Remotely debug Python code on Linux in Visual Studio

In this article, you explore how to configure your Visual Studio installation to support debugging Python code on remote Linux computers. This walkthrough is based on Visual Studio 2019 version 16.6.

Visual Studio can launch and debug Python applications locally and remotely on a Windows computer. Visual Studio also supports debugging remotely on a different operating system, device, or Python implementation other than CPython by using the debugpy library.

Visual Studio 2019 version 16.4 and earlier uses the ptvsd library. In Visual Studio 2019 version 16.5 and later, the debugpy library replaces ptvsd. When you use debugpy, the Python code being debugged hosts the debug server to which Visual Studio can attach. This hosting requires a small modification to your code to import and enable the server. You might also need to adjust the network or firewall configurations on the remote computer to allow TCP connections.

Prerequisites

• Visual Studio installed with support for Python workloads. For more information, see Install Python support in Visual Studio.

• A remote computer running Python on an operating system like Mac OSX or Linux.

• Port 5678 (inbound) open on the remote computer's firewall, which is the default for remote debugging.

Set up a Linux computer

You can easily create a Linux virtual machine on Azure and access it by using Remote Desktop from Windows. Ubuntu for the virtual machine is convenient because Python is installed by default. If you have a different configuration, see Install Python interpreters for other Python download locations.

Configure the firewall

The inbound port 5678 must be open on the remote computer's firewall to support remote debugging.

For details on how create a firewall rule for an Azure virtual machine, see the following articles:

• Filter network traffic with a network security group by using the Azure portal
• Route network traffic with a route table using the Azure portal
• Deploy and configure Azure Firewall using the Azure portal

Prepare the script for debugging

Follow these steps to prepare a script for debugging your Python code on Linux.

1. On the remote computer, create a Python file named guessing-game.py with the following code:

   import random
   
   guesses_made = 0
   name = input('Hello! What is your name?\n')
   number = random.randint(1, 20)
   print('Well, {0}, I am thinking of a number between 1 and 20.'.format(name))
   
   while guesses_made < 6:
       guess = int(input('Take a guess: '))
       guesses_made += 1
       if guess < number:
           print('Your guess is too low.')
       if guess > number:
           print('Your guess is too high.')
       if guess == number:
           break
   if guess == number:
       print('Good job, {0}! You guessed my number in {1} guesses!'.format(name, guesses_made))
   else:
       print('Nope. The number I was thinking of was {0}'.format(number))
   

2. Install the debugpy package into your environment by using the pip3 install debugpy command.

   Note

   It's a good idea to record the version of debugpy that's installed in case you need it for troubleshooting. The debugpy listing also shows available versions.

3. Enable remote debugging by adding the following code at the top of the guessing-game.py file before other code. (Though not a strict requirement, it's impossible to debug any background threads spawned before the listen function is called.)

   import debugpy
   debugpy.listen(('0.0.0.0', 5678))
   

4. Save the file and run the program:

   python3 guessing-game.py
   

   The call to the listen function runs in the background and waits for incoming connections as you interact with the program. If desired, you can call the wait_for_client function after you call the listen function to block the program until the debugger attaches.

Attach remotely from Python Tools

The following steps show how to set a breakpoint to stop the remote process.

1. Create a copy of the remote file on the local computer and open it in Visual Studio. It doesn't matter where the file is located, but its name should match the name of the script on the remote computer.

2. (Optional) To have IntelliSense for debugpy on your local computer, install the debugpy package into your Python environment.

3. Select Debug > Attach to Process.

4. In the Attach to Process dialog, set Connection Type to Python remote (debugpy).

5. In the Connection Target field, enter the command tcp://<ip_address>:5678.

   • tcp:// specifies the connection type as Transmission Control Protocol (TCP).
   • <ip_address> is the IP address of the remote computer, which can be an explicit address or a name like myvm.cloudapp.net.
   • :5678 is the remote debugging port number.

6. Select Enter to populate the list of available debugpy processes on that computer:

   

   If you happen to start another program on the remote computer after populating this list, select the Refresh button.

7. Select the process to debug and select Attach, or double-click the process.

8. Visual Studio switches into debugging mode while the script continues to run on the remote computer, providing all the usual debugging capabilities.

   You can set a breakpoint on the if guess < number: line, then switch over to the remote computer and enter another guess. Visual Studio on your local computer stops at the breakpoint, shows local variables, and so on:

   

9. When you stop debugging, Visual Studio detaches from the program. The program continues to run on the remote computer. debugpy also continues listening for attaching debuggers, so you can reattach to the process again at any time.

Troubleshoot the connection

Review the following points to help troubleshoot issues with the connection.

• Make sure you select Python remote (debugpy) for the Connection Type.

• Confirm the secret in the Connection Target exactly matches the secret in the remote code.

• Confirm the IP address in the Connection Target matches that of the remote computer.

• Verify the remote debugging port on the remote computer is open, and the connection target includes the port suffix, such as :5678.

  To use a different port, specify the port number in the call to the listen function, as in debugpy.listen((host, port)). In this case, be sure to open the specific port in the firewall.

• Confirm the debugpy version installed on the remote computer (as returned by the pip3 list command) matches the Visual Studio Python Tools (PTVS) version.

  The following table lists the valid version pairs. As necessary, update the version of debugpy on the remote computer.

  Visual Studio	Python Tools	debugpy
  2019 16.6	1.0.0b5	1.0.0b5
  2019 16.5	1.0.0b1	1.0.0b1

Use ptvsd 3.x for legacy debugging

The ptvsd 3.x legacy debugger is the default in Visual Studio 2017 version 15.7 and earlier.

Depending on your Visual Studio configuration, you might need to use ptvsd 3.x for remote debugging:

• Visual Studio 2017 version 15.7 and earlier with Python 2.6, 3.1 to 3.4, or IronPython
• Visual Studio 2019 version 16.5 and later with Python 2.6, 3.1 to 3.4, or IronPython
• Early 4.x versions

If your configuration implements an older version scenario, Visual Studio shows the error, Debugger does not support this Python environment.

Setup remote debugging

To prepare for remote debugging with ptvsd 3.x, complete the following steps:

1. Set up your secret, which is used to restrict access to the running script.

   In ptvsd 3.x, the enable_attach function requires you to pass a "secret" as the first argument.

   • When you attach the remote debugger, enter the secret with the enable_attach(secret="<secret>") command.

   Although you can allow anyone to connect by using the enable_attach(secret=None) command, this option isn't recommended.

2. Create your connection target URL in the form tcp://<secret>@<ip_address>:5678.

   • tcp:// specifies the connection type as TCP.
   • <secret> is the string passed with the enable_attach function in the Python code.
   • <ip_address> is the IP address of the remote computer, which can be an explicit address or a name like myvm.cloudapp.net.
   • :5678 is the remote debugging port number.

Secure connection with TCPS protocol

By default, the connection to the ptvsd 3.x remote debug server is secured by the secret only, and all data is passed in plain text. For a more secure connection, ptvsd 3.x supports SSL by using the secure form of the TCP protocol, or TCPS.

Use the following steps to configure ptvsd 3.x to work with the TCPS protocol:

1. On the remote computer, use the openssl command to generate separate files for the key and the self-signed certificate:

   openssl req -new -x509 -days 365 -nodes -out cert.cer -keyout cert.key
   

   • At the openssl prompt, enter the hostname or IP address that you use to connect for the Common Name.

   For more information, see Self-signed certificates in the Python ssl module documentation. Note that the command described in the Python documentation generates only a single combined file.

2. In the code, modify the call to the enable_attach function to include certfile and keyfile arguments by using the filenames as the values. These arguments have the same meaning as for the standard ssl.wrap_socket Python function.

   ptvsd.enable_attach(secret='my_secret', certfile='cert.cer', keyfile='cert.key')
   

   You can also make the same change in the code file on the local computer. Because this code isn't actually run, it isn't strictly necessary.

3. Restart the Python program on the remote computer so it's ready for debugging.

4. Secure the channel by adding the certificate to Trusted Root CA on the Windows computer with Visual Studio:

   1. Copy the certificate file from the remote computer to the local computer.

   2. Open Control Panel and go to Windows Tools > Manage computer certificates.

   3. In the certlm [Certificates - local computer] dialog, expand the Trusted Root Certification Authorities node, right-click Certificates, and select All Tasks > Import.

   4. Browse to and select the .cer file copied from the remote computer.

   5. Continue through the dialog prompts to complete the import process.

5. Repeat the attach process in Visual Studio, as described earlier in Attach remotely from Python Tools.

   For this instance, define tcps:// as the protocol for the Connection Target (or Qualifier).

   

Address connection issues

During the connection attempt, Visual Studio might encounter issues. Review the following scenarios and take the appropriate action, as needed.

• Visual Studio warns about potential certificate issues when connecting over SSL.

  Action: You can ignore the message and proceed.

  Caution

  Keep in mind that although the channel is still encrypted against eavesdropping, it can be open to man-in-the-middle attacks.

• Visual Studio displays the remote certificate is not trusted warning.

  Issue: The certificate isn't properly added to the Trusted Root CA.

  Action: Recheck the steps to add the certificate to Trusted Root CA on the Windows computer, and try the connection again.

  

• Visual Studio displays the remote certificate name does not match hostname warning.

  Issue: The proper hostname or IP address isn't specified for the Common Name for the certificate.

  Action: Recheck the steps in Secure the connection with TCPS. Be sure to use the correct Common Name when you create the certificate, and try the connection again.

  

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