12.2.3 Generating MIME documents

One of the most common tasks is to generate the flat text of the email message represented by a message object tree. 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 tree 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 an object tree via the Parser class, and back to flat text, is idempotent (the input is identical to the output).

Here are the public methods of the Generator class:

class 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 2.0 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 front of the line). This is the only guaranteed portable way to avoid having such lines be mistaken for Unix-From headers (see WHY THE CONTENT-LENGTH FORMAT IS BAD for details).

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 broken on semicolons and continued as per RFC 2822. If no semicolon is found, then the header is left alone. Set to zero to disable wrapping headers. Default is 78, as recommended (but not required) by RFC 2822.

The other public Generator methods are:

__call__(msg[, unixfrom])

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

Optional unixfrom is a flag that forces the printing of the Unix-From (a.k.a. envelope header or From_ header) delimiter before the first RFC 2822 header of the root message object. If the root object has no Unix-From header, a standard one is crafted. By default, this is set to 0 to inhibit the printing of the Unix-From delimiter.

Note that for sub-objects, no Unix-From header is ever printed.

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.

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