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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: HeaderParseError
Date: Mon, 04 Jul 2011 13:20:42 +0200
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:8767

Thomas Guettler wrote:

> On 04.07.2011 11:51, Peter Otten wrote:
>> Thomas Guettler wrote:
>> 
>>> I get a HeaderParseError during decode_header(), but Thunderbird can
>>> display the name.
>>>
>>>>>> from email.header import decode_header
>>>>>>
>> 
decode_header('=?iso-8859-1?B?QW5tZWxkdW5nIE5ldHphbnNjaGx1c3MgU_xkcmluZzNwLmpwZw==?=')
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>>   File "/usr/lib64/python2.6/email/header.py", line 101, in
>>>   decode_header
>>>     raise HeaderParseError
>>> email.errors.HeaderParseError

>> The characters after "...Netzanschluss " look like garbage. What does
>> Thunderbird display?
> 
> Hi Peter, Thunderbird shows this:
> 
> Anmeldung Netzanschluss Südring3p.jpg

>>> a = u"Anmeldung Netzanschluss 
Südring3p.jpg".encode("iso-8859-1").encode("base64")

>>> b = "QW5tZWxkdW5nIE5ldHphbnNjaGx1c3MgU_xkcmluZzNwLmpwZw==?="
>>> for i, (x, y) in enumerate(zip(a, b)):
...     if x != y: print i, x, y
...
33 / _
52
?
>>> b.decode("base64")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/encodings/base64_codec.py", line 42, in 
base64_decode
    output = base64.decodestring(input)
  File "/usr/lib/python2.6/base64.py", line 321, in decodestring
    return binascii.a2b_base64(s)
binascii.Error: Incorrect padding
>>> b.replace("_", "/").decode("base64")
'Anmeldung Netzanschluss S\xfcdring3p.jpg'

Looks like you encountered a variant of base64 that uses "_" instead of "/" 
for chr(63). The wikipedia page http://en.wikipedia.org/wiki/Base64
calls that base64url.

You could try and make the email package accept that with a monkey patch 
like the following:

#untested
import binascii
def a2b_base64(s):
    return binascii.a2b_base64(s.replace("_", "/"))

from email import base64mime
base64mime.a2b_base64 = a2b_base64

Alternatively monkey-patch the binascii module before you import the email 
package.