Python - email header decoding UTF-8

Question

is there any Python module which helps to decode the various forms of encoded mail headers, mainly Subject, to simple - say - UTF-8 strings?

Here are example Subject headers from mail files that I have:

Subject: [ 201105311136 ]=?UTF-8?B?IMKnIDE2NSBBYnM=?=. 1 AO;
Subject: [ 201105161048 ] GewSt:=?UTF-8?B?IFdlZ2ZhbGwgZGVyIFZvcmzDpHVmaWdrZWl0?=
Subject: [ 201105191633 ]
  =?UTF-8?B?IERyZWltb25hdHNmcmlzdCBmw7xyIFZlcnBmbGVndW5nc21laHJhdWZ3ZW5kdW4=?=
  =?UTF-8?B?Z2VuIGVpbmVzIFNlZW1hbm5z?=

text - encoded sting - text

text - encoded string

text - encoded string - encoded string

Encodig could also be something else like ISO 8859-15.

Update 1: I forgot to mention, I tried email.header.decode_header

    for item in message.items():
    if item[0] == 'Subject':
            sub = email.header.decode_header(item[1])
            logging.debug( 'Subject is %s' %  sub )

This outputs

DEBUG:root:Subject is [('[ 201101251025 ] ELStAM;=?UTF-8?B?IFZlcmbDvGd1bmcgdm9tIDIx?=. Januar 2011', None)]

which does not really help.

Update 2: Thanks to Ingmar Hupp in the comments.

the first example decodes to a list of two tupels:

print decode_header("""[ 201105161048 ] GewSt:=?UTF-8?B?IFdlZ2ZhbGwgZGVyIFZvcmzDpHVmaWdrZWl0?=""")
[('[ 201105161048 ] GewSt:', None), (' Wegfall der Vorl\xc3\xa4ufigkeit', 'utf-8')]

is this always [(string, encoding),(string, encoding), ...] so I need a loop to concat all the [0] items to one string or how to get it all in one string?

Subject: [ 201101251025 ] ELStAM;=?UTF-8?B?IFZlcmbDvGd1bmcgdm9tIDIx?=. Januar 2011

does not decode well:

print decode_header("""[ 201101251025 ] ELStAM;=?UTF-8?B?IFZlcmbDvGd1bmcgdm9tIDIx?=. Januar 2011""")

[('[ 201101251025 ] ELStAM;=?UTF-8?B?IFZlcmbDvGd1bmcgdm9tIDIx?=. Januar 2011', None)]

Answer 1

This type of encoding is known as MIME encoded-word and the email module can decode it:

from email.header import decode_header
print decode_header("""=?UTF-8?B?IERyZWltb25hdHNmcmlzdCBmw7xyIFZlcnBmbGVndW5nc21laHJhdWZ3ZW5kdW4=?=""")

This outputs a list of tuples, containing the decoded string and the encoding used. This is because the format supports different encodings in a single header. To merge these into a single string you need to convert them into a shared encoding and then concatenate this, which can be accomplished using Python's unicode object:

from email.header import decode_header
dh = decode_header("""[ 201105161048 ] GewSt:=?UTF-8?B?IFdlZ2ZhbGwgZGVyIFZvcmzDpHVmaWdrZWl0?=""")
default_charset = 'ASCII'
print ''.join([ unicode(t[0], t[1] or default_charset) for t in dh ])

Update 2:

The problem with this Subject line not decoding:

Subject: [ 201101251025 ] ELStAM;=?UTF-8?B?IFZlcmbDvGd1bmcgdm9tIDIx?=. Januar 2011
                                                                     ^

Is actually the senders fault, which violates the requirement of encoded-words in a header being separated by white-space, specified in RFC 2047, section 5, paragraph 1: an 'encoded-word' that appears in a header field defined as '*text' MUST be separated from any adjacent 'encoded-word' or 'text' by 'linear-white-space'.

If need be, you can work around this by pre-processing these corrupt headers with a regex that inserts a whitespace after the encoded-word part (unless it's at the end), like so:

import re
header_value = re.sub(r"(=\?.*\?=)(?!$)", r"\1 ", header_value)