# Python object serialization
# The 'pickle' module implements binary protocols for serializing and de-serializing 
# a Python object structure. 
# “Pickling” is the process whereby a Python object hierarchy is converted into a byte stream, 
# and “unpickling” is the inverse operation, whereby a byte stream (from a binary file or bytes-like 
# object) is converted back into an object hierarchy. 
# Pickling (and unpickling) is alternatively known as “serialization”, “marshalling,” or “flattening”; 
# however, to avoid confusion, the terms used here are “pickling” and “unpickling”.
# sqlite3 — DB-API 2.0 interface for SQLite databases
# SQLite is a C library that provides a lightweight disk-based database that doesn’t require a separate 
# server process and allows accessing the database using a nonstandard variant of the SQL query language. 
# Some applications can use SQLite for internal data storage. 
# It’s also possible to prototype an application using SQLite and then port the code to a larger database 
# such as PostgreSQL or Oracle.
# The 'create_aggregate(name, num_params, aggregate_class)' creates a user-defined aggregate function.
# The aggregate class must implement a step method, which accepts the number of parameters num_params 
# (if num_params is -1, the function may take any number of arguments), and a finalize method which will
# return the final result of the aggregate.
# The finalize method can return any of the types supported by SQLite: bytes, str, int, float and None.

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])
