Build a Python App with CockroachDB and psycopg2

$ pip install psycopg2-binary

$ git clone https://github.com/cockroachlabs/hello-world-python-psycopg2/

#!/usr/bin/env python3
"""
Test psycopg with CockroachDB.
"""

import logging
import os
import random
import time
import uuid
from argparse import ArgumentParser, RawTextHelpFormatter

import psycopg2
from psycopg2.errors import SerializationFailure
import psycopg2.extras


def create_accounts(conn):
    psycopg2.extras.register_uuid()
    ids = []
    id1 = uuid.uuid4()
    id2 = uuid.uuid4()
    with conn.cursor() as cur:
        cur.execute(
            "CREATE TABLE IF NOT EXISTS accounts (id UUID PRIMARY KEY, balance INT)"
        )
        cur.execute(
            "UPSERT INTO accounts (id, balance) VALUES (%s, 1000), (%s, 250)", (id1, id2))
        logging.debug("create_accounts(): status message: %s",
                      cur.statusmessage)
    conn.commit()
    ids.append(id1)
    ids.append(id2)
    return ids


def delete_accounts(conn):
    with conn.cursor() as cur:
        cur.execute("DELETE FROM accounts")
        logging.debug("delete_accounts(): status message: %s",
                      cur.statusmessage)
    conn.commit()


def print_balances(conn):
    with conn.cursor() as cur:
        cur.execute("SELECT id, balance FROM accounts")
        logging.debug("print_balances(): status message: %s",
                      cur.statusmessage)
        rows = cur.fetchall()
        conn.commit()
        print(f"Balances at {time.asctime()}:")
        for row in rows:
            print("account id: {0}  balance: ${1:2d}".format(row['id'], row['balance']))


def transfer_funds(conn, frm, to, amount):
    with conn.cursor() as cur:

        # Check the current balance.
        cur.execute("SELECT balance FROM accounts WHERE id = %s", (frm,))
        from_balance = cur.fetchone()['balance']
        if from_balance < amount:
            raise RuntimeError(
                f"insufficient funds in {frm}: have {from_balance}, need {amount}"
            )

        # Perform the transfer.
        cur.execute(
            "UPDATE accounts SET balance = balance - %s WHERE id = %s", (
                amount, frm)
        )
        cur.execute(
            "UPDATE accounts SET balance = balance + %s WHERE id = %s", (
                amount, to)
        )

    conn.commit()
    logging.debug("transfer_funds(): status message: %s", cur.statusmessage)


def run_transaction(conn, op, max_retries=3):
    """
    Execute the operation *op(conn)* retrying serialization failure.

    If the database returns an error asking to retry the transaction, retry it
    *max_retries* times before giving up (and propagate it).
    """
    # leaving this block the transaction will commit or rollback
    # (if leaving with an exception)
    with conn:
        for retry in range(1, max_retries + 1):
            try:
                op(conn)

                # If we reach this point, we were able to commit, so we break
                # from the retry loop.
                return

            except SerializationFailure as e:
                # This is a retry error, so we roll back the current
                # transaction and sleep for a bit before retrying. The
                # sleep time increases for each failed transaction.
                logging.debug("got error: %s", e)
                conn.rollback()
                logging.debug("EXECUTE SERIALIZATION_FAILURE BRANCH")
                sleep_ms = (2**retry) * 0.1 * (random.random() + 0.5)
                logging.debug("Sleeping %s seconds", sleep_ms)
                time.sleep(sleep_ms)

            except psycopg2.Error as e:
                logging.debug("got error: %s", e)
                logging.debug("EXECUTE NON-SERIALIZATION_FAILURE BRANCH")
                raise e

        raise ValueError(
            f"transaction did not succeed after {max_retries} retries")


def main():
    opt = parse_cmdline()
    logging.basicConfig(level=logging.DEBUG if opt.verbose else logging.INFO)
    try:
        # Attempt to connect to cluster with connection string provided to
        # script. By default, this script uses the value saved to the
        # DATABASE_URL environment variable.
        # For information on supported connection string formats, see
        # https://www.cockroachlabs.com/docs/stable/connect-to-the-database.html.
        db_url = opt.dsn
        conn = psycopg2.connect(db_url, 
                                application_name="$ docs_simplecrud_psycopg2", 
                                cursor_factory=psycopg2.extras.RealDictCursor)
    except Exception as e:
        logging.fatal("database connection failed")
        logging.fatal(e)
        return
    ids = create_accounts(conn)
    print_balances(conn)

    amount = 100
    toId = ids.pop()
    fromId = ids.pop()

    try:
        run_transaction(conn, lambda conn: transfer_funds(
            conn, fromId, toId, amount))

    except ValueError as ve:
        # Below, we print the error and continue on so this example is easy to
        # run (and run, and run...).  In real code you should handle this error
        # and any others thrown by the database interaction.
        logging.debug("run_transaction(conn, op) failed: %s", ve)
        pass

    print_balances(conn)

    delete_accounts(conn)

    # Close communication with the database.
    conn.close()


def parse_cmdline():
    parser = ArgumentParser(description=__doc__,
                            formatter_class=RawTextHelpFormatter)

    parser.add_argument("-v", "--verbose",
                        action="store_true", help="print debug info")

    parser.add_argument(
        "dsn",
        default=os.environ.get("DATABASE_URL"),
        nargs="?",
        help="""\
database connection string\
 (default: value of the DATABASE_URL environment variable)
            """,
    )

    opt = parser.parse_args()
    if opt.dsn is None:
        parser.error("database connection string not set")
    return opt


if __name__ == "__main__":
    main()

Balances at Fri Oct 30 18:27:00 2020:
(1, 1000)
(2, 250)
Balances at Fri Oct 30 18:27:00 2020:
(1, 900)
(2, 350)