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The Header.decode_header function eats up spaces in
non-encoded part of a header.
See the following source:
# -*- coding: iso-8859-1 -*-
from email.Header import Header, decode_header
h = Header('Essai ', None)
h.append('éè', 'iso-8859-1')
print h
print decode_header(h)
This prints:
Essai =?iso-8859-1?q?=E9=E8?=
[('Test', None), ('\xe9\xe8', 'iso-8859-1')]
This should print:
Essai =?iso-8859-1?q?=E9=E8?=
[('Test ', None), ('\xe9\xe8', 'iso-8859-1')]
 ^ This space disappears
This appears in Python 2.3 but the source code of the
function didn't change in 2.4 so the same problem
should still exist. Bug "[ 1372770 ] email.Header
should preserve original FWS" may be linked to that one
although I'm not sure this is exactly the same.
This patch (not extensively tested though) seems to
solve this problem:
--- /usr/lib/python2.3/email/Header.py 2005年09月05日
00:20:03.000000000 +0200
+++ Header.py 2006年04月10日 12:27:27.000000000 +0200
@@ -90,7 +90,7 @@
 continue
 parts = ecre.split(line)
 while parts:
- unenc = parts.pop(0).strip()
+ unenc = parts.pop(0).rstrip()
 if unenc:
 # Should we continue a long line?
 if decoded and decoded[-1][1] is None:

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user_id=254738
I can confirm this bug and have been bitten by it as well.

Hello,
Any news about this bug. It seems still there in 2.5 after a one year notice...
Regards,

I propose the attached patch. RFC 2047 specifies to ignore whitespace between encoded-words, but IMHO not between ordinary text and encoded-words.
File Added: emailheader.diff

IIRC, I tried the OP's patch and it broke too many of the email package's test suite. I made an attempt at fixing the problem to be much more RFC compliant, but couldn't get the test suite to pass completely. This points to a much deeper problem with email package header management. I don't think the problem is a bug, I think it's a design flaw.

Would someone like to comment on Georg's patch.

Georg's patch no longer applies to py3k. I ported it, but the result is not functional. It causes extra spaces during header generation, because it is there that email4/5 "deals" with "ignoring" spaces between encoded words by *adding* spaces when they are adjacent to non-encoded words. (In general email4/5 compresses runs of whitespace into single spaces.) I tried fixing that, but then ran in to the fact that header parsing/generation currently depends on the whitespace compression in order to handle the header folding cases. So, the logic used for header parsing and generation in emai5 does not allow for an easy patch to fix this bug. I'm deferring it to email6, where I an rewriting the header parser/generator.

I've been bitten by this too (in python up to 2.7 in roundup the bug-tracker). We're currently using a workaround that re-inserts spaces, see git on roundup.sourceforge.net file mailgw.py method _decode_header_to_utf8
RFC2047 even has a test-case at the end, it specifies:
encoded form displayed as
(=?ISO-8859-1?Q?a?= b) (a b)
note the space between 'a' and 'b' above. Spaces between non-encoded and encoded parts should be preserved. And it's probably a good idea to put the examples from the RFC into the regression test.

Antoine, I marked this for Python 3.3 only because there is no good way to fix it in 2.7/3.2. (If someone comes up with a way I'll be happy to review it, though!)

This is fixed by the fix in issue 1079. Ralf found a *relatively* backward compatible way to fix it, but since the point is preserving whitespace that wasn't preserved before, there is an unavoidable behavior change, so it can't be backported.