[Python-Dev] Stream codecs and _multibytecodec (original) (raw)

Thomas Wouters thomas at python.org
Fri Apr 21 01:06:30 CEST 2006

While merging the trunk changes into the p3yk branch, I discovered what I think is a bug in the stream codecs. It's easily reproduced in the trunk: in Lib/codecs.py, make the 'Codec' class new-style. Then, suddenly, test_codecs will crash with an exception like this:

====================================================================== ERROR: test_basics (test.test_codecs.BasicUnicodeTest)

Traceback (most recent call last): File "/home/thomas/python/python/trunk/Lib/test/test_codecs.py", line 1089, in test_basics writer = codecs.getwriter(encoding)(q) File "/home/thomas/python/python/trunk/Lib/codecs.py", line 296, in init self.stream = stream TypeError: readonly attribute

Looking at the class, it's hard to tell how it's suddenly a read-only attribute, until you figure out which codec it breaks with: big5. Which is defined in encodings.big5, as a subclass of codecs.Codec, _multibytecodec.MultibyteStreamWriter and codecs.StreamWriter. And _multibytecodec.MultibyteStreamWriter is a new-style class (as it's defined in C) with a read-only 'stream' attribute. The conflicting attribute is silently masked by Python, since classic classes ignore two-thirds of the descriptor protocol (setting and deleting), but it jumps right out when all classes become new-style.

I'm not sure whether this attribute conflict is on purpose or not, but since it will break in the future, I suggest it gets fixed. It looks like renaming the _MultibyteStreamWriter attribute is the easiest solution, but I don't know which API has precedence. (One of the two was added fairly recently, at least, since it didn't break in the p3yk branch until I merged in the last few months' worth of changes :)

-- Thomas Wouters <thomas at python.org>

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