Talks - Tian Gao: What makes a Python debugger possible and how can we make it 100x faster

Python debuggers like PDB traditionally cause ~100x slowdown due to triggering events for every line execution

sys.settrace is the core mechanism behind Python debuggers, allowing callback functions to handle events like:

• Function calls
• Line executions
• Returns
• Exceptions

Python 3.13 introduces sys.monitoring which provides:

• Selective event enabling/disabling
• Function-level granularity control
• ~100x performance improvement when properly implemented
• Ability to only trigger necessary events

Frame objects provided by CPython contain crucial debugging information:

• Local variables (frame.f_locals)
• Call stack information
• Line numbers (frame.f_linenumber)
• Code objects (frame.f_code)

Major debugging tool vendors are adopting the new sys.monitoring:

• PyCharm’s PyDevD
• Microsoft’s VS Code debugging backend
• Coverage.py for performance improvements

Core debugger requirements include:

• Execution control (step into/over/out)
• Variable inspection
• Call stack examination
• Dynamic code execution (eval/exec)

The granularity problem of sys.settrace:

• Can only enable/disable events globally
• Cannot selectively control events for specific functions
• Results in unnecessary event overhead