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18.1.3. email: 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). 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)
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 other public Generator methods are:
- flatten(msg, unixfrom=False, linesep='\n')
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. It defaults to \n because that is the most useful value for Python application code (other library packages expect \n separated lines). linesep=\r\n can be used to generate output with RFC-compliant line separators.
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.
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)
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 other public BytesGenerator methods are:
- flatten(msg, unixfrom=False, linesep='n')
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 input that created the msg contained bytes with the high bit set and those bytes have not been modified, they will be copied faithfully to the output, even if doing so is not strictly RFC compliant. (To produce strictly RFC compliant output, use the Generator class.)
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. It defaults to \n because that is the most useful value for Python application code (other library packages expect \n separated lines). linesep=\r\n can be used to generate output with RFC-compliant line separators.
- 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]