Docker Compose for Python and PostgreSQL: Setup Guide

Use a docker-compose.yml file to define services for Python and PostgreSQL. The Python service runs your app, and the PostgreSQL service runs the database, linked via a network and environment variables.

Syntax

A docker-compose.yml file defines multiple services that run together. Each service has a name and configuration like the image to use, ports to expose, environment variables, and volumes for data persistence.

Key parts:

• services: Lists containers to run.
• image: Docker image to use.
• build: Build context for custom images.
• ports: Maps container ports to host ports.
• environment: Sets environment variables inside containers.
• volumes: Shares files or folders between host and container.

yaml

version: '3.8'
services:
  python-app:
    build: ./app
    ports:
      - "8000:8000"
    environment:
      - DATABASE_URL=postgresql://user:password@db:5432/mydb
    depends_on:
      - db
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
    volumes:
      - pgdata:/var/lib/postgresql/data
volumes:
  pgdata: {}

Example

This example shows a Python app connecting to a PostgreSQL database using Docker Compose. The db service runs PostgreSQL, and the python-app service runs the Python code that connects to it.

The Python app uses environment variables to get database connection info.

yaml

version: '3.8'
services:
  python-app:
    build: ./app
    ports:
      - "8000:8000"
    environment:
      - DATABASE_URL=postgresql://user:password@db:5432/mydb
    depends_on:
      - db
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
    volumes:
      - pgdata:/var/lib/postgresql/data
volumes:
  pgdata: {}

# app/Dockerfile
# ----------------
# FROM python:3.11-slim
# WORKDIR /app
# COPY requirements.txt ./
# RUN pip install -r requirements.txt
# COPY . ./
# CMD ["python", "app.py"]

# app/app.py
# ----------------
# import os
# import psycopg2
# import time
#
# def connect_db():
#     while True:
#         try:
#             conn = psycopg2.connect(os.getenv('DATABASE_URL'))
#             print("Connected to database")
#             conn.close()
#             break
#         except Exception as e:
#             print(f"Waiting for DB: {e}")
#             time.sleep(2)
#
# if __name__ == '__main__':
#     connect_db()

Output

Connected to database

Common Pitfalls

Common mistakes include:

• Not setting depends_on so Python tries to connect before PostgreSQL is ready.
• Missing environment variables for database credentials.
• Not using volumes for PostgreSQL data, causing data loss on container restart.
• Incorrect database URLs or ports.

Always check logs with docker-compose logs to debug connection issues.

yaml

version: '3.8'
services:
  python-app:
    build: ./app
    ports:
      - "8000:8000"
    environment:
      - DATABASE_URL=postgresql://user:password@db:5432/mydb
    # Missing depends_on here causes race condition
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
    # Missing volume means data is lost on restart

volumes: {}

# Corrected version adds depends_on and volume as shown in previous examples.

Quick Reference

• Use depends_on to control startup order.
• Set environment variables for database credentials and connection strings.
• Use volumes to persist database data.
• Map ports to access services from your host machine.
• Build Python app with a Dockerfile specifying dependencies.

Key Takeaways

Define Python and PostgreSQL as separate services in docker-compose.yml for easy management.

Use environment variables to pass database connection info securely to the Python app.

Add depends_on to ensure PostgreSQL starts before the Python app tries to connect.

Use volumes to keep PostgreSQL data safe across container restarts.

Check logs with docker-compose logs to troubleshoot connection or startup issues.