Currently, the sqlite3 DB-API 2.0 attribute 'threadsafety' is hard-coded to 1, meaning "threads may share the module, but not connections". This is not always true, since it depends on the default SQLite threaded mode, selected at compile-time with the SQLITE_THREADSAFE define.

SQLite's default compile-time threaded mode is SQLITE_THREADSAFE=1, also known as "serialized threaded mode", meaning SQLite can be safely used by multiple threads with no restriction. This mode equals DB-API 2.0 threadsafety level 3: threads may share the module, connections and cursors.

On macOS 11.6, the system supplied Python 3 and SQLite 3 uses SQLITE_THREADSAFE=2 (also known as "multi-thread mode"), meaning SQLite can be safely used by multiple threads provided that no single database connection is used simultaneously in two or more threads. This mode equals DB-API 2.0 threadsafety level 1: threads may share the module, but not connections.

With SQLITE_THREADSAFE=0 (also known as "single-thread mode"), meaning all mutexes are disabled and SQLite is unsafe to use in more than a single thread at once. This mode equals DB-API 2.0 threadsafety level 0: threads may not share the module.


Suggesting to set the 'treadsafety' dynamically at sqlite3 module load time, either via Lib/sqlite3/dbapi2.py, or in C during module init (slightly faster).


See also:

- https://www.python.org/dev/peps/pep-0249/
- https://sqlite.org/threadsafe.html
- https://discuss.python.org/t/is-sqlite3-threadsafety-the-same-thing-as-sqlite3-threadsafe-from-the-c-library/11463/11

+1 on setting the attributes dynamically. Other DB-API modules use a
similar approach.

FYI, it is also possible to change the threaded mode using sqlite3_config(), however that API is not exposed to the sqlite3 module, and there is no plans for doing so in the near future :)

BTW, I've verified that the overhead of adding this query is negligible.

$ python -m pyperf compare_to --min-speed 1 main.json patched.json 
Benchmark hidden because not significant (1): sqlite_synth

git switch -
Switched to branch 'main'
$ ./python.exe -m timeit "import sqlite3; sqlite3.threadsafety"
1000000 loops, best of 5: 225 nsec per loop
$ git switch -
Switched to branch 'sqlite-threadsafety'
$ ./python.exe -m timeit "import sqlite3; sqlite3.threadsafety"
1000000 loops, best of 5: 221 nsec per loop

New changeset c2739867113a2b650db300c03ef06cf18dcee3f4 by Erlend Egeberg Aasland in branch 'main':
bpo-45613: Set `sqlite3.threadsafety` dynamically (GH-29227)
https://github.com/python/cpython/commit/c2739867113a2b650db300c03ef06cf18dcee3f4