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# 19.1.3. `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.

`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=policy.default)

  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. The default policy maintains backward compatibility.

  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`‘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.

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