Remote debugging with debugpy works from code but not from command line

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05-10-2024

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Debugging woes: Why debugpy works in code but not from the command line?

Ever faced the frustrating situation where remote debugging with debugpy works perfectly fine within your code editor, but throws an error when initiated from the command line? This seemingly contradictory behavior can leave you scratching your head, wondering what's going on under the hood.

Let's break down this common issue, explore the underlying reasons, and provide solutions to get your remote debugging back on track.

The Scenario: Debugging from Code vs. Command Line

Imagine you're developing a Python application that interacts with a remote server. You've set up remote debugging using debugpy, and everything runs smoothly when you initiate the debugger from your integrated development environment (IDE). You hit breakpoints, examine variables, and step through your code with ease.

However, when you attempt to start the debugging session directly from the command line using python -m debugpy --listen 5678 your_script.py, the process fails to connect to the debugger. This is a common experience for many developers, and it can be confusing to understand why it works one way but not the other.

The Culprit: Environmental Mismatches

The culprit behind this behavior often lies in the discrepancies between the environments where you're executing your code and launching the debugpy process. Here's a breakdown of the potential areas of misalignment:

1. Environment Variables:

• The Python Interpreter: Your IDE might be using a different Python interpreter than the one you're specifying on the command line.
• System Paths: The PATH environment variable, which defines the directories where your operating system searches for executables, might be configured differently in your IDE and command line environments.
• Other Required Libraries: Ensure that all the necessary libraries for your application and debugpy are correctly installed in both environments.

2. Network Configuration:

• Firewalls: Firewalls can sometimes block the debugger's connection attempt. Make sure the necessary ports are open in both the client and server environments.
• Network Security: Check if any network security settings, like corporate proxies or VPNs, might be interfering with the connection.

Solutions: Bridging the Gap

1. Environment Consistency:

   • Virtual Environments: Use a virtual environment for your project to ensure a consistent set of packages and dependencies across both environments.
   • IDE Settings: Configure your IDE to use the same Python interpreter and environment variables as your command-line environment.

2. Network Troubleshooting:

   • Firewall Rules: Check your firewall configuration and create rules allowing access to the debugpy port (typically 5678).
   • Network Connectivity: Verify that the client and server machines can communicate over the network and that the port is accessible.
   • Proxy Settings: Ensure that any proxies or VPNs are correctly configured and not blocking the debugpy connection.

3. Command Line Syntax:

   • IP Address: If you're debugging remotely, make sure you're specifying the correct IP address of the server machine in your --listen argument.
   • Alternative Ports: Experiment with different port numbers if you're encountering port conflicts.

Example: Debugging a Flask App

Let's consider an example where you're debugging a Flask web application. Here's how you might configure your environment for successful remote debugging:

1. Create a Virtual Environment:

   python3 -m venv my_venv
   source my_venv/bin/activate 
   

2. Install Dependencies:

   pip install Flask debugpy
   

3. Modify your Flask application:

   from flask import Flask
   import debugpy
   
   app = Flask(__name__)
   
   @app.route('/')
   def index():
       return 'Hello, World!'
   
   if __name__ == '__main__':
       # Start the debugpy server
       debugpy.listen(("0.0.0.0", 5678)) 
       debugpy.wait_for_client()
       app.run(debug=True)
   

4. Start the debugger from the command line:

   python -m debugpy --listen 5678 your_flask_app.py 
   

5. Connect from your IDE:

   • In your IDE, set up a remote debug configuration targeting the server's IP address and port (5678).

By carefully addressing the potential causes and following these guidelines, you can streamline your remote debugging experience with debugpy and eliminate the frustrating inconsistencies between code and command-line debugging.