Jack: I've updated the PR, improving compatibility with the "blake3" package on PyPI.  I took your notes, and also looked at the C module you wrote.

The resulting commit is here:

https://github.com/python/cpython/pull/31686/commits/37ce72b0444ad63fd1989ad36be5f7790e51f4f1

Specifically:

* derive_key_context is now a string, which I internally encode into UTF-8.

* I added the AUTO member to the module, set to -1.

* max_threads may not be zero; it can be >= 1 or AUTO.

* I added the reset() method.


Some additional thoughts, both on what I did and on what you did:

* In your new() method, your error string says "keys must be 32 bytes".  I went with "key must be exactly 32 bytes"; the name of the parameter is "key", and I think "exactly" makes the message clearer.

* In my new() method, I complain if the derive_key_context is zero-length.  In your opinion, is that a good idea, or is that overly fussy?

* In your copy() method, you hard-code Blake3Type.  It's considered good form to use type(self) here, in case the user is calling copy on a user-created subclass of Blake3Type.

* In your copy() method, if the original has a lock created, you create a lock in the copy too.  I'm not sure why you bother; I leave the lock member uninitialized, and let the existing logic create the lock in the copy on demand.

* In the Blake3_methods array, you list the "update" method twice.  I suspect this is totally harmless, but it's unnecessary.