Prepare a MIME message

def prepare_message(user, useralias, addresses, subject, contents, attachments, headers, encoding, newline_to_break=True):
    # check if closed!!!!!! XXX
    """ Prepare a MIME message """
    if isinstance(contents, text_type):
        contents = [contents]
    if isinstance(attachments, text_type):
        attachments = [attachments]

    # merge contents and attachments for now.
    if attachments is not None:
        for a in attachments:
            if not os.path.isfile(a):
                raise TypeError("'{0}' is not a valid filepath".format(a))
        contents = attachments if contents is None else contents + attachments

    has_included_images, content_objects = prepare_contents(contents, encoding)
    msg = MIMEMultipart()
    if headers is not None:
        # Strangely, msg does not have an update method, so then manually.
        for k, v in headers.items():
            msg[k] = v
    if headers is None or "Date" not in headers:
        msg["Date"] = formatdate()

    msg_alternative = MIMEMultipart("alternative")
    msg_related = MIMEMultipart("related")
    msg_related.attach("-- HTML goes here --")
    msg.attach(msg_alternative)
    add_subject(msg, subject)
    add_recipients_headers(user, useralias, msg, addresses)
    htmlstr = ""
    altstr = []
    if has_included_images:
        msg.preamble = "This message is best displayed using a MIME capable email reader."

    if contents is not None:
        for content_object, content_string in zip(content_objects, contents):
            if content_object["main_type"] == "image":
                # all image objects need base64 encoding, so do it now
                email.encoders.encode_base64(content_object["mime_object"])
                # aliased image {'path' : 'alias'}
                if isinstance(content_string, dict) and len(content_string) == 1:
                    for key in content_string:
                        hashed_ref = str(abs(hash(key)))
                        alias = content_string[key]
                    # pylint: disable=undefined-loop-variable
                    content_string = key
                else:
                    alias = os.path.basename(str(content_string))
                    hashed_ref = str(abs(hash(alias)))

                # TODO: I should probably remove inline now that there is "attachments"
                # if string is `inline`, inline, else, attach
                # pylint: disable=unidiomatic-typecheck
                if type(content_string) == inline:
                    htmlstr += '<img src="cid:{0}" title="{1}"/>'.format(hashed_ref, alias)
                    content_object["mime_object"].add_header(
                        "Content-ID", "<{0}>".format(hashed_ref)
                    )
                    altstr.append("-- img {0} should be here -- ".format(alias))
                    # inline images should be in related MIME block
                    msg_related.attach(content_object["mime_object"])
                else:
                    # non-inline images get attached like any other attachment
                    msg.attach(content_object["mime_object"])

            else:
                if content_object["encoding"] == "base64":
                    email.encoders.encode_base64(content_object["mime_object"])
                    msg.attach(content_object["mime_object"])
                elif content_object["sub_type"] not in ["html", "plain"]:
                    msg.attach(content_object["mime_object"])
                else:
                    if newline_to_break:
                        content_string = content_string.replace("\n", "<br>")
                    try:
                        htmlstr += "<div>{0}</div>".format(content_string)
                    except UnicodeEncodeError:
                        htmlstr += u"<div>{0}</div>".format(content_string)
                    altstr.append(content_string)

    msg_related.get_payload()[0] = MIMEText(htmlstr, "html", _charset=encoding)
    msg_alternative.attach(MIMEText("\n".join(altstr), _charset=encoding))
    msg_alternative.attach(msg_related)
    return msg