> Anyone know why it uses a C extension?

Performance.  A number of people (among them the authors of the _csv
extension and me, a contributor to the Python csv module that fronts
it) routinely read and write large (several megabytes) CSV files.  We
had all had experience with earlier Python-only CSV readers and
writers.  Their performance was just too poor.

If you wrote a new module in Python that's compatible with the
existing module -- and performed acceptably -- I see no reason it
couldn't replace the current module.  There are already a number
of test cases.  You'd certainly have to embellish them, but if the
current set passed that would be a good indication your code was at
least on the right track compatibility-wise.

There are other reasons to desire a Python-based solution other
than Unicode support.  It would be much more likely that such a
module would work with other Python implementations (e.g., PyPy,
IronPython and Jython).

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