Subject prefixes

Mailing lists can define a subject prefix which gets added to the front of any Subject text. This can be used to quickly identify which mailing list the message was posted to.

>>> mlist = create_list('[email protected]m')

The default list style gives the mailing list a default prefix.

>>> print(mlist.subject_prefix)
[Test]

This can be changed to anything, but typically ends with a trailing space.

>>> mlist.subject_prefix = '[XTest] '
>>> process = config.handlers['subject-prefix'].process

No Subject

If the original message has no Subject, then a canned one is used.

>>> msg = message_from_string("""\
... From: [email protected]
...
... A message of great import.
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest] (no subject)

Inserting a prefix

If the original message had a Subject header, then the prefix is inserted at the beginning of the header’s value.

>>> msg = message_from_string("""\
... From: [email protected]
... Subject: Something important
...
... A message of great import.
... """)
>>> msgdata = {}
>>> process(mlist, msg, msgdata)
>>> print(msg['subject'])
[XTest] Something important

The original Subject is available in the metadata.

>>> print(msgdata['original_subject'])
Something important

If a Subject header already has a prefix, usually following a Re: marker, another one will not be added but the prefix will be moved to the front of the header text.

>>> msg = message_from_string("""\
... From: [email protected]
... Subject: Re: [XTest] Something important
...
... A message of great import.
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest] Re: Something important

If the Subject header has a prefix at the front of the header text, that’s where it will stay.

>>> msg = message_from_string("""\
... From: [email protected]
... Subject: [XTest] Re: Something important
...
... A message of great import.
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest] Re: Something important

Sometimes the incoming Subject header has a pathological sequence of Re: like markers. These should all be collapsed up to the first non-Re: marker.

>>> msg = message_from_string("""\
... From: [email protected]
... Subject: [XTest] Re: RE : Re: Re: Re: Re: Re: Something important
...
... A message of great import.
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest] Re: Something important

Internationalized headers

Internationalization adds some interesting twists to the handling of subject prefixes. Part of what makes this interesting is the encoding of i18n headers using RFC 2047, and lists whose preferred language is in a different character set than the encoded header.

>>> msg = message_from_string("""\
... Subject: =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'].encode())
[XTest] =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
>>> print(str(msg['subject']))
[XTest] メールマン

Prefix numbers

Subject prefixes support a placeholder for the numeric post id. Every time a message is posted to the mailing list, a post id gets incremented. This is a purely sequential integer that increases monotonically. By added a %d placeholder to the subject prefix, this post id can be included in the prefix.

>>> mlist.subject_prefix = '[XTest %d] '
>>> mlist.post_id = 456
>>> msg = message_from_string("""\
... Subject: Something important
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest 456] Something important

This works even when the message is a reply, except that in this case, the numeric post id in the generated subject prefix is updated with the new post id.

>>> msg = message_from_string("""\
... Subject: [XTest 123] Re: Something important
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest 456] Re: Something important

If the Subject header had old style prefixing, the prefix is moved to the front of the header text.

>>> msg = message_from_string("""\
... Subject: Re: [XTest 123] Something important
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest 456] Re: Something important

And of course, the proper thing is done when posting id numbers are included in the subject prefix, and the subject is encoded non-ASCII.

>>> msg = message_from_string("""\
... Subject: =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'].encode())
[XTest 456] =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
>>> print(msg['subject'])
[XTest 456] メールマン

Even more fun is when the internationalized Subject header already has a prefix, possibly with a different posting number.

>>> msg = message_from_string("""\
... Subject: [XTest 123] Re: =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'].encode())
[XTest 456] Re: =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
>>> print(msg['subject'])
[XTest 456] Re: メールマン

As before, old style subject prefixes are re-ordered.

>>> msg = message_from_string("""\
... Subject: Re: [XTest 123] =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'].encode())
[XTest 456] Re:
  =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
>>> print(msg['subject'])
[XTest 456]  Re: メールマン

In this test case, we get an extra space between the prefix and the original subject. It’s because the original is crooked. Note that a Subject starting with ‘n ‘ is generated by some version of Eudora Japanese edition.

>>> mlist.subject_prefix = '[XTest] '
>>> msg = message_from_string("""\
... Subject:
...  Important message
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'])
[XTest]  Important message

And again, with an RFC 2047 encoded header.

>>> msg = message_from_string("""\
... Subject:
...  =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
...
... """)
>>> process(mlist, msg, {})
>>> print(msg['subject'].encode())
[XTest] =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=
>>> print(msg['subject'])
[XTest]  メールマン