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Debugging Python Scripts with UDB

This guide explains how to use UDB to debug Python scripts using the python addon.

Prerequisites

You will need:

  • A working UDB installation
  • The path to your Python interpreter (e.g. /usr/bin/python)
  • The Python script you want to debug

Step 1: Start UDB with the Python Interpreter

Rather than running your Python script directly, launch UDB with your Python interpreter as the target program:

udb /path/to/python
    - or -
udb `which python`

Note: If using pyenv, use:

    udb `pyenv which python`

Step 2: Install the Python Addon

At the UDB prompt, install the Python debugging addon:

not running> extend python

UDB will download and set up the addon automatically.

⚠️ Note: The python addon is experimental and may change in incompatible ways in future releases.

Step 3: Start Your Script

Use the upy start command to load and begin recording your Python script:

not running> upy start /path/to/your_script.py [arguments]

For example:

not running> upy start /home/user/scripts/fizzbuzz.py 20

UDB will initialize the Python environment and begin recording execution. You'll see a prompt like:

Python has been initialized.
recording 5,048,331>

Navigating the Recording

Once your script is running, you have two sets of commands available:

Standard UDB commands

Use these to navigate at the C level (the underlying interpreter execution). For example:

  • continue, next, step — move forward
  • reverse-continue, reverse-next — move backward
  • break — set a C-level breakpoint
  • layout dashboard — enable the dashboard TUI layout
  • Last <expression> — travel backwards to the last time <expression> changed

Python-level commands (prefixed with upy)

Use these to navigate at the Python level (your script's source code). For example:

Command Description
upy continue Run forward until the next breakpoint or end of program
upy reverse-continue Run backward until the previous breakpoint
upy next Step forward to the next Python line
upy reverse-next Step backward to the previous Python line
upy break <function> Set a Python breakpoint at a function
upy break <file.py:line> Set a Python breakpoint at a line in file.py
upy start <script> [args] Start a Python script

Example session — running a script, setting a breakpoint, and stepping back through it:

recording> upy continue
# ... script output ...

end> upy break fizzbuzz
Python breakpoint 1 at fizzbuzz ()

end> upy reverse-continue
Python breakpoint 1, fizzbuzz () at /home/user/scripts/fizzbuzz.py:4
  #0 File "fizzbuzz.py", line 4, in fizzbuzz
    for i in range(1, max + 1):

> upy next
  #0 File "fizzbuzz.py", line 5, in fizzbuzz
    if i % 3 == 0 and i % 5 == 0:

TUI Layout

For a visual, source-level debugging interface, type:

layout python

This opens a terminal UI showing your Python source code as you step through it, similar to layout dashboard for C-level debugging.

Getting Help

To see all available Python debugging commands, type:

help upy

Quick Reference

udb `which python`         # Launch UDB with Python
udb /path/to/python        # Launch UDB with Python
extend python              # Load the Python addon 
upy start script.py [args] # Start and record your script
upy continue               # Run forward
upy reverse-continue       # Run backward
upy next                   # Step to next Python line
upy break <function>       # Set a Python breakpoint
upy break <file.py:line>   # Set a Python breakpoint
upy watch <expression>     # Set a Python watchpoint
upy info locals            # Display Python local variables
upy backtrace              # Display Python backtrace
layout python              # Open Python TUI view
layout dashboard           # Open C/C++ TUI view
help upy                   # Show all upy commands