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For some yet unknown reason, test_codecs fails when it is run after
test_zipimport and subsequently test_mailbox:
./python Lib/test/regrtest.py -w test_zipimport test_mailbox test_codecs
[SNIP]
======================================================================
ERROR: test_basics (test.test_codecs.BasicUnicodeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/alex/src/python.org/py3k/Lib/test/test_codecs.py", line
1360, in test_basics
 encodedresult += encoder.encode(c)
 File "/home/alex/src/python.org/py3k/Lib/encodings/ascii.py", line 22,
in encode
 return codecs.ascii_encode(input, self.errors)[0]
AttributeError: 'NoneType' object has no attribute 'ascii_encode'
======================================================================
ERROR: test_decoder_state (test.test_codecs.BasicUnicodeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/alex/src/python.org/py3k/Lib/test/test_codecs.py", line
1445, in test_decoder_state
 self.check_state_handling_decode(encoding, u, u.encode(encoding))
 File "/home/alex/src/python.org/py3k/Lib/test/test_codecs.py", line
30, in check_state_handling_decode
 part1 = d.decode(s[:i])
 File "/home/alex/src/python.org/py3k/Lib/encodings/ascii.py", line 26,
in decode
 return codecs.ascii_decode(input, self.errors)[0]
AttributeError: 'NoneType' object has no attribute 'ascii_decode'
----------------------------------------------------------------------
Ran 121 tests in 0.204s
FAILED (errors=2)
test test_codecs failed -- errors occurred; run in verbose mode for details

When the zipimport test is run, encodings.ascii is imported. Then it is
removed from sys.modules by the test cleanup. When the mailbox test is
run, it is reimported. However, when the codecs test tries to use it,
the encodings.ascii module has been finalized (as if during interpreter
shutdown) by having all its globals set to None. Thus the test failure.
I haven't figured out why this is happening yet, but it seems likely
that the issue is either with zipimport or with the zipimport test
cleanup. I'm guessing that anything that triggers encodings.ascii to be
loaded before test_codecs tries to use it will trigger the bug, but I
haven't tested that theory yet.

A simple solution could be to import encodings.ascii in regrtest.py,
before it saves the list of modules.
Or, since codecs are stored in a global cache dictionary that cannot be
cleared (interp->codec_search_cache), avoid unloading modules from the
encodings package.

Here is a patch.

The patch looks good to me.

Thanks. The patch is now committed in py3k and 3.1.