18.1.3. "email.generator": Generating MIME documents
****************************************************

One of the most common tasks is to generate the flat text of the email
message represented by a message object structure.  You will need to
do this if you want to send your message via the "smtplib" module or
the "nntplib" module, or print the message on the console.  Taking a
message object structure and producing a flat text document is the job
of the "Generator" class.

Again, as with the "email.parser" module, you aren’t limited to the
functionality of the bundled generator; you could write one from
scratch yourself.  However the bundled generator knows how to generate
most email in a standards-compliant way, should handle MIME and non-
MIME email messages just fine, and is designed so that the
transformation from flat text, to a message structure via the "Parser"
class, and back to flat text, is idempotent (the input is identical to
the output) [1].  On the other hand, using the Generator on a
"Message" constructed by program may result in changes to the
"Message" object as defaults are filled in.

Here are the public methods of the "Generator" class, imported from
the "email.generator" module:

class email.generator.Generator(outfp[, mangle_from_[, maxheaderlen]])

   The constructor for the "Generator" class takes a file-like object
   called *outfp* for an argument.  *outfp* must support the "write()"
   method and be usable as the output file in a Python extended print
   statement.

   Optional *mangle_from_* is a flag that, when "True", puts a ">"
   character in front of any line in the body that starts exactly as
   "From", i.e. "From" followed by a space at the beginning of the
   line.  This is the only guaranteed portable way to avoid having
   such lines be mistaken for a Unix mailbox format envelope header
   separator (see WHY THE CONTENT-LENGTH FORMAT IS BAD for details).
   *mangle_from_* defaults to "True", but you might want to set this
   to "False" if you are not writing Unix mailbox format files.

   Optional *maxheaderlen* specifies the longest length for a non-
   continued header. When a header line is longer than *maxheaderlen*
   (in characters, with tabs expanded to 8 spaces), the header will be
   split as defined in the "Header" class.  Set to zero to disable
   header wrapping. The default is 78, as recommended (but not
   required) by **RFC 2822**.

   The other public "Generator" methods are:

   flatten(msg[, unixfrom])

      Print the textual representation of the message object structure
      rooted at *msg* to the output file specified when the
      "Generator" instance was created.  Subparts are visited depth-
      first and the resulting text will be properly MIME encoded.

      Optional *unixfrom* is a flag that forces the printing of the
      envelope header delimiter before the first **RFC 2822** header
      of the root message object.  If the root object has no envelope
      header, a standard one is crafted.  By default, this is set to
      "False" to inhibit the printing of the envelope delimiter.

      Note that for subparts, no envelope header is ever printed.

      2.2.2 新版功能.

   clone(fp)

      Return an independent clone of this "Generator" instance with
      the exact same options.

      2.2.2 新版功能.

   write(s)

      Write the string *s* to the underlying file object, i.e. *outfp*
      passed to "Generator"’s constructor.  This provides just enough
      file-like API for "Generator" instances to be used in extended
      print statements.

As a convenience, see the methods "Message.as_string()" and
"str(aMessage)", a.k.a. "Message.__str__()", which simplify the
generation of a formatted string representation of a message object.
For more detail, see "email.message".

The "email.generator" module also provides a derived class, called
"DecodedGenerator" which is like the "Generator" base class, except
that non-*text* parts are substituted with a format string
representing the part.

class email.generator.DecodedGenerator(outfp[, mangle_from_[, maxheaderlen[, fmt]]])

   This class, derived from "Generator" walks through all the subparts
   of a message.  If the subpart is of main type *text*, then it
   prints the decoded payload of the subpart. Optional *_mangle_from_*
   and *maxheaderlen* are as with the "Generator" base class.

   If the subpart is not of main type *text*, optional *fmt* is a
   format string that is used instead of the message payload. *fmt* is
   expanded with the following keywords, "%(keyword)s" format:

   * "type" – Full MIME type of the non-*text* part

   * "maintype" – Main MIME type of the non-*text* part

   * "subtype" – Sub-MIME type of the non-*text* part

   * "filename" – Filename of the non-*text* part

   * "description" – Description associated with the non-*text* part

   * "encoding" – Content transfer encoding of the non-*text* part

   The default value for *fmt* is "None", meaning

      [Non-text (%(type)s) part of message omitted, filename %(filename)s]

   2.2.2 新版功能.

在 2.5 版更改: The previously deprecated method "__call__()" was
removed.

-[ 脚注 ]-

[1] This statement assumes that you use the appropriate setting
    for the "unixfrom" argument, and that you set maxheaderlen=0
    (which will preserve whatever the input line lengths were).  It is
    also not strictly true, since in many cases runs of whitespace in
    headers are collapsed into single blanks.  The latter is a bug
    that will eventually be fixed.
