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 thewrite()
method and be usable as the output file for theprint()
function.Optional mangle_from_ is a flag that, when
True
, puts a>
character in front of any line in the body that starts exactly asFrom
, 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 toTrue
, but you might want to set this toFalse
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 toflatten
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
‘spolicy
.Because strings cannot represent non-ASCII bytes, if the policy that applies when
flatten
is run hascte_type
set to8bit
,Generator
will operate as if it were set to7bit
. This means that messages parsed with a Bytes parser that have a Content-Transfer-Encoding of8bit
will be converted to a use a7bit
Content-Transfer-Encoding. Non-ASCII bytes in the headers will be RFC 2047 encoded with a charset ofunknown-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, *, policy=None) The constructor for the
BytesGenerator
class takes a binary file-like object called outfp for an argument. outfp must support awrite()
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 asFrom
, 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 toTrue
, but you might want to set this toFalse
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 toflatten
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 thepolicy
optioncte_type
is8bit
(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. Ifcte_type
is7bit
, 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 MIMEunknown-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‘spolicy
.
-
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 theBytesGenerator
‘s constructor. This provides just enough file-like API forBytesGenerator
instances to be used in theprint()
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 theGenerator
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 partmaintype
– Main MIME type of the non-text partsubtype
– Sub-MIME type of the non-text partfilename
– Filename of the non-text partdescription
– Description associated with the non-text partencoding
– 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. |