Python and sqlite3 problem

The following python script runs to completion on Linux and creates a database file “test_db” in the same directory as the script.

import sqlite3
con = sqlite3.connect('test_db')
cur = con.cursor()
cur.execute("CREATE TABLE tab1 (name, phone)")
cur.execute("INSERT INTO tab1 VALUES('Bob',1234)")
con.close()
print("reached end of script")

  File "testSQL/py", line 4, in <module>
    cur.execute("CREATE TABLE tab1 (name, phone)")
    sqlite3.DatabaseError: file is not a database

It does create a file but it does not put it in the same directory as the python script, but in the parent directory. It also mangles the name.
The created file has the parent directory name appended to it using a / separator.
Eg the script file is “testSQL/py” in directory “sqlite”
The new database file is not in “sqlite”, but in the directory above and has a filename of “sqlite/test_db”
It looks as if the unix to RISC OS file conversion algorithm is going wrong.