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12.4.1 MimeWriter Objects

`MimeWriter` instances have the following methods:

`addheader`(key, value[, prefix])
  Add a header line to the MIME message. The key is the name of the header, where the value obviously provides the value of the header. The optional argument prefix determines where the header is inserted; "`0`" means append at the end, "`1`" is insert at the start. The default is to append.

`flushheaders`()
  Causes all headers accumulated so far to be written out (and forgotten). This is useful if you don't need a body part at all, e.g. for a subpart of type message/rfc822 that's (mis)used to store some header-like information.

`startbody`(ctype[, plist[, prefix]])
  Returns a file-like object which can be used to write to the body of the message. The content-type is set to the provided ctype, and the optional parameter plist provides additional parameters for the content-type declaration. prefix functions as in `addheader()` except that the default is to insert at the start.

`startmultipartbody`(subtype[, boundary[, plist[, prefix]]])
  Returns a file-like object which can be used to write to the body of the message. Additionally, this method initializes the multi-part code, where subtype provides the multipart subtype, boundary may provide a user-defined boundary specification, and plist provides optional parameters for the subtype. prefix functions as in `startbody()`. Subparts should be created using `nextpart()`.

`nextpart`()
  Returns a new instance of `MimeWriter` which represents an individual part in a multipart message. This may be used to write the part as well as used for creating recursively complex multipart messages. The message must first be initialized with `startmultipartbody()` before using `nextpart()`.

`lastpart`()
  This is used to designate the last part of a multipart message, and should always be used when writing multipart messages.