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 (serialized) version of the email message represented by a message object structure. You will need to do this if you want to send your message via smtplib.SMTP.sendmail() or the nntplib module, or print the message on the console. Taking a message object structure and producing a serialized representation is the job of the generator classes.

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 bytes-oriented parsing and generation operations are inverses, assuming the same non-transforming policy is used for both. That is, parsing the serialized byte stream via the BytesParser class and then regenerating the serialized byte stream using BytesGenerator should produce output identical to the input [1]. (On the other hand, using the generator on an EmailMessage constructed by program may result in changes to the EmailMessage object as defaults are filled in.)

The Generator class can be used to flatten a message into a text (as opposed to binary) serialized representation, but since Unicode cannot represent binary data directly, the message is of necessity transformed into something that contains only ASCII characters, using the standard email RFC Content Transfer Encoding techniques for encoding email messages for transport over channels that are not “8 bit clean”.

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

Return a BytesGenerator object that will write any message provided to the flatten() method, or any surrogateescape encoded text provided to the write() method, to the file-like object outfp. outfp must support a write method that accepts binary data.

If optional mangle_from_ is True, put a > character in front of any line in the body that starts with the exact string "From ", that is From followed by a space at the beginning of a line. mangle_from_ defaults to the value of the mangle_from_ setting of the policy (which is True for the compat32 policy and False for all others). mangle_from_ is intended for use when messages are stored in unix mbox format (see mailbox and WHY THE CONTENT-LENGTH FORMAT IS BAD).

If maxheaderlen is not None, refold any header lines that are longer than maxheaderlen, or if 0, do not rewrap any headers. If manheaderlen is None (the default), wrap headers and other message lines according to the policy settings.

If policy is specified, use that policy to control message generation. If policy is None (the default), use the policy associated with the Message or EmailMessage object passed to flatten to control the message generation. See email.policy for details on what policy controls.

New in version 3.2.

Changed in version 3.3: Added the policy keyword.

Changed in version 3.6: The default behavior of the mangle_from_ and maxheaderlen parameters is to follow the policy.

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.

If the policy option cte_type is 8bit (the default), copy any headers in the original parsed message that have not been modified to the output with any bytes with the high bit set reproduced as in the original, and preserve the non-ASCII Content-Transfer-Encoding of any body parts that have them. If cte_type is 7bit, convert the bytes with the high bit set as needed using an ASCII-compatible Content-Transfer-Encoding. That is, transform parts with non-ASCII Cotnent-Transfer-Encoding (Content-Transfer-Encoding: 8bit) to an ASCII compatibile Content-Transfer-Encoding, and encode RFC-invalid non-ASCII bytes in headers using the MIME unknown-8bit character set, thus rendering them RFC-compliant.

If unixfrom is True, print the envelope header delimiter used by the Unix mailbox format (see mailbox) before the first of the RFC 5322 headers of the root message object. If the root object has no envelope header, craft a standard one. The default is False. Note that for subparts, no envelope header is ever printed.

If linesep is not None, use it as the separator character between all the lines of the flattened message. If linesep is None (the default), use the value specified in the policy.

clone(fp)

Return an independent clone of this BytesGenerator instance with the exact same option settings, and fp as the new outfp.

write(s)

Encode s using the ASCII codec and the surrogateescape error handler, and pass it to the write method of the outfp passed to the BytesGenerator‘s constructor.

As a convenience, EmailMessage provides the methods as_bytes() and bytes(aMessage) (a.k.a. __bytes__()), which simplify the generation of a serialized binary representation of a message object. For more detail, see email.message.

Because strings cannot represent binary data, the Generator class must convert any binary data in any message it flattens to an ASCII compatible format, by converting them to an ASCII compatible Content-Transfer_Encoding. Using the terminology of the email RFCs, you can think of this as Generator serializing to an I/O stream that is not “8 bit clean”. In other words, most applications will want to be using BytesGenerator, and not Generator.

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

Return a Generator object that will write any message provided to the flatten() method, or any text provided to the write() method, to the file-like object outfp. outfp must support a write method that accepts string data.

If optional mangle_from_ is True, put a > character in front of any line in the body that starts with the exact string "From ", that is From followed by a space at the beginning of a line. mangle_from_ defaults to the value of the mangle_from_ setting of the policy (which is True for the compat32 policy and False for all others). mangle_from_ is intended for use when messages are stored in unix mbox format (see mailbox and WHY THE CONTENT-LENGTH FORMAT IS BAD).

If maxheaderlen is not None, refold any header lines that are longer than maxheaderlen, or if 0, do not rewrap any headers. If manheaderlen is None (the default), wrap headers and other message lines according to the policy settings.

If policy is specified, use that policy to control message generation. If policy is None (the default), use the policy associated with the Message or EmailMessage object passed to flatten to control the message generation. See email.policy for details on what policy controls.

Changed in version 3.3: Added the policy keyword.

Changed in version 3.6: The default behavior of the mangle_from_ and maxheaderlen parameters is to follow the policy.

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.

If the policy option cte_type is 8bit, generate the message as if the option were set to 7bit. (This is required because strings cannot represent non-ASCII bytes.) Convert any bytes with the high bit set as needed using an ASCII-compatible Content-Transfer-Encoding. That is, transform parts with non-ASCII Cotnent-Transfer-Encoding (Content-Transfer-Encoding: 8bit) to an ASCII compatibile Content-Transfer-Encoding, and encode RFC-invalid non-ASCII bytes in headers using the MIME unknown-8bit character set, thus rendering them RFC-compliant.

If unixfrom is True, print the envelope header delimiter used by the Unix mailbox format (see mailbox) before the first of the RFC 5322 headers of the root message object. If the root object has no envelope header, craft a standard one. The default is False. Note that for subparts, no envelope header is ever printed.

If linesep is not None, use it as the separator character between all the lines of the flattened message. If linesep is None (the default), use the value specified in the policy.

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, and fp as the new outfp.

write(s)

Write s to the write method of the outfp passed to the Generator‘s constructor. This provides just enough file-like API for Generator instances to be used in the print() function.

As a convenience, EmailMessage provides the 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.

The email.generator module also provides a derived class, DecodedGenerator, which is like the Generator base class, except that non-text parts are not serialized, but are instead represented in the output stream by a string derived from a template filled in with information about the part.

class email.generator.DecodedGenerator(outfp, mangle_from_=None, maxheaderlen=None, fmt=None, *, policy=None)

Act like Generator, except that for any subpart of the message passed to Generator.flatten(), if the subpart is of main type text, print the decoded payload of the subpart, and if the main type is not text, instead of printing it fill in the string fmt using information from the part and print the resulting filled-in string.

To fill in fmt, execute fmt % part_info, where part_info is a dictionary composed of the following keys and values:

  • 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

If fmt is None, use the following default fmt:

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

Optional _mangle_from_ and maxheaderlen are as with the Generator base class.

Footnotes

[1]This statement assumes that you use the appropriate setting for unixfrom, and that there are no policy settings calling for automatic adjustments (for example, refold_source must be none, which is not the default). It is also not 100% true, since if the message does not conform to the RFC standards occasionally information about the exact original text is lost during parsing error recovery. It is a goal to fix these latter edge cases when possible.