How to Change Port in Flask: A Comprehensive Guide

In Flask, the default port for running your application is 5000. However, you might need to change this port if it’s already in use by another application or if you want to run multiple Flask apps simultaneously. Changing the port is straightforward and can be done using command-line options, environment variables, or directly in your code.

Using Command-Line Options

To change the port in a Flask application using command-line options, follow these steps:

1. Navigate to your Flask application directory:

   cd path/to/your/flask/app
   

2. Run the Flask application with the desired port:

   flask run --port 8000
   

3. Specify both host and port if needed:

   flask run --host 0.0.0.0 --port 8000
   

4. Using an environment variable:

   export FLASK_RUN_PORT=8000
   flask run
   

Replace 8000 with your desired port number.

Setting Environment Variables

Here are the steps to change the port in a Flask application by setting environment variables:

Windows

1. Open Command Prompt.
2. Set the environment variables:

   set FLASK_RUN_PORT=7000
   set FLASK_RUN_HOST=127.0.0.1
   

3. Run your Flask application:

   flask run
   

macOS/Linux

1. Open Terminal.
2. Set the environment variables:

   export FLASK_RUN_PORT=7000
   export FLASK_RUN_HOST=127.0.0.1
   

3. Run your Flask application:

   flask run
   

Using a .flaskenv File

1. Create a file named .flaskenv in your project directory.
2. Add the following lines:

   FLASK_RUN_PORT=7000
   FLASK_RUN_HOST=127.0.0.1
   

3. Run your Flask application:

   flask run
   

These steps will configure your Flask application to run on port 7000 and host 127.0.0.1.

Modifying Configuration Files

Here’s how you can change the port in a Flask application by modifying the configuration files:

1. Create a Flask App:

   from flask import Flask
   app = Flask(__name__)
   
   @app.route('/')
   def hello_world():
       return 'Hello, World!'
   
   if __name__ == '__main__':
       app.run()
   

2. Modify the Configuration File:
   Create a configuration file (e.g., config.py) and set the desired port:

   class Config:
       PORT = 8080  # Change this to your desired port number
   

3. Load Configuration in Your Flask App:
   Update your Flask app to load the configuration from config.py:

   from flask import Flask
   from config import Config
   
   app = Flask(__name__)
   app.config.from_object(Config)
   
   @app.route('/')
   def hello_world():
       return 'Hello, World!'
   
   if __name__ == '__main__':
       app.run(port=app.config['PORT'])
   

4. Run Your Flask App:
   Simply run your Flask app as usual:

   python app.py
   

Your Flask application will now run on the port specified in the configuration file (in this case, port 8080).

Common Issues and Troubleshooting

Here are some common issues you might encounter when changing the port in Flask, along with troubleshooting tips:

Common Issues

1. Port Already in Use:

   • Symptom: OSError: [Errno 98] Address already in use
   • Solution: Check if another application is using the port. Use netstat -tuln | grep <port_number> on Linux or netstat -a -n -o | find "<port_number>" on Windows to identify the process. Kill the process or choose a different port.

2. Firewall Blocking the Port:

   • Symptom: Unable to access the Flask app externally.
   • Solution: Ensure the port is open in your firewall settings. On Linux, use sudo ufw allow <port_number>. On Windows, adjust the firewall settings to allow traffic on the specified port.

3. Incorrect Port Binding:

   • Symptom: Flask app not accessible on the specified port.
   • Solution: Ensure you are binding to the correct port in your Flask app. Use app.run(port=<port_number>) in your code.

4. Permission Issues:

   • Symptom: PermissionError: [Errno 13] Permission denied
   • Solution: Ports below 1024 require root privileges. Use a port number above 1024 or run the Flask app with elevated privileges.

5. Environment Conflicts:

   • Symptom: Flask app runs on the default port despite specifying a different one.
   • Solution: Check for environment variables that might be overriding your settings. Ensure FLASK_RUN_PORT is set correctly if using environment variables.

Troubleshooting Tips

• Check Logs: Always check the Flask server logs for detailed error messages.
• Use Debug Mode: Run your Flask app in debug mode (app.run(debug=True, port=<port_number>)) to get more detailed error information.
• Test Locally: Before deploying, test your Flask app locally to ensure the port change works as expected.
• Update Configuration: If using a configuration file, ensure the port setting is correctly updated.

To Change the Port in Flask

You can use one of the following methods:

• Use the `port` parameter when calling `app.run()`: app.run(port=8080)
• Set the `FLASK_RUN_PORT` environment variable: export FLASK_RUN_PORT=8080
• Update the configuration file (e.g., `config.py`) to set the port number
• Use a WSGI server like Gunicorn or uWSGI, which allows you to specify the port when running the application

Choosing a Method

When choosing a method, consider the following factors:

• Development: For development purposes, using the `port` parameter in `app.run()` is often the simplest and most convenient approach.
• Production: In production environments, it’s recommended to use a WSGI server like Gunicorn or uWSGI, which provides better performance, reliability, and security features.
• Environment variables: If you need to change the port frequently or want to make it configurable, setting the `FLASK_RUN_PORT` environment variable is a good option.

Regardless of the method chosen, ensure that the port number is correctly set in your configuration file (if using one) and that any necessary firewall rules are updated to allow incoming traffic on the specified port.