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])

con.close()