## 12.2.1 Representing an email message

The central class in the `email` package is the `Message` class; it is the base class for the `email` object model. `Message` provides the core functionality for setting and querying header fields, and for accessing message bodies.

Conceptually, a `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 Unix-From header, also known as the envelope header or the `From_` header. The payload is either a string in the case of simple message objects, a list of `Message` objects for multipart MIME documents, or a single `Message` instance for message/rfc822 type objects.

`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 `Message` class:

class `Message`()
  The constructor takes no arguments.

`as_string`([unixfrom])
  Return the entire formatted message as a string. Optional unixfrom, when true, specifies to include the Unix-From envelope header; it defaults to 0.

`__str__()`()
  Equivalent to `aMessage.as_string(unixfrom=1)`.

`is_multipart`()
  Return 1 if the message's payload is a list of sub-`Message` objects, otherwise return 0. When `is_multipart()` returns 0, the payload should either be a string object, or a single `Message` instance.

`set_unixfrom`(unixfrom)
  Set the Unix-From (a.k.a envelope header or `From_` header) to unixfrom, which should be a string.

`get_unixfrom`()
  Return the Unix-From header. Defaults to `None` if the Unix-From header was never set.

`add_payload`(payload)
  Add payload to the message object's existing payload. If, prior to calling this method, the object's payload was `None` (i.e. never before set), then after this method is called, the payload will be the argument payload.

  If the object's payload was already a list (i.e. `is_multipart()` returns 1), then payload is appended to the end of the existing payload list.

  For any other type of existing payload, `add_payload()` will transform the new payload into a list consisting of the old payload and payload, but only if the document is already a MIME multipart document. This condition is satisfied if the message's Content-Type: header's main type is either multipart, or there is no Content-Type: header. In any other situation, `MultipartConversionError` is raised.

`attach`(payload)
  Synonymous with `add_payload()`.

`get_payload`([i[, decode]])
  Return the current payload, which will be a list of `Message` objects when `is_multipart()` returns 1, or a scalar (either a string or a single `Message` instance) when `is_multipart()` returns 0.

  With optional i, `get_payload()` will return the i-th element of the payload, counting from zero, if `is_multipart()` returns 1. 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 scalar (i.e. `is_multipart()` returns 0) 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). If the message is a multipart and the decode flag is true, then `None` is returned.

`set_payload`(payload)
  Set the entire message object's payload to payload. It is the client's responsibility to ensure the payload invariants.

The following methods implement a mapping-like interface for accessing the message object'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, there is an explicit order. These semantic differences are intentional and are biased toward maximal convenience.

Note that in all cases, any optional Unix-From header the message may have 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 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, first use `__delitem__()` to delete all named fields, 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.

`has_key`(name)
  Return 1 if the message contains a header field named name, otherwise return 0.

`keys`()
  Return a list of all the message's header field names. These keys will be sorted in the order in which they were added to the message via `__setitem__()`, and may contain duplicates. Any fields deleted and then subsequently re-added are always appended to the end of the header list.

`values`()
  Return a list of all the message's field values. These will be sorted in the order in which they were added to the message via `__setitem__()`, and may contain duplicates. Any fields deleted and then subsequently re-added are always appended to the end of the header list.

`items`()
  Return a list of 2-tuples containing all the message's field headers and values. These will be sorted in the order in which they were added to the message via `__setitem__()`, and may contain duplicates. Any fields deleted and then subsequently re-added are always appended to the end of the header list.

`get`(name[, failobj])
  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])
  Return a list of all the values for the field named name. These will be sorted in the order in which they were added to the message via `__setitem__()`. Any fields deleted and then subsequently re-added are always appended to the end of the list.

  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 to set 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"

`get_type`([failobj])
  Return the message's content type, as a string of the form maintype/subtype as taken from the Content-Type: header. The returned string is coerced to lowercase.

  If there is no Content-Type: header in the message, failobj is returned (defaults to `None`).

`get_main_type`([failobj])
  Return the message's main content type. This essentially returns the maintype part of the string returned by `get_type()`, with the same semantics for failobj.

`get_subtype`([failobj])
  Return the message's sub-content type. This essentially returns the subtype part of the string returned by `get_type()`, with the same semantics for failobj.

`get_params`([failobj[, header]])
  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 always unquoted with `Utils.unquote()`.

  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:.

`get_param`(param[, failobj[, header]])
  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:.

`get_charsets`([failobj])
  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_filename`([failobj])
  Return the value of the `filename` parameter of the Content-Disposition: header of the message, or failobj if either the header is missing, or has no `filename` parameter. The returned string will always be unquoted as per `Utils.unquote()`.

`get_boundary`([failobj])
  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 `Utils.unquote()`.

`set_boundary`(boundary)
  Set the `boundary` parameter of the Content-Type: header to boundary. `set_boundary()` will always quote boundary so you should not quote it yourself. 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.

`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 ... in` loop; each iteration returns the next subpart.

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

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

`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.

  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.

  One note: when generating the flat text for a multipart message that has no epilogue (using the standard `Generator` class), no newline is added after the closing boundary line. If the message object has an epilogue and its value does not start with a newline, a newline is printed after the closing boundary. This seems a little clumsy, but it makes the most practical sense. The upshot is that if you want to ensure that a newline get printed after your closing multipart boundary, set the epilogue to the empty string.