Build a Python App with CockroachDB and Psycopg 3

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Warning:

As of May 16, 2023, CockroachDB v21.2 is no longer supported. For more details, refer to the Release Support Policy.

Tip:

Check out our developer courses at Cockroach University.

This tutorial shows you how build a simple Python application with CockroachDB and the Psycopg 3 driver.

Step 1. Start CockroachDB

Create a free cluster

If you haven't already, sign up for a CockroachDB Cloud account.
Log in to your CockroachDB Cloud account.
On the Clusters page, click Create Cluster.
On the Create your cluster page, select Serverless.
Click Create cluster.

Your cluster will be created in a few seconds and the Create SQL user dialog will display.

Create a SQL user

The Create SQL user dialog allows you to create a new SQL user and password.

Enter a username in the SQL user field or use the one provided by default.
Click Generate & save password.
Copy the generated password and save it in a secure location.
Click Next.

Currently, all new users are created with full privileges. For more information and to change the default settings, see [Manage SQL users on a cluster.

Get the root certificate

The Connect to cluster dialog shows information about how to connect to your cluster.

Select General connection string from the Select option dropdown.
Open a new terminal on your local machine, and run the CA Cert download command provided in the Download CA Cert section. The client driver used in this tutorial requires this certificate to connect to CockroachDB Cloud.

Get the connection string

Open the General connection string section, then copy the connection string provided and save it in a secure location.

Note:

The connection string is pre-populated with your username, password, cluster name, and other details. Your password, in particular, will be provided only once. Save it in a secure place (Cockroach Labs recommends a password manager) to connect to your cluster in the future. If you forget your password, you can reset it by going to the SQL Users page for the cluster, found at https://cockroachlabs.cloud/cluster/<CLUSTER ID>/users.

If you haven't already, download the CockroachDB binary.
Run the cockroach start-single-node command:
```
$ cockroach start-single-node --advertise-addr 'localhost' --insecure
```
This starts an insecure, single-node cluster.

Take note of the following connection information in the SQL shell welcome text:

CockroachDB node starting at 2021-08-30 17:25:30.06524 +0000 UTC (took 4.3s)
build:               CCL v21.1.6 @ 2021/07/20 15:33:43 (go1.15.11)
webui:               http://localhost:8080
sql:                 postgresql://root@localhost:26257?sslmode=disable

You'll use the sql connection string to connect to the cluster later in this tutorial.

Warning:

The --insecure flag used in this tutorial is intended for non-production testing only. To run CockroachDB in production, use a secure cluster instead.

Step 2. Get the sample code

Clone the sample code's GitHub repo:

$ git clone https://github.com/cockroachdb/example-app-python-psycopg3

The sample code in example.py does the following:

Creates an accounts table and inserts some rows
Transfers funds between two accounts inside a transaction
Deletes the accounts from the table before exiting so you can re-run the example code

To handle transaction retry errors, the code uses an application-level retry loop that, in case of error, sleeps before trying the funds transfer again. If it encounters another retry error, it sleeps for a longer interval, implementing exponential backoff.

Step 3. Install the Psycopg 3 driver

psycopg[binary] is the sample app's only third-party module dependency.

To install psycopg[binary], run the following command:

$ pip3 install "psycopg[binary]"

For other ways to install Psycopg, see the official documentation.

Step 4. Run the code

Set the DATABASE_URL environment variable to the connection string to your cluster:
```
$ export DATABASE_URL="postgresql://root@localhost:26257?sslmode=disable"
```
```
$ export DATABASE_URL="{connection-string}"
```
Where {connection-string} is the connection string you copied earlier.
The app uses the connection string saved to the DATABASE_URL environment variable to connect to your cluster and execute the code.

Run the code:

$ cd example-app-python-psycopg3

$ python3 example.py

The output should show the account balances before and after the funds transfer:

Balances at Thu Aug  4 15:51:03 2022:
account id: 2e964b45-2034-49a7-8ab8-c5d0082b71f1  balance: $1000
account id: 889cb1eb-b747-46f4-afd0-15d70844147f  balance: $250
Balances at Thu Aug  4 15:51:03 2022:
account id: 2e964b45-2034-49a7-8ab8-c5d0082b71f1  balance: $900
account id: 889cb1eb-b747-46f4-afd0-15d70844147f  balance: $350

What's next?

Read more about using the Python psycopg3 driver.

You might also be interested in the following pages:

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