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

**Source code:** Lib/email/generator.py

======================================================================

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.

"bytes" output can be generated using the "BytesGenerator" class. If
the message object structure contains non-ASCII bytes, this
generator’s "flatten()" method will emit the original bytes.  Parsing
a binary message and then flattening it with "BytesGenerator" should
be idempotent for standards compliant messages.

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

class email.generator.Generator(outfp, mangle_from_=True, maxheaderlen=78, *, policy=None)

   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 for the "print()"
   function.

   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 *policy* keyword specifies a "policy" object that controls a
   number of aspects of the generator’s operation.  If no *policy* is
   specified, then the *policy* attached to the message object passed
   to "flatten" is used.

   Changed in version 3.3: Added the *policy* keyword.

   The other public "Generator" methods are:

   flatten(msg, unixfrom=False, linesep=None)

      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.

      Optional *linesep* specifies the line separator character used
      to terminate lines in the output.  If specified it overrides the
      value specified by the *msg*’s or "Generator"’s "policy".

      Because strings cannot represent non-ASCII bytes, if the policy
      that applies when "flatten" is run has "cte_type" set to "8bit",
      "Generator" will operate as if it were set to "7bit".  This
      means that messages parsed with a Bytes parser that have a
      *Content-Transfer-Encoding* of "8bit" will be converted to a use
      a "7bit" Content-Transfer-Encoding.  Non-ASCII bytes in the
      headers will be **RFC 2047** encoded with a charset of "unknown-
      8bit".

      Changed in version 3.2: Added support for re-encoding "8bit"
      message bodies, and the *linesep* argument.

   clone(fp)

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

   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 the
      "print()" function.

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

class email.generator.BytesGenerator(outfp, mangle_from_=True, maxheaderlen=78, *, policy=None)

   The constructor for the "BytesGenerator" class takes a binary
   *file-like object* called *outfp* for an argument.  *outfp* must
   support a "write()" method that accepts binary data.

   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 *policy* keyword specifies a "policy" object that controls a
   number of aspects of the generator’s operation.  If no *policy* is
   specified, then the *policy* attached to the message object passed
   to "flatten" is used.

   Changed in version 3.3: Added the *policy* keyword.

   The other public "BytesGenerator" methods are:

   flatten(msg, unixfrom=False, linesep=None)

      Print the textual representation of the message object structure
      rooted at *msg* to the output file specified when the
      "BytesGenerator" instance was created.  Subparts are visited
      depth-first and the resulting text will be properly MIME
      encoded.  If the "policy" option "cte_type" is "8bit" (the
      default), then any bytes with the high bit set in the original
      parsed message that have not been modified will be copied
      faithfully to the output.  If "cte_type" is "7bit", the bytes
      will be converted as needed using an ASCII-compatible Content-
      Transfer-Encoding.  In particular, RFC-invalid non-ASCII bytes
      in headers will be encoded using the MIME "unknown-8bit"
      character set, thus rendering them RFC-compliant.

      Messages parsed with a Bytes parser that have a *Content-
      Transfer-Encoding* of 8bit will be reconstructed as 8bit if they
      have not been modified.

      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.

      Optional *linesep* specifies the line separator character used
      to terminate lines in the output.  If specified it overrides the
      value specified by the "Generator"or *msg*’s "policy".

   clone(fp)

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

   write(s)

      Write the string *s* to the underlying file object.  *s* is
      encoded using the "ASCII" codec and written to the *write*
      method of the  *outfp* *outfp* passed to the "BytesGenerator"’s
      constructor.  This provides just enough file-like API for
      "BytesGenerator" instances to be used in the "print()" function.

   New in version 3.2.

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_=True, maxheaderlen=78, fmt=None)

   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]

-[ Footnotes ]-

[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.
