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Trying to implement UTF-7 codec in Python I found some warts in error handling.
1. Non-ASCII bytes.
No errors:
>>> 'a€b'.encode('utf-7')
b'a+IKw-b'
>>> b'a+IKw-b'.decode('utf-7')
'a€b'
Terminating '-' at the end of the string is optional.
>>> b'a+IKw'.decode('utf-7')
'a€'
And sometimes it is optional in the middle of the string (if following char is not used in BASE64).
>>> b'a+IKw;b'.decode('utf-7')
'a€;b'
But if following char is not ASCII, it is accepted as well, and this looks as a bug.
>>> b'a+IKw\xffb'.decode('utf-7')
'a€ÿb'
In all other cases non-ASCII byte causes an error:
>>> b'a\xffb'.decode('utf-7')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython/Lib/encodings/utf_7.py", line 12, in decode
 return codecs.utf_7_decode(input, errors, True)
UnicodeDecodeError: 'utf7' codec can't decode byte 0xff in position 1: unexpected special character
>>> b'a\xffb'.decode('utf-7', 'replace')
'a�b'
2. Ending lone high surrogate.
Lone surrogates are silently accepted by utf-7 codec.
>>> '\ud8e4\U0001d121'.encode('utf-7')
b'+2OTYNN0h-'
>>> '\U0001d121\ud8e4'.encode('utf-7')
b'+2DTdIdjk-'
>>> b'+2OTYNN0h-'.decode('utf-7')
'\ud8e4𝄡'
>>> b'+2OTYNN0h'.decode('utf-7')
'\ud8e4𝄡'
>>> b'+2DTdIdjk-'.decode('utf-7')
'𝄡\ud8e4'
Except at the end of unterminated shift sequence:
>>> b'+2DTdIdjk'.decode('utf-7')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython/Lib/encodings/utf_7.py", line 12, in decode
 return codecs.utf_7_decode(input, errors, True)
UnicodeDecodeError: 'utf7' codec can't decode bytes in position 0-8: unterminated shift sequence
3. Incorrect shift sequence.
Strange behavior happens when shift sequence ends with wrong bits.
>>> b'a+IKx-b'.decode('utf-7', 'ignore')
'a€b'
>>> b'a+IKx-b'.decode('utf-7', 'replace')
'a€�b'
>>> b'a+IKx-b'.decode('utf-7', 'backslashreplace')
'a€\\x2b\\x49\\x4b\\x78\\x2db'
The decoder first decodes as much characters as can, and then pass all shift sequence (including already decoded bytes) to error handler. Not sure this is a bug, but this differs from common behavior of other decoders.

There is a reason for behavior in case 2. This is likely a truncated data and it is safer to raise an exception than silently produce lone surrogate. Current UTF-7 encoder always adds '-' after ending shift sequence. I suppose this is not a bug.
However there are yet three bugs.
4. Decoder can emit lone low surrogate before replacement character in case of error.
>>> b'+2DTdI-'.decode('utf-7', 'replace')
'\ud834�'
A low surrogate is a part of incomplete astral character and shouldn't emitted in case of error in encoded astral character.
5. According to RFC 2152: "A "+" character followed immediately by any character other than members of set B or "-" is an ill-formed sequence." But this is accepted by current decoder as empty shift sequence that is decoded to empty string.
>>> b'a+,b'.decode('utf-7')
'a,b'
>>> b'a+'.decode('utf-7')
'a'
6. Replacement character '\ufffd' can be replaced with character 'ý' ('\xfd'):
>>> b'\xff'.decode('utf-7', 'replace')
'�'
>>> b'a\xff'.decode('utf-7', 'replace')
'a�'
>>> b'a\xffb'.decode('utf-7', 'replace')
'a�b'
>>> b'\xffb'.decode('utf-7', 'replace')
'ýb'
This bug is reproduced only in 3.4+.
Following patch fixes bugs 1 and 4 and adds more tests.
Corner cases 2 and 3 are likely not bugs.
I doubt about fixing bug 5. iconv accepts such ill-formed sequences. In any case I think the fix of this bug can be applied only for default branch.
I have no idea how to fix bug 6. I afraid it can be a bug in _PyUnicodeWriter and therefore can affect other decoders.

Updated patch fixes also a bug in _PyUnicodeWriter. Other affected encoding is "unicode-escape":
>>> br'\u;'.decode('unicode-escape', 'replace')
'ý;'

New changeset 3c13567ea642 by Serhiy Storchaka in branch '3.4':
Issue #24848: Fixed bugs in UTF-7 decoding of misformed data:
https://hg.python.org/cpython/rev/3c13567ea642
New changeset a61fa2b08f87 by Serhiy Storchaka in branch '3.5':
Issue #24848: Fixed bugs in UTF-7 decoding of misformed data:
https://hg.python.org/cpython/rev/a61fa2b08f87
New changeset 037253b7cd6d by Serhiy Storchaka in branch 'default':
Issue #24848: Fixed bugs in UTF-7 decoding of misformed data:
https://hg.python.org/cpython/rev/037253b7cd6d
New changeset c6eaa722e2c1 by Serhiy Storchaka in branch '2.7':
Issue #24848: Fixed bugs in UTF-7 decoding of misformed data:
https://hg.python.org/cpython/rev/c6eaa722e2c1 

http://buildbot.python.org/all/builders/x86%20XP-4%202.7/builds/3431/steps/test/logs/stdio
======================================================================
FAIL: test_errors (test.test_codecs.UTF7Test)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "d:\cygwin\home\db3l\buildarea2円.7.bolen-windows\build\lib\test\test_codecs.py", line 709, in test_errors
 self.assertEqual(raw.decode('utf-7', 'replace'), expected)
AssertionError: u'a\u20ac\ufffd' != u'a\u20ac\ufffdb'
- a\u20ac\ufffd
+ a\u20ac\ufffdb
? +
======================================================================
FAIL: test_lone_surrogates (test.test_codecs.UTF7Test)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "d:\cygwin\home\db3l\buildarea2円.7.bolen-windows\build\lib\test\test_codecs.py", line 743, in test_lone_surrogates
 self.assertEqual(raw.decode('utf-7', 'replace'), expected)
AssertionError: u'a\ufffd' != u'a\ufffdb'
- a\ufffd
+ a\ufffdb
? +

Have no ideas why tests are failed and only on this buildbot.

> Have no ideas why tests are failed and only on this buildbot.
test_codecs always crash on Python 3.6 with Python compiled in debug mode:
test_errors (test.test_codecs.UTF7Test) ... python: Objects/unicodeobject.c:1263: _copy_characters: Assertion `ch <= to_maxchar' failed.
Fatal Python error: Aborted
Current thread 0x00007f1489057700 (most recent call first):
 File "/home/haypo/prog/python/default/Lib/encodings/utf_7.py", line 12 in decode
 File "/home/haypo/prog/python/default/Lib/test/test_codecs.py", line 1021 in test_errors
 File "/home/haypo/prog/python/default/Lib/unittest/case.py", line 600 in run
 File "/home/haypo/prog/python/default/Lib/unittest/case.py", line 648 in __call__
 File "/home/haypo/prog/python/default/Lib/unittest/suite.py", line 122 in run
 File "/home/haypo/prog/python/default/Lib/unittest/suite.py", line 84 in __call__
 File "/home/haypo/prog/python/default/Lib/unittest/suite.py", line 122 in run
 File "/home/haypo/prog/python/default/Lib/unittest/suite.py", line 84 in __call__
 File "/home/haypo/prog/python/default/Lib/unittest/suite.py", line 122 in run
 File "/home/haypo/prog/python/default/Lib/unittest/suite.py", line 84 in __call__
 File "/home/haypo/prog/python/default/Lib/unittest/runner.py", line 176 in run
 File "/home/haypo/prog/python/default/Lib/test/support/__init__.py", line 1775 in _run_suite
 File "/home/haypo/prog/python/default/Lib/test/support/__init__.py", line 1809 in run_unittest
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/runtest.py", line 159 in test_runner
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/runtest.py", line 160 in runtest_inner
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/runtest.py", line 124 in runtest
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/main.py", line 285 in run_tests_sequential
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/main.py", line 344 in run_tests
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/main.py", line 380 in main
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/main.py", line 421 in main
 File "/home/haypo/prog/python/default/Lib/test/libregrtest/main.py", line 443 in main_in_temp_cwd
 File "/home/haypo/prog/python/default/Lib/test/__main__.py", line 3 in <module>
 File "/home/haypo/prog/python/default/Lib/runpy.py", line 85 in _run_code
 File "/home/haypo/prog/python/default/Lib/runpy.py", line 170 in _run_module_as_main
Abandon (core dumped)

Oops, ignore my comment, I forgot to recompile Python. "make" and the bug is done :-)

Test failure is random. With build 3435 tests are successful, with all other are failed. The same with other buildbot: http://buildbot.python.org/all/builders/x86%20Windows7%202.7/ . 3345 and 3347 are green, others are red.

The difference between 2.7 and 3.x is that 2.7 uses isalnum() in IS_BASE64, and 3.x test concrete ranges. Therefore depending on platform and locale 2.7 can accept wrong bytes as BASE64 characters and return incorrect result. Following patch makes 2.7 code the same as 3.x. Tests are changed to fail with large probability with unpatched code ('\xe1' is an alnum on almost all 8-bit locales).

The patch looks good to me.

New changeset ff1366ff2761 by Serhiy Storchaka in branch '2.7':
Issue #24848: Fixed yet one bug in UTF-7 decoder. Testing for BASE64 character
https://hg.python.org/cpython/rev/ff1366ff2761