, then that item in the returned list
      will be *failobj*.


   .. method:: walk()

      The :meth:`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 :meth:`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

   .. versionchanged:: 2.5
      The previously deprecated methods :meth:`get_type`, :meth:`get_main_type`, and
      :meth:`get_subtype` were removed.

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


   .. attribute:: 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 :class:`~email.parser.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
      :class:`~email.generator.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 :mod:`email.parser` and
      :mod:`email.generator` for details.

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


   .. attribute:: 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.

      .. versionchanged:: 2.5
         You do not need to set the epilogue to the empty string in order for the
         :class:`Generator` to print a newline at the end of the file.


   .. attribute:: defects

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

      .. versionadded:: 2.4