import sqlite3

class MySum:

    def __init__(self):

        self.count = 0

    def step(self, value):

        self.count += value

    def finalize(self):

        return self.count

con = sqlite3.connect(":memory:")

con.create_aggregate("mysum", 1, MySum)

cur = con.cursor()

cur.execute("create table test(i)")

cur.execute("insert into test(i) values (1)")

cur.execute("insert into test(i) values (2)")

cur.execute("select mysum(i) from test")

print cur.fetchone()[0]