While working on bpo-45126 / GH-28227 (and while working on the AC conversion, bpo-40956), I've been slightly frustrated on the implementation of sqlite3 "isolation level". The code is messy, and we've got two connection members that carry pretty much the same type of information (self->isolation_level, and self->begin_statement).

I would like to make the following enhancements:
  - merge 'isolation_level' and 'begin_statement' members in some kind of way
  - split pysqlite_connection_set_isolation_level():
    + one method for parsing and storing the 'isolation_level' member
    + one method for carrying out any needed SQLite API operation

This should result in a cleaner connection __init__() method.


Another slightly related performance enhancement could be to cache the "begin" (and "commit") statements as sqlite3_stmt pointers on the connection object, but that is a digression.

Hmm, now I understood what you intended,
The code was hard to read without knowing the condition of the begin_statement.
Would this be a trade-off in terms of code readability?

IMO, GH-29053 improves readability, mainly because of these factors:

- Argument Clinic now takes care of the ref count dance, and as a bonus, it is more self-documenting (stating clearly that it accepts str and None)
- Begin statement validation has been refactored out, which IMO helps understanding how isolation level and begin statement is related
- The isolation level setter is now greatly simplified; it fits well in a screenful, there are fewer ref count dances
- Using get_begin_statement() is much less noisier than the previous string compare code in pysqlite_connection_set_isolation_level()


The only thing that still bugs me is the "magical" 6 byte offset needed to convert between isolation level and begin statement. Perhaps a set of conversion functions would make it more readable.

New changeset b567b9d74bd9e476a3027335873bb0508d6e450f by Erlend Egeberg Aasland in branch 'main':
bpo-45512: Simplify isolation_level handling in `sqlite3` (GH-29053)
https://github.com/python/cpython/commit/b567b9d74bd9e476a3027335873bb0508d6e450f

Need to amend GH-29053, so I'm opening until the fix is merged.

New changeset 822c3dcce3996e411c1ff5c432c6ac7d2845cfd6 by Erlend Egeberg Aasland in branch 'main':
bpo-45512: Raise exception if sqlite3.Connection.__init__ is called with bad isolation level (#29561)
https://github.com/python/cpython/commit/822c3dcce3996e411c1ff5c432c6ac7d2845cfd6

New changeset 5f9247e36a0213b0dcfd43533db5cf6570895cfd by Erlend Egeberg Aasland in branch 'main':
bpo-45512: Extend `sqlite3` test suite regarding isolation levels (GH-29576)
https://github.com/python/cpython/commit/5f9247e36a0213b0dcfd43533db5cf6570895cfd

@erlendaasland

Now we close this issue right?

New changeset e002bbc6cce637171fb2b1391ffeca8643a13843 by Dong-hee Na in branch 'main':
bpo-45512: Simplify manage isolation level (GH-29562)
https://github.com/python/cpython/commit/e002bbc6cce637171fb2b1391ffeca8643a13843

> Now we close this issue right?

Not yet ;) I have an AC tweak I'd like to add. Wait for a PR anytime soon.

New changeset 0920b61a0cb30128287ebafab1df8cad3a3dffdb by Erlend Egeberg Aasland in branch 'main':
bpo-45512: Use Argument Clinic to set sqlite3 isolation level (GH-29593)
https://github.com/python/cpython/commit/0920b61a0cb30128287ebafab1df8cad3a3dffdb

Thanks for reviews and improvements, Dong-hee :)