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# 18.1.1. `email.message`: Representing an email message

The central class in the `email` package is the `Message` class, imported from the `email.message` module. 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 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 `Message` objects for MIME container documents (e.g. multipart/* and message/rfc822).

`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 `email.message.``Message`

  The constructor takes no arguments.

  `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`. 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 mangles lines that begin with `From`. For more flexibility, instantiate a `Generator` instance and use its `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()

  `__str__`()

    Equivalent to `as_string(unixfrom=True)`.

  `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.

  `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.

  `get_payload`([i[, decode]])

    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, 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`.

  `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 `set_charset()` for details.

    2.2.2 版更變: charset argument added.

  `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.

    The message will be assumed to be of type text/*, with the payload either in unicode or encoded with charset.input_charset. It will be encoded or converted to charset.output_charset and transfer encoded properly, if needed, when generating the plain text representation of the message. MIME headers (MIME-Version, Content-Type, Content-Transfer-Encoding) will be added as needed.

    2.2.2 版新加入.

  `get_charset`()

    Return the `Charset` instance associated with the message’s payload.

    2.2.2 版新加入.

  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.

  `__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.

  `has_key`(name)

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

  `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])

    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. 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 must 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.

    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.

    2.2.2 版新加入.

  `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.

    2.2.2 版新加入.

  `get_content_maintype`()

    Return the message’s main content type. This is the maintype part of the string returned by `get_content_type()`.

    2.2.2 版新加入.

  `get_content_subtype`()

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

    2.2.2 版新加入.

  `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.

    2.2.2 版新加入.

  `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.

    2.2.2 版新加入.

  `get_params`([failobj[, header[, unquote]]])

    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.

    2.2.2 版更變: unquote argument added.

  `get_param`(param[, failobj[, header[, unquote]]])

    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`.

    2.2.2 版更變: unquote argument added, and 3-tuple return value possible.

  `set_param`(param, value[, header[, requote[, charset[, language]]]])

    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.

    2.2.2 版新加入.

  `del_param`(param[, header[, requote]])

    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.

    2.2.2 版新加入.

  `set_type`(type[, header][, requote])

    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.

    2.2.2 版新加入.

  `get_filename`([failobj])

    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])

    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])

    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.

    2.2.2 版新加入.

  `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.

  `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

  2.5 版更變: The previously deprecated methods `get_type()`, `get_main_type()`, and `get_subtype()` were removed.

  `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.

    2.5 版更變: 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.

    2.4 版新加入.