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Learn how to seamlessly connect your Python application to PostgreSQL using Docker-Compose while avoiding common pitfalls.
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This video is based on the question https://stackoverflow.com/q/73355454/ asked by the user 'Sebastián Martínez' ( https://stackoverflow.com/u/11588229/ ) and on the answer https://stackoverflow.com/a/73355669/ provided by the user 'Sami Tahri' ( https://stackoverflow.com/u/10034177/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Connect Your Python App to Postgres Using Docker-Compose

In recent times, many developers have adopted using containers to simplify their application deployments. However, connecting services like a Python application to a PostgreSQL database in a Docker environment can sometimes lead to confusion and errors, especially when dealing with configuration files. In this guide, we will explore how to connect your Python application to PostgreSQL through Docker Compose, focusing on solving common issues along the way.

The Problem

Consider this scenario: you have set up a simple Python application meant to connect to a PostgreSQL database, and both are running within Docker containers. However, upon trying to establish the connection via SQLAlchemy, you encounter the following error:

[[See Video to Reveal this Text or Code Snippet]]

This error typically indicates that your application cannot resolve the hostname for the database service. Understanding why this happens and how to fix it is crucial.

Step-by-Step Solution

To solve the connection issue, follow these steps:

1. Understanding the Containers

When you run services in Docker using Docker Compose, each service can communicate with others using the service name as a hostname. Here’s how the relevant parts of your docker-compose.yml file looked:

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In the configuration above, your PostgreSQL service is defined as db.

2. Configuring .env File

Your current .env file looks like this:

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Issue:

The DB_HOST variable is set to localhost. However, since both services are running in separate containers, localhost refers to the Python application’s own environment, not the database container.

Solution:

Update the DB_HOST variable to point to the service name db like this:

[[See Video to Reveal this Text or Code Snippet]]

3. Adjusting the Python Connection Code

With the updated .env, your Python connection code will look something like this:

[[See Video to Reveal this Text or Code Snippet]]

4. Rebuilding Your Containers

After making these updates, make sure to rebuild your containers for the changes to take effect:

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Conclusion

By following these steps and properly configuring your environment variables, your Python application should now be able to connect to the PostgreSQL database running in Docker without issues. Remember, whenever working with Docker Compose, always refer to the service names you defined when specifying hostnames.

If you continue to encounter problems, verify your connection string and consult the logs of your containers to debug further. Happy coding!