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On a side note, maybe it makes sense for a new module like this one to promote and use the modern (>=2.2) ways of defining C types.

What I have in mind is using tp_methods instead of Py_FindMethod, and generally not reverting to strcmp().  In this case, the constants like 'digest_size' would be best stored as class attributes instead, if possible.  Indeed, allowing expressions like "hashlib.md5.digest_size" conveys the idea that the result doesn't depend on a particular instance, unlike "hashlib.md5().digest_size".  (Of course class attributes are also readable from the instance, as usual.)

I can give it a try if you don't want to invest more time in this patch than you already did (for which we are grateful to you :-)