Comment kindly provided by D. Richard Hipp himself:
"""""
I don't have a login for the python bug tracker so I cannot comment
there.  But I think I see the problem.  This is as Aviv Polivoda
remarks at https://bugs.python.org/issue28518#msg279808

I think this is a error in sqlite as the documentation says:
"ransaction control statements such as BEGIN, COMMIT, ROLLBACK,
SAVEPOINT, and RELEASE cause sqlite3_stmt_readonly() to return true,"

Except it is not a bug in SQLite, but rather an ambiguity in the
documentation.  In his quote, Aviv omitted the second clause from the
documentation:  "since the statements themselves do not actually
modify the database but rather they control the timing of when other
statements modify the database."  (Full text here:
https://www.sqlite.org/c3ref/stmt_readonly.html)

For a plain BEGIN statement, there are no changes to the database
file, so sqlite3_stmt_readonly() does indeed return TRUE.  But for a
BEGIN IMMEDIATE statement, there are database changes, because the
extra IMMEDIATE keyword causes the statement to go ahead and start
transaction "immediately".

So technically, the documentation is correct, though I can certainly
understand that it is misleading and ambiguous as currently worded.
Clarification will be checked in shortly and will appear in the 3.16.0
release.

Note also that behavior of sqlite3_stmt_readonly() has not changed
since that interface was first added for the 3.7.5 release on
2011-02-01.  So this is not an SQLite regression.  Rather, I presume
that Python has recently started using the sqlite3_stmt_readonly()
interface in a new way.



--
D. Richard Hipp
drh@sqlite.org
"""""