import sqlite3
import sys

def custom_convert(data):
    print("CONVERTING", data)
    return "Custom({})".format(data.decode() )

def show(sql):
    print(sql)
    print("-" * len(sql))
    cs.execute(sql)
    for row in cs:
        print(row)
    print("")

if __name__ == "__main__":
    sqlite3.register_converter("CUSTOM", custom_convert)

    db = sqlite3.connect(":memory:", detect_types=sqlite3.PARSE_DECLTYPES)
    cs = db.cursor()
    cs.execute("create table demo (alpha custom)")
    cs.executemany(
        "insert into demo (alpha) values (?)",
        [["foo"]])

    print("")
    show("select alpha from demo union all select alpha from demo")
    show("select alpha from demo union select alpha from demo")
"""
$ python3.4 demo3.py

select alpha from demo union all select alpha from demo
-------------------------------------------------------
CONVERTING b'foo'
CONVERTING b'foo'
('Custom(foo)',)
('Custom(foo)',)

select alpha from demo union select alpha from demo
---------------------------------------------------
('foo',)
"""